CMBS Prepayment: Conduit vs SASB, Yield Maintenance, and Defeasance — A Practitioner's Guide

Key Takeaways

The Prepayment Question Practitioners Need Answered

Every CMBS borrower eventually confronts the same question: what does it cost to prepay this loan? The answer depends on which side of the lockout window the loan is in, whether it's in the yield-maintenance period or the defeasance period (most modern CMBS uses defeasance), and what the Treasury yield curve looks like on the day the prepayment calculation runs. The math is straightforward in the abstract and obscure in practice — because every defeasance consultant in the market gates the calculation behind a sales conversation.

Chatham Financial, Commercial Defeasance LLC ("Defease With Ease"), AST Defeasance, and Waterstone all offer free quote calculators that produce a single number; none of them publishes the formula or the worked example that produced it. Practitioner-grade CMBS borrowers have to commit to a $5K–$15K engagement before seeing the math — or they have to derive it themselves. This article does the deriving.

The audience is institutional borrowers approaching CMBS maturity, capital markets desks running prepay vs hold analysis, asset managers evaluating refinance optionality, and debt brokers structuring CMBS placements. The article works through CMBS conduit vs SASB structural differences, the formal yield maintenance formula, the defeasance portfolio construction, a worked $50M example, and the 2026 specific story — the return of negative-defeasance opportunities for the first time since 2006 as the 2014–2016 CMBS vintage hits maturity.

CMBS Structure: Conduit vs SASB

Commercial mortgage-backed securities come in two structural flavors that matter for prepayment mechanics:

Conduit CMBS pools loans from multiple borrowers, multiple property types, and multiple markets into a single securitization. The pool diversification provides credit enhancement; the resulting securities are tranched A-AAA through B-piece. Conduit loans are typically $5M–$75M, 10-year fixed-rate, sized to 65–75% LTV at origination. Borrowers in a conduit pool can't choose their pool-mates, can't negotiate the prepayment terms (they're standard market), and have limited ongoing communication with the master servicer (the entity that collects payments and enforces the loan terms).

SASB (Single Asset, Single Borrower) CMBS securitizes one large loan to one borrower as a standalone deal. Loan sizes range from $200M to multi-billion-dollar. The borrower negotiates the loan structure directly; prepayment terms can include shorter lockouts, partial-release mechanics, and yield- maintenance instead of defeasance. SASB borrowers maintain direct relationships with the servicer and have more flexibility on operational provisions. Per the Structured Finance Association's SASB research, SASB issuance grew from ~20% of CMBS volume in 2015 to ~45% in 2024–2025 as institutional borrowers shifted toward larger, more flexible deals.

Per Trepp's CMBS issuance data and the KBRA structured finance outlook, total CMBS issuance is projected at approximately $183B in 2026, up from $115B in 2024. The growth reflects the 2014–2016 vintage maturity wave (10-year loans originated during the post-GFC recovery are reaching balloon) plus a steady SASB pipeline from institutional borrowers refinancing 2014–2015 vintage SASB deals.

The Prepayment Timeline

A typical 10-year CMBS conduit loan has a four-phase prepayment structure:

The lockout exists because the CMBS investors need predictable cash flow during the early years of the securitization. The penalty window protects the investors against rate-driven prepayment that would force them to reinvest at lower yields. The open period at the end provides operational flexibility for the borrower to refinance during the last few months before balloon maturity without paying penalty.

Yield Maintenance: The Formula

Yield maintenance is a closed-form prepayment penalty that pays the lender the present value of the lost coupon. The formula in its standard form:

The intuition: the lender is being paid out today the present value of every dollar of coupon they would have received between today and maturity, computed at the current reinvestment rate. If the loan coupon is 4.0% and the Treasury reinvestment rate is 4.2%, the lender can reinvest the prepayment proceeds at a higher rate than the loan coupon — so YM should be zero (the max(0, ...) floor protects this). If the loan coupon is 4.0% and the reinvestment rate is 2.0%, the lender loses 200 bps of coupon for every month remaining; YM compensates for that loss.

The Treasury yield used in the reinvestment-rate calculation comes from the Fed's H.15 daily yield curve, interpolated to the remaining loan tenor. The spread above Treasury (the "make-whole spread") is documented in the loan agreement and typically runs 25–50 bps for institutional CMBS.

Defeasance: How It Actually Works

Defeasance is structurally different from yield maintenance. Instead of paying the lender a cash penalty, the borrower substitutes a portfolio of U.S. Treasury securities — sized and timed to exactly replicate the remaining loan cash flows — for the underlying real estate collateral. The loan continues to exist on the books; what changes is what's behind it.

The mechanics:

• 1. The borrower instructs a defeasance consultant. The consultant identifies the U.S. Treasury securities required to match the loan's remaining cash flows (monthly principal and interest through the open period, plus the balloon at maturity). The consultant negotiates the Treasury purchase in the secondary market, typically on a same-day or next-day basis.

• 2. The borrower funds the Treasury purchase. The cost of the Treasury portfolio is the defeasance cost. When Treasury yields are below the loan coupon (the typical case), the Treasury portfolio costs more than the loan balance; the borrower pays the difference. When Treasury yields are above the loan coupon (the rare "negative defeasance" case), the Treasury portfolio costs less than the loan balance; the borrower receives the difference.

• 3. The collateral substitution occurs. The defeasance consultant, the master servicer (typically Wells Fargo, KKR Real Estate Finance, or another major servicer), and the borrower's counsel close the substitution. The real estate collateral is released; the Treasury portfolio becomes the loan collateral. The borrower is now free to sell or refinance the real estate.

The advantage of defeasance over yield maintenance: defeasance can produce material cost savings (or even negative cost) in certain rate environments. The disadvantage: the transaction takes 30–45 days end-to-end and requires specialized consultant fees ($5K–$15K for the consultant plus $3K–$10K for the borrower's defeasance counsel). For small loans or quick prepayment scenarios, yield maintenance is simpler; for larger loans and material rate environments, defeasance is the institutional norm.

Worked Example: $50M CMBS Loan

A $50M CMBS conduit loan originated in 2018 at a 4.50% coupon, 10-year term, 30-year amortization. In May 2026 (month 96 of 120), the borrower wants to prepay to refinance into agency debt at 5.75%. The loan is in the defeasance period (months 25–117); the loan balance is approximately $42.8M (factoring 8 years of amortization). The borrower has 24 months remaining until the open period.

The defeasance calculation:

• Required Treasury cash flows: 24 monthly payments of approximately $190K of interest plus modest amortization, plus a $42.4M balloon at month 120. Total required cash flows: ~$42.4M balloon + ~$4.6M of monthly P&I = ~$47.0M.
• Treasury portfolio cost: At the May 2026 Treasury yield curve (~4.20% on the 2-year per the Fed H.15), the PV of $47.0M of cash flows ending in 24 months is approximately $43.5M.
• Defeasance cost: Treasury portfolio cost ($43.5M) minus loan balance ($42.8M) = $0.7M, or about 1.6% of loan balance. Plus consultant fees ($5K–$15K) and borrower's counsel fees ($3K–$10K).

The defeasance cost is unusually low in this example because the May 2026 Treasury yield (4.20%) is below the loan coupon (4.50%) by only 30 bps — producing modest PV of lost coupon. If the same loan were defeased when Treasury yields were 100 bps below the loan coupon (a more typical condition), the defeasance cost would be approximately 3–4% of loan balance. If Treasury yields rose above the loan coupon (the "negative defeasance" case discussed below), the defeasance cost would be negative — the borrower would receive cash at prepayment rather than pay it.

Negative Defeasance: A 2026 Phenomenon

For the first time since 2006, the U.S. Treasury yield curve in 2026 is producing "negative defeasance" opportunities on certain loan vintages. The mechanics: when the Treasury yield for a remaining term exceeds the loan coupon, the Treasury portfolio required to defease the loan costs less than the loan balance — so the borrower receives cash on prepayment rather than paying it.

The specific cohort affected is the 2014–2016 CMBS vintage with coupons in the 3.5–4.0% range, approaching maturity in 2024–2026. With current Treasury yields at 4.20% on the 2-year (per the Fed H.15) and 4.10% on the 10-year, these loans can be defeased at a net cash receipt of 0.5–1.5% of loan balance — effectively being paid to prepay. Chatham Financial's research arm has flagged this condition as the first negative-defeasance window since the 2006–2007 cycle.

For 2014–2016 vintage borrowers, this changes the prepayment calculus materially. A 2014 vintage CMBS loan with two years remaining used to be a "wait it out" decision because defeasance was expensive; in 2026 it's potentially a "prepay now and take the cash" decision. The Trepp CMBS distress monitor tracks the vintage as it works through the maturity wave; KBRA's CRE CLO outlook flags the negative-defeasance opportunity as a structural feature of the current cycle that will compress as the yield curve normalizes.

When YM, When Defeasance, When Hold

Three considerations drive the prepayment decision:

• Where is the loan in its life? If the loan is in the lockout period, prepayment isn't an option; the only choices are workout (which requires lender consent and typically isn't available for a performing loan) or assumption (transferring the loan to a buyer at sale). If the loan is in the open period, prepayment is at par with no penalty; just do it.

• What are current yield-curve conditions? When Treasury yields are well below the loan coupon (the typical case in declining-rate environments), defeasance and YM both have meaningful cost. When Treasury yields are at or above the loan coupon (rare; the May 2026 condition for 2014–2016 vintage loans), defeasance can produce material savings or even net cash receipt.

• What's the refinance arbitrage? Even with a 3% defeasance cost, prepaying makes sense if the refinanced loan terms (lower rate, longer term, cash-out, removed recourse) produce more value than the prepayment cost. Run the math both ways: present value of remaining payments under the existing loan vs prepayment cost plus present value of remaining payments under the refinanced loan.

For SASB loans with negotiated terms, the prepayment options are often broader than for conduit loans — partial releases, restructure provisions, and modified penalty curves are sometimes available. The Structured Finance Association publishes occasional research on SASB structural variation that's worth referencing for institutional borrowers.

Five Mistakes Borrowers Make

• Computing YM with the wrong reinvestment rate. The make-whole spread (Treasury + 25–50 bps typical) is in the loan documents and varies by deal. Using the wrong spread can over- or under-state YM by 50–100 bps of loan balance.

• Assuming defeasance is "free" in a negative-defeasance market. Even when the Treasury portfolio cost is below loan balance, defeasance still involves consultant fees, counsel fees, and 30-45 days of timeline. The "negative cost" is a balance-sheet calculation; the operational cost still exists.

• Not modeling the open period in refinance optionality. If the loan is 6 months from the open period, often the right answer is to wait. Paying 3% defeasance to refinance into a slightly better rate doesn't usually clear the math if free prepayment is six months away.

• Failing to consult the master servicer early. The master servicer (Wells Fargo, KKR Real Estate, or a specialty servicer) controls the prepayment process. Their cooperation, timeline, and document requirements vary. Start the conversation 90+ days before intended prepayment date.

• Underestimating the operational burden. Defeasance involves 30–45 days of coordination among defeasance consultant, master servicer, borrower's counsel, servicer's counsel, trustee, rating agency (occasionally), and the Treasury broker. Plan for the operational lift, not just the balance-sheet number.

Do It in Apers

Related Articles

• Construction Loans: Draws and Interest Reserves — the construction-period mechanics that precede a CMBS takeout.
• Bridge Loans: Floating-Rate Risk and Exit Assumptions — the bridge debt that often precedes CMBS placement.
• Cap Rate Calculator and Formula — the exit-cap assumption that drives refinance value.
• IRR Calculator and Formula for Real Estate — the return metric that prepayment cost feeds into.

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