Office Lease Analysis: Gross vs NNN vs Modified Gross, and the Expense Stop / Base Year Mechanics That Decide Who Pays What

Key Takeaways

Why Lease Structure Is a Daily Underwriting Decision

In multi-tenant office, the lease structure determines who absorbs operating expense inflation, who carries vacancy-related expense distortion, and how Year 1 NOI is calculated. Property tax, insurance, and utility costs can each move on their own path, and the lease structure decides whether the landlord or the tenant bears each increase. This article uses an illustrative $18/SF to $24/SF OpEx path rather than market statistics, so the arithmetic can be followed line by line.

A tenant on a triple-net structure absorbs that inflation by contract. A tenant on a modified gross structure with an old base year (signed in 2018 against a $18/SF Year 1 OpEx that has grown to $24/SF in 2026) absorbs the cumulative escalation against that 2018 floor. A landlord on a full-service gross structure with no escalation mechanism absorbs the inflation itself and watches NOI compress. The market prices the difference: Investment Grade'ssummary of a Q1 2026 net lease reportputs single-tenant office asking cap rates at 7.90%, down 10 basis points from Q4 2025, against 6.80% for single-tenant net lease product overall.

The institutional discipline is to trust nothing in the broker's lease summary until you have read the economic terms back to the rent roll, the base year, the expense stop, the gross-up provision, and the OpEx pass-through clause. The same physical building under three different lease-structure assumptions produces three different Year 1 NOI numbers. In this article's worked example, with assumed face rents, the all-FSG versus all-NNN spread is about $31M of value at a 7% cap rate. Lease structure is a valuation input. This article walks the math.

The Three Structures: FSG, MG, NNN

The three structures define a spectrum of risk transfer between landlord and tenant. Full-service gross leaves the landlord fully exposed; triple-net pushes the entire expense base to the tenant; modified gross is the negotiated middle where the structure depends on the specific clauses. Holland & Knight'sMarch 2026 "Who Pays for What" guidedefines NNN as a pass-through of operating expenses, taxes, and insurance, and notes that a gross lease can increase the landlord's economic risk because of unpredictable increases in property expenses.

Full-service gross (FSG). Tenant pays one number, the face rent, and the landlord absorbs every operating expense behind it: property tax, insurance, utilities, janitorial, security, management fee, repairs, maintenance, landscaping, snow removal, HVAC and elevator service, administrative. Tenant has perfect cost certainty; landlord prices the expected OpEx into the face rent and absorbs the variance. In a stable OpEx environment, FSG is competitive because the landlord earns the spread between underwritten OpEx and actual. When actual OpEx outruns the underwriting assumptions priced into the face rent, FSG compresses landlord NOI. In practice, Holland & Knight notes that a lease described as "full-service gross" is often a modified gross lease; this article uses FSG to mean a gross lease with no escalation.

Modified gross (MG). The contested middle. Tenant pays base rent plus escalations above a defined baseline. The baseline is either a base year (the actual Year 1 OpEx, used as the floor) or an expense stop (a negotiated $/SF figure that may or may not equal Year 1 actual). Above the floor, the tenant absorbs escalations on a pro-rata share basis. Holland & Knight describes modified gross as commonly used in multi-tenant projects and typically built around a base-year concept. MG admits sub-types: base-year stop, fixed-dollar expense stop, hybrid with separate stops for property tax, insurance, and CAM. Two MG leases in the same building can have wildly different economics depending on the base year vintage, the stop type, the gross-up clause, and the exclusion list. Always read every MG lease clause by clause.

Triple net (NNN). Tenant pays base rent plus its pro-rata share of all three operating expense categories: property tax, insurance, and common area maintenance (CAM, the catch-all for everything else). The "three nets" framing is the structural origin of the name. Holland & Knight notes NNN leases are most common in retail; the structure also appears in single-tenant and multi-tenant office. Sub-variants in office: absolute net (tenant carries even structural and capital expenses, rare in office), double net (tenant carries tax and insurance but landlord retains CAM, occasional in single-tenant office), and modified NNN (tenant carries all three but with capped escalations or specific category exclusions).

The comparison at a glance:

Table 1. The three office lease structures on the spectrum from full landlord risk to full tenant risk. Modified gross occupies the contested middle where two leases in the same building can have materially different economics.

Base Year and the Institutional Games

The base year is the dollar amount of operating expenses in the lease's defined first year, used as the floor against which subsequent-year escalations are calculated. In this article's examples the tenant pays base rent flat each year (a simplification; real leases usually carry rent steps), and the landlord recovers the dollar difference between the current-year OpEx and the base-year OpEx as additional rent. The math is simple at the surface: $18/SF base year, $24/SF current year, $6/SF escalation that the tenant pays in addition to base rent. The institutional reading is harder, because the games landlords play with the base year compound the further into a lease term the tenant goes.

The institutional games begin at lease signing and compound over the term.

The low-floor base year. Landlord signs the tenant against a base year that understates stabilized OpEx: a partially occupied year, a pre-reassessment tax year, or a trough-cost year. The tenant's floor is artificially low, so it pays the normalization as escalation from Day 1. The market convention is that the base year is the lease's first full year at a stabilized operating level; sophisticated tenant reps negotiate a "year of occupancy" base year and push back against floors set in an unrepresentative year.

Tax base year manipulation. The property tax bill in a building's first year of operation is often abnormally low: either the building was under construction for part of the year, the assessment was based on land-only value, or the assessor hasn't yet reassessed at the stabilized value. Landlords sometimes sign tenants against that pre-stabilization tax base year, knowing the assessed value will normalize upward in the following years and the tenant will absorb the full normalization as escalation. The institutional discipline is to pull assessor records, identify when the property was first assessed at its stabilized value, and require that the base year tax bill reflect the stabilized assessment, not the construction-period assessment.

Insurance carrier swap. Landlord places insurance in the base year with the cheapest carrier it can find, then switches to a higher-cost carrier in Year 2 and passes the differential through as inflation. The tenant has no visibility into the carrier change, and the escalation against the artificially low base-year floor reflects the landlord's own placement decision rather than market-wide insurance inflation. The carrier-change disclosure is the discovery point.

Asymmetric gross-up. Landlord grosses up the operating expenses in escalation years (to full occupancy, capturing what variable OpEx would have been if the building were full) but does not gross up the base year. The escalation arithmetic now compares a grossed-up current year against a raw base year. The tenant pays an inflated escalation that captures the building's vacancy as if it were occupancy. The institutional discipline is to require symmetric gross-up application: either both years grossed up or neither.

Tenant protections. Sophisticated tenant counsel negotiates three protections. Therolling base year: the base year resets each year to the prior actual, so the tenant only absorbs year-over-year inflation, never cumulative inflation against a stale floor. Theconstant-dollar base year: the base year is inflation-adjusted using CPI or a negotiated index, neutralizing the low-floor base-year game. The fair-and-equitable gross-up symmetry clause: the gross-up applies symmetrically across the base year and all escalation years. Stacked, these three protections address the main base-year games. They are typically negotiated at the LOI stage.

Expense Stops vs Base Year

The expense stop is the base year's negotiated cousin. Rather than defining the floor as "the actual Year 1 OpEx," the expense stop defines it as "$X per square foot, regardless of actual." The distinction matters at the clause level because the expense stop decouples the floor from the landlord's actual cost: actual OpEx may land above or below the stop. ThePropertyMetricsguide, quoting the CCIM Institute, defines an expense stop as "the level (or maximum amount) up to which the landlord will pay certain operating expenses," with amounts above the stop the tenant's responsibility. TheCommercial Real Estate Loans glossaryframes it the same way: the most a landlord will pay for expenses in any given year. That framing is accurate, but the institutional reading goes one level deeper into the sub-types.

Base-year stop. The stop is set equal to the actual Year 1 OpEx. This is the form that gets conflated with "base year" in the broker-tier literature. Operationally identical to a base year for the first year; the distinction surfaces in subsequent-year reconciliation mechanics and audit rights, where stop language and base-year language can have different procedural implications. Tenant-friendly when Year 1 is a high-OpEx year (the floor is set at the peak); landlord- friendly when Year 1 is a low-OpEx year (the floor is set at the trough).

Fixed-dollar stop. The stop is a negotiated number independent of actual, for example $8.50/SF. It suits parties who want a negotiated number rather than a historical one. If actual Year 1 OpEx comes in at $9.50/SF, the tenant pays the $1/SF above the stop and the landlord bears only the first $8.50/SF. If actual comes in at $7.50/SF, the landlord pays only $7.50 against rent priced for $8.50 and keeps the $1/SF spread. The landlord's exposure is capped at the stop, and the tenant carries every dollar above it. Tenants sometimes negotiate stops that escalate (for example 2–3% a year, an illustrative figure) so that less of normal inflation passes through to them.

Component stops. Separate stops for each category: property tax, insurance, and CAM. The sophisticated tenant negotiation; protects against one expense category running materially hot while the others stay tame. For example, the tenant can negotiate an insurance stop set at the most recent year's insurance cost rather than a trailing three-year average, so a recent insurance spike sits in the floor the landlord carries. Experienced tenant counsel typically negotiates these on larger office leases.

The clause-level distinction between expense stop and base year matters for audit rights, gross-up application, and the renewal economics question. A base year resets every time the tenant signs a new lease (or renewal); an expense stop is negotiated separately and can be carried forward across renewals at the original number if the lease provides for it (rare; usually the renewal resets the stop). The institutional discipline on diligence: confirm the lease's defined floor type, the floor's vintage, the sub-type (fixed-dollar vs base-year vs component), and the renewal carry-forward language. Three questions, three minutes, materially different economic implications.

The Gross-Up Provision

The gross-up provision is the lease's mechanism for neutralizing vacancy-related distortion in operating expense reconciliation. The problem the gross-up solves: in a 70%-occupied building, the variable operating expenses (electricity in occupied suites, janitorial service, management fees that scale with revenue, supplies) are artificially low because the vacant 30% of the building is not consuming them. The base year of a tenant who signed when the building was 70% occupied would lock in an artificially low floor, and the landlord would be exposed when occupancy improved and variable OpEx normalized upward against the low floor. The gross-up adjusts the variable OpEx to what itwould have been at a higher occupancy level, typically 95% or 100% (the exact percentage is negotiated, per Holland & Hart), then charges each tenant their pro-rata share of the grossed-up number.

The canonical legal reference isHolland & Hart's gross-up provisions white paper, which establishes the variable-vs-fixed distinction at the legal level. The institutional practitioner reference isAQUILA Commercial's tenant-rep-side analysis, which walks the mechanics at the clause level from the tenant's perspective.

The institutional discipline checks four things in every gross-up clause:

One: variable-only application. Gross-up applies only to variable expenses (utilities, janitorial, management fees, supplies, repairs that scale with occupancy). Fixed expenses such as property tax and insurance premiums do not vary with occupancy and should not be grossed up (Holland & Hart and AQUILA both draw this line). Landlords sometimes apply gross-up to fixed expenses (intentionally or through poor reconciliation software). This is a pure overcharge; property tax does not increase because the building leased up. Severity: material concern, and it often surfaces on CAM audits where the gross-up clause is loosely drafted.

Two: the gross-up trigger and target. Two numbers matter: the occupancy below which the clause applies (AQUILA: typically below 95% to 100%) and the occupancy to which expenses are grossed up (AQUILA: 95% to 100%). A higher target (100%) grosses up more of the vacant variable cost and increases the tenant's bill; a lower target (for example 75% or 80%, a compromise Holland & Hart describes) reduces it. Verify both in the lease language.

Three: symmetric application across the base year. The gross-up must apply both to the base year (or the expense stop reconciliation) and to escalation years. Asymmetric application, which grosses up escalation years but not the base year, inflates the tenant's escalation by capturing the building's vacancy gap as if it were inflation. This is a common seller-side game on MG and NNN reconciliations, particularly on buildings that leased up materially between the base year and the audit year. Symmetric application is the institutional standard.

Four: occupancy measurement methodology. The gross-up calculation requires the landlord to define "occupied": is it leased, occupied, or revenue-generating? Each gives a different answer. A suite that is leased but in tenant improvement build-out is not consuming variable OpEx; counting it as occupied misstates the occupancy used in the gross-up. The institutional discipline is to require "revenue-generating occupancy" as the gross-up base: the suite has a paying tenant in occupancy.

CAM, Pro-Rata Share, and the Pass-Through Math

Common area maintenance (CAM) is the catch-all for the operating expenses that aren't property tax or insurance. In a multi-tenant office building, CAM includes utilities for common areas (lobby, corridors, elevators, garage), janitorial for common areas, security, repairs and maintenance of common-area systems (HVAC, elevator, fire/life safety), landscaping, snow removal, exterior cleaning, management fee, administrative cost, supplies, and lobby attendant or concierge salaries. CAM is the engine of MG and NNN pass-throughs; the institutional reading walks the math.

Pro-rata share. Tenant SF divided by building SF, as the lease defines both. Many leases measure rentable SF (which includes a load factor for common areas) rather than usable SF (the tenant's direct space) or a landlord-defined leasable figure. ANSI/BOMA Z65.1 is the BOMA International standard for office floor measurement and defines rentable area precisely; landlords sometimes use a different denominator than the lease intends in order to inflate the tenant's share. Verify the lease's pro-rata share definition references a stated measurement standard and a fixed building area, not "the building's total square footage as determined by Landlord."

Estimate / true-up cycle. Landlords typically collect monthly CAM estimates against the budget, then reconcile to actuals at year-end. The reconciliation produces either an additional payment due from the tenant (if actuals exceeded budget) or a credit to the tenant (if budget exceeded actuals). The lease sets when the reconciliation must be delivered and how long the tenant has to dispute it, so the audit window is a diligence item. Disputes are common because the line between operating and capital is fuzzy, the gross-up methodology is rarely documented, and the inclusion/exclusion list is often interpreted ambiguously by landlord accounting.

Common inclusions. Property tax, insurance premiums, utilities (electric, water, sewer, gas), janitorial, security, management fee (this article assumes 2–4% of EGI as an illustrative range), repairs and maintenance, landscaping, snow removal, HVAC service contracts, elevator service contracts, fire/life safety, supplies, lobby attendant, administrative cost.

Common exclusions. Capital expenditures (HVAC replacement, roof, elevator modernization, building system replacement), leasing commissions, tenant improvement allowances, free rent (concessions), executive compensation, marketing and promotional costs, depreciation, debt service, penalties and violations, capital reserves, advertising, ground lease payments (unless the lease provides for their recovery), and the landlord's corporate-level overhead. The exclusion list is where the games happen: landlords push capital items into operating, particularly HVAC replacement (typically the capital line item with the largest dollar magnitude on an older building).

The institutional CAM reconciliation discipline: every reconciliation should be reconciled to (a) the prior year's actual reconciliation (line-by-line variance), (b) published OpEx benchmark data for the building's class and metro, (c) the lease's explicit inclusion/exclusion list, and (d) the budget vs actual variance with line-item explanation. The diligence packet is four documents and the institutional reading is line-by-line.

The OpEx Inflation Overlay

Property tax, insurance, and utilities can each move on their own path, and the flow-through differs sharply by lease structure. Rather than lean on market averages, this section uses the OpEx path assumed in the worked example below ($18/SF in 2018 growing to $24/SF in 2026) and shows who bears each dollar of that increase under each structure.

Table 2. The OpEx path assumed in the worked example. These are illustrative assumptions, not market statistics. The 3.66% annual rate is (24 / 18)^(1/8) − 1, and the 2028 figure extends the same rate two more years.

The flow-through by structure:

• FSG. Landlord absorbs 100% of the increase. In the worked example, an all-FSG building at $65/SF face rent ($13.0M revenue) sees OpEx rise from $3.6M to $4.8M, which takes NOI from $9.4M to $8.2M: a 12.8% compression against original underwriting (9.2% measured on revenue), absent renegotiation. The leverage point: as FSG tenants roll, the landlord can convert to MG (or to NNN with appropriate face-rent compensation) and arrest the compression. Buildings unable to convert because the tenant has the leverage to demand FSG renewal at face are the structural losers when OpEx rises.

• MG with an old base year (2018). Tenant pays escalations against an old floor. On a 2018 base year MG with a 2026 reconciliation, the tenant absorbs the cumulative 2018–2026 increase (33% in the example) as escalation. Renewal economics are contested: tenants negotiate hard against rolling the lease forward at face because they know a renewal on a new MG with a current base year resets their floor and, in the example below, lowers their effective rent by about $6/SF. The landlord's renewal economics are hostage to the base year reset.

• MG with a current base year (recently signed). Tenant pays escalations against a recent floor, which is how the structure was originally designed to work.

• NNN. Tenant pays 100% pass-through. Tenant credit risk rises as OpEx absorbs a larger share of the tenant's cash flow. The landlord retains base rent durability but the tenant's downside is now structural OpEx exposure. Investment Grade's summary of a Q1 2026 net lease report puts single-tenant office net lease asking cap rates at 7.90%, versus 6.80% for all single-tenant net lease product, a premium consistent with how the market prices credit and lease-term risk.

The incentive structure also differs by lease type. Landlords on FSG absorb 100% of any OpEx savings from capital investments such as energy and building-controls upgrades, so they have the largest incentive to deploy. Landlords on NNN pass savings through to the tenant in subsequent-year reconciliation, which affects capital allocation across a stabilized portfolio.

The CAM Reconciliation Red Flag Taxonomy

The institutional reading uses a structured taxonomy of CAM reconciliation red flags. Each red flag maps to the lease clause where it surfaces, the cross-check document that confirms or refutes it, the severity grade (Diligence / Material / Deal-Killer), and the specific reconciliation or underwriting adjustment. The twelve flags below are a working checklist for multi-tenant office CAM reconciliations.

Table 3. The CAM reconciliation red flag taxonomy. Severity grades: Diligence (price into a buffer); Material Concern (specific reconciliation adjustment); Deal-Killer (renegotiate or walk at lease signing). Sophisticated tenant reps audit every reconciliation against the full taxonomy.

The seller-side patterns to look for on multi-tenant Class A office reconciliations: HVAC replacement billed as repairs in OpEx (capital-in-operating, flag #1); management fees above the lease limit (flag #2); gross-up applied to property tax (flag #4); pro-rata denominator using leasable rather than rentable area (flag #6); base-year property tax captured from the under-construction year before stabilization (flag #9, the highest-severity); insurance carrier swapped without adjustment to the base-year insurance figure (flag #10). The institutional CAM auditor walks the twelve-flag table once per reconciliation, and every flag that survives review is a refund line.

Worked Example: 200,000 SF Class A Boston CBD, Three Tenants, Three Structures

The integrated worked example. Setup before reading the lease pool:

Tenant A: Law Firm. 50,000 SF (25% of building). FSG @ $65/SF, 10-year lease commencing 2026.The law firm signs a fresh FSG lease in 2026 at $65/SF face rent. No escalation; one number, all-in. Landlord absorbs the tenant's 25% pro-rata share of the building's 2026 OpEx: 25% × $4,800,000 = $1,200,000 absorbed annually. Tenant pays $65/SF × 50,000 SF = $3,250,000 in face rent. Landlord nets $3,250,000 − $1,200,000 = $2,050,000 from Tenant A. Tenant's effective rent: $65/SF (no escalation absorbed). The landlord has effectively quoted a face rent that prices in expected 2026 OpEx plus a margin, but in a 33% cumulative inflation environment, the law firm has perfect cost certainty and the landlord absorbs any further 2027–2035 OpEx growth on the absorbed share.

Tenant B: Financial Services. 75,000 SF (37.5% of building). MG with 2018 base year @ $58/SF base rent, Year 9 of a 10-year lease (2018–2027). The financial services tenant signed in 2018 against the 2018 building OpEx of $18/SF as the base year. Tenant pays $58/SF base rent flat each year. Escalation in 2026: ($24 − $18) × 75,000 SF = $450,000 annually, or $6/SF on top of base rent. Tenant's effective gross rent in 2026: $58 + $6 = $64/SF. Landlord's cost of Tenant B's 37.5% share of 2026 OpEx is 37.5% × $4,800,000 = $1,800,000. Landlord receives $58/SF × 75,000 SF = $4,350,000 base rent + $450,000 escalation = $4,800,000 from Tenant B. Landlord nets $4,800,000 − $1,800,000 = $3,000,000 from Tenant B ($40/SF). Of that $1,800,000 cost, $450,000 is recovered through escalation, so the landlord's unrecovered share is $1,350,000, the 2018 base-year level (37.5% × $3,600,000). The 2018 base year is doing the work: Tenant B absorbs eight years of accumulated inflation against the 2018 floor.

Tenant C: Tech Company. 75,000 SF (37.5% of building). NNN @ $52/SF base + pro-rata OpEx, Year 3 of 12-year lease commencing 2024. The tech tenant signed in 2024 on a triple-net structure at $52/SF base rent plus pro-rata share of all operating expenses. 2026 pro-rata pass-through: 37.5% × $4,800,000 = $1,800,000, or $24/SF on top of base rent. Tenant's effective gross rent: $52 + $24 = $76/SF. Landlord absorbs zero of the tenant's pro-rata share (the pass-through is a wash). Landlord receives $52/SF × 75,000 SF = $3,900,000 base rent + $1,800,000 pass-through = $5,700,000 from Tenant C. Landlord nets $5,700,000 − $1,800,000 = $3,900,000 from Tenant C (the pass-through revenue cancels the OpEx the landlord paid and then recovered). Tenant C carries the full $24/SF OpEx exposure; if 2027 OpEx grows to $26/SF, Tenant C absorbs the full $150,000 incremental.

Table 4. The three-tenant, three-structure building. Total landlord NOI (before vacancy adjustment and shared common-area OpEx) is $8.95M on the mixed lease pool: $13.75M of revenue less $4.8M of OpEx. The structural composition matters: the same physical OpEx flows through differently to each tenant.

The renewal economics for Tenant B. Tenant B's 10-year term ends in 2027, so the renegotiation is a renewal beginning in 2028. If Tenant B renews on a new MG with a base year set at the first year of the new term, the floor resets to about $25.8/SF (the example's 3.66% trend extended from $24 in 2026). Tenant B's face rent stays roughly at market ($58–$62/SF; assume $60/SF), but the escalation now applies only to 2029+ inflation, not the 2018–2028 accumulated increase of about $7.80/SF. On the old lease, Tenant B's 2028 effective rent would have been about $65.80 ($58 plus $7.80); on the renewal it is $60. The $7.80/SF escalation loss is partly offset by the $2 face-rent increase, so effective rent falls about $5.80/SF, or about $435K a year across 75,000 SF, and that directly hits NOI. This is the renewal trap that landlords on MG buildings with multiple old base years face simultaneously: the base year reset on each renewal structurally compresses landlord NOI by the cumulative inflation between the original base year and the renewal date. The institutional underwriter must model the base-year reset explicitly on every MG lease rolling within the hold period.

The reading for the institutional analyst: lease structure mix is part of the asset, not metadata. A building with 60% FSG, 30% old-base-year MG, and 10% NNN is a different asset for valuation purposes than the same building with 20% FSG, 30% MG-current-base-year, and 50% NNN. The cap rate the buyer applies will be different; the NOI durability under OpEx stress will be different; the renewal economics will be different. For the underlying valuation mechanics that this NOI flows into, see thecap rate calculator and formula articleand theIRR calculator and formula article. For the TI/LC and effective rent overlay that runs alongside this lease-structure analysis, see theoffice underwriting: TI/LC, free rent, effective rent article. For repositioning class and trophy strategies that intersect with lease pool composition, see thetrophy Class A vs Class B repositioning article.

The Institutional Underwriting Overlay

How an institutional underwriter reads the lease pool of a multi-tenant office building. The discipline is a structured cascade.

Step one: pull the lease abstract for every tenant. Verify the lease type, the base year (if MG) and its vintage, the expense stop (if MG) and its type, the gross-up clause language and the threshold, the audit rights clause, the inclusion/exclusion list, the pro-rata share definition, the permitted use clause, and the renewal options. Map each lease to theA.CRE overview of NNN, base-year, and expense-stop office leasesand flag deviations.

Step two: reconcile the rent roll's lease type column against actual lease documents.Sample every one of the largest tenants by SF and a share of the long tail (an illustrative 25%). Rent roll abstracts, particularly those produced by lease administration vendors, can miscode lease structure (an MG-with-tax-stop coded as full NNN; a modified-NNN coded as plain NNN; a fixed-dollar expense stop coded as a base-year stop). The miscodes are not small: they can move Year 1 NOI materially on individual leases.

Step three: build a pro-forma OpEx flowing through each lease structure. For each tenant, calculate the tenant's share of current-year building OpEx (what the landlord pays) and the revenue (what landlord collects via base rent + escalation + pass-through). Net to the tenant-level NOI contribution. The sum is the building's underwritten Year 1 NOI before vacancy and shared common-area OpEx adjustments.

Step four: stress test 2026 OpEx +5% / +10% / +15%. Identify which tenants absorb the increase (NNN tenants and MG-above-base-year tenants), which the landlord absorbs (FSG tenants and the base-year-locked portion of MG tenants). Build the NOI sensitivity table. A building with 60% FSG / 40% NNN has dramatically different sensitivity to OpEx stress than a building with 20% FSG / 80% NNN.

Step five: look at the lease maturity ladder. Tenants on old-base-year MG leases rolling in the next 24 months will renew at lower effective rents (the base-year reset structurally drops their effective rent because the new base year captures the cumulative 2018–2026 inflation). Model this as a downward effective-rent adjustment at renewal. On a building with substantial old-base-year MG leases rolling in Year 1–2 of the hold, this is a Year-2–3 NOI step-down that the broker's proforma will not show.

Step six: look at lease structure mix vs market. A predominantly-FSG building in a market that has shifted to NNN signals tenant-rep negotiation leverage at renewal: tenants will push for NNN with lower face rent at renewal to match the prevailing market structure, and the landlord may absorb a structural NOI step-down on the conversion. Conversely, a predominantly-NNN building in a market where Class A is moving back to MG (rare but possible in tenant-leverage markets) signals structural NOI upside.

Step seven: cross-check OpEx assumptions against institutional benchmarks. Published office OpEx surveys report benchmarks by class, metro, and building size, and broker research adds metro-specific context. The institutional discipline: every OpEx line item in the proforma should be cross-checked against at least two independent references.

Step eight: debt sizing. Lenders apply vacancy and OpEx stresses on top of underwritten figures. Lease structure determines whether the OpEx stress hits NOI (FSG) or is passed through to tenants (NNN). For office collateral going toCMBS conduit or SASB financing, a predominantly-FSG lease pool leaves the borrower exposed to OpEx growth, which weakens DSCR under stress.

Five Mistakes Practitioners Make

• Treating "modified gross" as a single structure. MG is a category. Two MG leases in the same building can have wildly different economics depending on base year vintage, stop type, gross-up clause symmetry, and the exclusion list. Always read every MG lease clause by clause. Lease-administration vendor outputs that aggregate lease type at the building level systematically misrepresent MG variance.

• Ignoring the base year vintage. A 2018 base year in a 2026 lease is structurally penal to the tenant: the prior eight years of OpEx inflation are baked into immediate escalations against an unchanged floor. Always verify the base year vintage on every MG lease and model the base-year reset on every renewal within the hold period. The reset is a NOI step-down for the landlord that the broker's proforma will not show.

• Missing the gross-up provision asymmetry. Landlord grosses up escalation years but not the base year (or vice versa). Asymmetric application inflates the tenant's share by capturing the building's vacancy gap as if it were inflation. Always verify the gross-up clause applies symmetrically across base year and escalation years, and verify it applies only to variable expenses, never to fixed.

• Conflating face rent and effective rent. Two tenants on $58/SF face rent in the same building can be paying $58/SF effective (FSG) and $82/SF effective (NNN with full pass-through). Always net face rent to comparable effective rent using the building's current OpEx and the lease's pass-through mechanics. The institutional comp-set discipline is on effective rent, not face. For the full effective-rent walkthrough with TI/LC and free rent overlay, see theoffice underwriting: TI/LC, free rent, effective rent article.

• Skipping the CAM reconciliation history. A landlord with three consecutive years of contested CAM reconciliations signals a pattern, not an exception. Pull three years of reconciliations on diligence; if the building has a chronic dispute history, that history travels with the asset. Cross-check to the 12-flag red-flag taxonomy; recurring flags across multiple years are diagnostic of the landlord's reconciliation methodology and the audit volume to expect post-close.

From Lease Analysis to Institutional Pro Forma

The reading exercise above is the institutional discipline; the modeling step is what consumes it. Once you have walked the three-structure taxonomy, confirmed the base year vintage on every MG lease, audited the gross-up clause for symmetric application and variable-only scope, reconciled the pro-rata share definitions against BOMA-rentable, mapped the inclusion/exclusion list against the twelve-flag taxonomy, and stress-tested the OpEx environment by structure, the next step is the full institutional office pro forma. Year 1 NOI, debt sizing, value-add capex timing, stabilized Year 3, exit cap, levered IRR. We know you can build this in Excel from the lease abstracts and the OpEx benchmarks. The reader can do it in Apers in minutes.

Related Articles

• Office Underwriting: TI/LC, Free Rent, and Effective Rent: the leasing concessions and net effective rent overlay that runs alongside lease-structure analysis.
• Trophy Class A vs Class B Repositioning: the asset-class and repositioning strategies that intersect with lease pool composition.
• Cap Rate Calculator and Formula: the valuation mechanics that the lease-structure NOI flows into.
• IRR Calculator and Formula for Real Estate: the returns analysis the full pro forma supports.
• CMBS Conduit vs SASB: Defeasance and Yield Maintenance: the office CMBS financing context for lease-pool-dependent DSCR underwriting.
• Multifamily Underwriting Fundamentals: The Rent Roll Reading Exercise: the parallel institutional reading exercise for multifamily.

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