Conduit CMBS vs SASB: The Two Sub-Markets

Introduction

Conduit and SASB are two products that share the CMBS name and almost nothing else. A conduit deal pools dozens of unrelated commercial mortgages so that no single borrower's trouble can sink the bonds, which makes diversification the product. A single-asset, single-borrower deal securitizes one large loan on one trophy property or a tightly held portfolio, so the bond effectively is the asset, and the only questions that matter are how good that asset is and how strong its sponsor. The two structures sit at opposite ends of the diversification spectrum, and over the past decade the market has swung hard toward the concentrated one. SASB issuance grew from roughly $10 billion in 2012 to about $70 billion in 2024, and it now makes up close to two-thirds of private-label CMBS issuance. Understanding why that shift happened, and what each structure is actually for, is core to reading the modern CMBS market.

A conduit deal aggregates a large number of smaller commercial mortgages, often 50 to 100 or more, from many different borrowers across varied property types and geographies. Individual loans typically run from $15 million to $75 million, and they are originated specifically to be pooled and securitized rather than held. The whole point is statistical: spread the credit risk across enough independent loans and no single default does much damage to the pool, so the senior bonds can carry high ratings on the strength of the structure rather than any one property.

Conduit CMBS

A securitization that pools many smaller commercial mortgages (often 50 to 100-plus) from unrelated borrowers across different property types and markets into a single deal. Diversification across the pool is the primary credit protection, and the bonds are typically fixed-rate.

Conduit loans are usually fixed-rate, which historically meant ten-year terms, though the market has shifted toward shorter maturities as borrowers and lenders both grew wary of locking in long-dated rate risk. Five-year loans now make up roughly 69% of conduit fixed-rate origination since 2023, against only about 11% in the prior decade. Because no single borrower controls the pool, conduit deals rely on the full tranche-and-subordination machinery: a stack of rated bonds, a first-loss B-piece, and a controlling class that directs workouts, all of which is detailed in CMBS structure, tranches, and subordination. The diversified pool is also what makes the B-piece buyer's credit work so demanding, since they must underwrite every loan in a 60-loan pool to size their first-loss bet.

The shrinking conduit

The conduit model has shrunk in more than just market share. The average conduit deal has fallen from roughly $3 billion before 2009 to around $750 million today, as tighter bank capital rules reduced the willingness of banks to warehouse loans and contribute to large diversified pools. The conduit is no longer the center of gravity it once was.

A single-asset, single-borrower deal is the opposite construction. Instead of diversifying away the risk of any one property, it concentrates the entire securitization on one large loan, backed by one trophy asset or a homogeneous portfolio held by a single institutional sponsor.

SASB CMBS

A single-asset, single-borrower securitization backed by one large commercial mortgage, typically $200 million or more and often $800 million to over $1 billion, on a single high-quality property or a tightly held portfolio under one sponsor. Credit rests on the asset's quality and the sponsor's strength rather than on diversification.

SASB loans are large by definition, usually at least $200 million and frequently running up to $800 million or beyond. Most SASB debt is floating-rate, though a meaningful slice, roughly 40%, is issued fixed. The structures tend to be shorter in duration and far more flexible on prepayment than conduit loans, which suits the business-plan-driven sponsors who use them: an owner financing a marquee office tower, a hotel, or a large logistics portfolio wants the option to refinance or sell when the plan plays out, not a decade-long lockout.

The bottom-up credit case

Because there is no diversification to fall back on, the credit analysis is entirely bottom-up. An investor in a SASB deal is underwriting one building and one sponsor, full stop, and the bonds rise and fall with that single asset. This makes SASB closer in spirit to a directly negotiated mortgage than to a diversified bond, which is precisely why sophisticated buyers like it: they can do deep diligence on one known asset rather than trusting a pool average.

The shift in the two structures' relative size is one of the defining stories of the post-crisis CMBS market. SASB accounted for roughly 68% of the $104 billion of private-label CMBS issued in 2024, and through the first three quarters of 2025 it ran at about two-thirds of issuance, with 97 deals totaling $67.47 billion. Total CMBS issuance reached $92.5 billion through September 2025, up 27% year over year, with the AAA spread tightening from a peak above 200 basis points in April 2023 to around 77 basis points by late 2025 as the market healed.

Two forces drove the rise of SASB and the relative decline of conduit.

The conduit-versus-SASB choice is really a question of asset size and profile. A sponsor with a single large, high-quality asset and a defined business plan is a SASB candidate; a lender with a book of smaller, ordinary loans to clear is feeding the conduit machine. The side-by-side comparison is worth carrying in your head.

Both structures remain essential, and a banker who understands real estate debt needs to know which problems each solves. The conduit is the diversified workhorse for the middle market; SASB is the bespoke instrument for the trophy assets and large sponsors that increasingly drive the market. Where the two leave off, the transitional and bridge loans that cannot be securitized in either format flow into the CRE CLO market for securitizing transitional loans, a third structure built for assets still in motion.