Financing the Missing Middle

Barriers and Emerging Opportunities in Funding ADUs and Small-Plex Housing

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James Carras, June 1, 2026

WHITE PAPER

Financing the Missing Middle

Barriers and Emerging Opportunities in Funding ADUs and Small-Plex Housing

June 2026

A national overview of how the lending system treats two-to-four-unit and accessory dwelling housing — and how that treatment has begun to change.

Abstract

Missing middle housing — duplexes, triplexes, fourplexes, and accessory dwelling units (ADUs) — is among the most efficient ways to add attainable homes to existing neighborhoods, yet it has been chronically underbuilt. The principal cause has been financial: a lending system organized around single-family homes and large commercial multifamily left two-to-four-unit and ADU projects in an underserved seam, with new construction especially dependent on personal wealth, home equity, or cash. That is now changing. Between 2023 and 2026, FHA, Fannie Mae, and Freddie Mac each began allowing borrowers to count ADU rental income toward qualifying and required appraisers to measure it; a wave of statewide ADU legalization has removed zoning and approval barriers; and mission and specialized lenders are building products around the value these units create. Meaningful gaps remain — speculative construction lending, four-unit and cooperative finance, and appraisal practice for three-to-four-unit properties — but the central question has shifted from whether the capital exists to how to connect it to the owners and small developers who need it. This paper maps the financing landscape, the recent reforms, and the agenda that remains.

James Carras

Adjunct Lecturer | Harvard Kennedy School

Principal | Carras Community Investment, Inc.

2026

Executive Summary

“Missing middle” housing — duplexes, triplexes, fourplexes, and accessory dwelling units (ADUs) — sits between the detached single-family home and the apartment building. It is among the most efficient ways to add attainable housing to existing neighborhoods: it uses infrastructure already in place, lets owners generate income and build wealth, and supports walkable, income-diverse communities. Yet for decades it has been chronically underbuilt, and the reason is less about zoning alone than about money.

This paper makes a simple argument. The principal obstacle to missing middle housing has been a financing system organized around two poles — single-family homes on one end and commercial multifamily of five-plus units on the other — that left two-to-four-unit and ADU projects in an underserved seam. New construction in particular fell outside products built on a single-family model, leaving owners dependent on personal wealth, home equity, or cash.

That picture is now changing. Three developments, most of them recent, are reshaping the financing landscape:

• The secondary market now recognizes ADU income. Between 2023 and 2026, FHA, Fannie Mae, and Freddie Mac each adopted rules allowing borrowers to count ADU rental income toward qualifying — and requiring appraisers to measure that income. This is the most consequential financing reform in a generation for these housing types.

• State preemption is unlocking the use. A wave of statewide ADU legalization has removed the local zoning barriers and the discretionary-review uncertainty that previously made these projects slow and risky to underwrite.

• Mission and specialized lenders are filling gaps. Community banks, credit unions, CDFIs, and a new class of ADU-focused lenders are building products around the after-built value these units create.

  
  

Real gaps remain — financing new units on speculative future rent, reaching four-unit and cooperative structures, and the commercial-appraisal lid on three-to-four-unit value. But the question has shifted from “does the capital exist?” to “how do we connect available capital to the owners and developers who need it?”

The Financing Problem: Caught Between Two Worlds

Missing middle housing of four units or less occupies a unique niche on the spectrum between residential and commercial lending. Commercial lenders generally classify multifamily as five units and above, and the bulk of new multifamily construction is far larger — often fifty units or more per building. These projects benefit from scale, specialized capital, and predictable underwriting. At the other end, single-family homes enjoy the deepest, most liquid mortgage market in the world.

Two-to-four-unit and ADU projects fall between. A point that surprises many buyers is that existing buildings of four units or less qualify for many of the same residential mortgages used to buy a single-family home. The break comes with new construction and with the income these units produce: traditional residential products were built on a single-family model, and historically did not credit the rent an added unit would generate. Owners who wanted to build were left with home-equity lines, cash-out refinancing, or cash — tools available disproportionately to those who already hold wealth, which is precisely the opposite of what an affordability strategy requires.

The Risk-Return “Barbell”

On the developer side, the same structural problem appears as a risk-return mismatch. Single-family development is generally seen as low-risk, low-return; large multifamily as higher-risk but high-return because volume carries it. A small-plex project can be perceived as nearly as complex and time-intensive to underwrite as a larger multifamily deal, but with far lower returns. Capital therefore flows to the two ends of the “barbell” and away from the middle, which is pursued less often than either single-family or high-density product.

The silver lining is that this same dynamic leaves room for small, local developers, entrepreneurs, and builders to take on projects that larger players overlook. What they lack is reliable financing — and the certainty of approvals that lenders price into a deal.

The Building-Code Wrinkle

A related barrier sits in building codes rather than finance, but it shapes feasibility. Under the model codes most U.S. jurisdictions use, the residential code (IRC) generally applies to two units or less, while triplexes and fourplexes cross into the commercial code (IBC), triggering fire-suppression, separation, and other requirements designed for far larger buildings. The added cost can render a three- or four-unit project infeasible, and the smaller, more affordable contractors who know the residential code often do not work in the commercial one. This code threshold is also misaligned with the four-unit ceiling of residential mortgage products — a seam within the seam.

The Residential Lending Landscape

Understanding what has changed requires a brief map of who sets the rules. Lenders originate mortgages in the primary market and frequently sell them into the secondary market. The Federal Housing Finance Agency (FHFA) oversees that secondary market and regulates Fannie Mae and Freddie Mac, the two government-sponsored enterprises (GSEs) that purchase loans and package them into mortgage-backed securities, providing the liquidity that keeps credit flowing. The Federal Home Loan Bank system supplies advances to member institutions, and the Federal Housing Administration (FHA), within HUD, insures loans for borrowers with lower credit scores and smaller down payments.

Because the GSEs and FHA effectively set the terms most of the market follows, their treatment of a housing type largely determines whether ordinary borrowers can finance it. For most of modern history, that treatment ignored the rental income an ADU produces and offered no clear path to finance new units. That is the specific thing that has now changed.

What Changed: The Market Now Recognizes ADU Income

The single most important development is that the institutions setting residential lending rules now allow ADU rental income to help borrowers qualify, and now require that income to be analyzed in the appraisal. For the first time, the income these units generate counts toward the borrower’s ability to carry the loan.

FHA — Mortgagee Letter 2023-17 (effective October 2023)

• Borrowers may count 75% of the actual or projected rent from an existing ADU toward qualifying income on a purchase or refinance.

• Borrowers adding a new ADU through the Standard 203(k) rehabilitation program may count 50% of the projected ADU rent — a rare path to finance new construction on anticipated income.

• ADU income may not exceed 30% of the borrower’s total monthly effective income and may not be used on cash-out refinances.

• A one-unit home with one ADU is treated as a one-unit property; a single-family home with an ADU became eligible for FHA new-construction financing.

• Appraisers must now analyze and report the ADU’s rental history or market rent using a Single-Family Comparable Rent Schedule.

HUD framed the change around the wealth-building potential of homeownership and expanding affordable supply. FHA’s owner-occupancy requirement still applies, which limits its usefulness for non-occupant investors.

Fannie Mae — Selling Guide Update (October 2025)

• For the first time, rental income from an ADU may count toward qualifying income on a one-unit principal residence, for purchases and limited cash-out refinances.

• Income may come from only one ADU even if several exist, capped at 30% of total qualifying income; properties with multiple ADUs are not eligible.

• Documentation follows standard rental-income rules; lenders generally apply 75% of market or lease rent to account for vacancy and expense. Automated-underwriting support followed in early 2026.

  
  

Freddie Mac

• Counts 75% of an existing ADU’s rent toward qualifying income on a purchase or no-cash-out refinance, subject to the same 30%-of-income cap.

• Extended ADU eligibility beyond one-unit homes to permit an ADU on certain two- and three-unit properties — reaching further into the small-plex range than the other channels.

  
  

Why This Matters

These changes directly answer the two barriers that most constrained the market. First, the appraisal gap: appraisers had routinely declined to value the income an ADU produces, in one study undervaluing such homes by nearly ten percent. Requiring a comparable-rent analysis begins to close that gap and helps the property appraise for what it is worth. Second, debt-to-income: an owner who could not previously qualify because the projected rent did not “count” may now clear the hurdle. The reform that advocates had urged for years — getting the secondary market to recognize missing middle rental income — has substantially arrived.

Loan Limits Scale With Units

FHA and conforming (Fannie/Freddie) loans are subject to mortgage maximums set annually by HUD and FHFA, and those maximums rise with the number of units — a structural feature that already accommodates missing middle housing. The 2026 baseline limits for most of the contiguous United States are below; high-cost counties carry substantially higher ceilings.

2026 Baseline Limits

One Unit

Two Units

Three Units

Four Units

FHA (low-cost “floor”)

$541,287

$693,050

$837,700

$1,041,125

Conforming (Fannie/Freddie)

$832,750

$1,066,250

$1,288,800

$1,601,750

High-cost “ceiling” (1-unit)

$1,249,125

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Sources: FHFA 2026 Conforming Loan Limit values and Fannie Mae Lender Letter LL-2025-04; FHA 2026 forward mortgage limits (HUD). FHA limits equal 65% of the conforming limit. Local (county-level) limits should be confirmed for any specific market.

FHA remains attractive for buyers with lower credit scores or seeking the 3.5% minimum down payment, while conforming loans on investor-held two-to-four-unit properties generally require larger down payments (commonly 15–25%). The headline is not the limits themselves but that the income from an added unit can now help a borrower meet the debt-to-income test those limits imply.

Construction and Gap Financing

For owners building new units rather than buying existing ones, a practical toolkit exists and has grown more viable as mainstream products acknowledge ADUs:

• Renovation and construction mortgages. FHA 203(k) and the GSEs’ renovation products can finance ADU construction or conversion as part of a purchase or refinance, now with explicit ADU treatment.

• Bridge loans. Short-term financing can cover the construction phase while a rental track record is established, bridging the gap between the start of construction and stabilized rent.

• CDFIs and mission lenders. Community development financial institutions continue to offer below-market and patient capital, frequently paired with affordability commitments — a proven model for ADU-specific products.

• Specialized and after-built-value lenders. A growing set of lenders and platforms underwrite to the after-renovation or after-built value of the property, letting owners borrow against the value the unit will create rather than only current equity.

• Community banks and credit unions. Locally rooted institutions, building on relationships and local market knowledge, remain the most natural partners to customize products as legalization unlocks volume.

• Grants, low-cost loans, and co-investment. Where community-development foundations, employers, or public funds exist, grants, below-market loans, shared-equity co-investment, and impact funds can fill the remaining gap.

  
  

The Policy Backdrop: State Preemption

Financing does not operate in a vacuum; lenders price the certainty of approvals into every deal. The most significant land-use development of recent years has been a shift from local experimentation to statewide preemption. By mid-2025, eighteen states had broadly legalized ADU construction — including the right to rent the unit — with most of those laws enacted within the prior four years, making ADU legalization the most common single housing-supply action state legislatures take.

Research consistently identifies three “poison-pill” rules that suppress ADU construction even where the units are nominally legal, and the strongest state laws prohibit all three:

• Owner-occupancy requirements, which shrink an owner’s future buyer pool and deter investment; financing rules now reward units that can be rented without restriction.

• Excess parking requirements, which can make a backyard or garage-conversion unit physically infeasible on a typical lot.

• Discretionary review, the public hearings and conditional-use permits that impose cost and uncertainty; by-right approval is the single most important enabling step and the one that most reduces lender-perceived risk.

  
  

For financing, the relevance is direct: in states with strong, by-right ADU laws, the discretionary-approval risk that once made these deals slow and uncertain to underwrite is largely removed, improving the “certainty of approvals” input that lenders weigh. Legalization and financing reform are complementary — and, as the uneven production results under some lot-split and duplex laws have shown, enabling a use does not guarantee it gets built unless the capital and the economics also align.

What Still Needs to Change

The progress is real but incomplete. The agenda for the next phase is now specific rather than general:

• Speculative new-construction lending. Income rules still lean on existing or contracted rent. Financing new units purely on projected market rent — the reform that would most help owners without a store of personal wealth — remains only partly addressed outside FHA’s 203(k) allowance.

• Four-unit and cooperative structures. ADU-income treatment does not extend to four-unit primary residences, and whole-co-op and share-loan financing for neighborhood-scale cooperatives remains underdeveloped.

• The commercial-appraisal lid. Three- and four-unit properties are still often appraised under commercial conventions that place an artificial ceiling on value, below the sum of the main dwelling plus the full value of the added units.

• Scale and standardization. A reliable secondary market for missing middle and small-plex loans — particularly loans made on future rental potential — could let mission lenders and CDFIs recycle capital and finance these projects at scale.

  
  

Recommendations

The reforms of the past three years have made missing middle housing financeable in ways it was not before. Turning that into built units depends on practical follow-through by the parties closest to each deal. The recommendations below are organized by audience.

For lenders and capital providers

• Adopt and operationalize the new ADU income rules. Update underwriting, train loan officers, and make clear to borrowers that ADU rent can now help them qualify under FHA and the GSEs.

• Build a small-plex and ADU product line. Pair renovation/construction mortgages (203(k), HomeStyle, CHOICERenovation) with bridge financing so owners can fund the gap between construction and stabilized rent.

• Underwrite to after-built value. Lend against the value the unit will create rather than only current equity, with appropriate controls — the model specialized ADU lenders have proven.

• Use CRA and mission capital deliberately. Treat two-to-four-unit and ADU lending as community-reinvestment-eligible activity, and partner with CDFIs to recycle capital through revolving funds.

For policymakers and civic leaders

• Legalize by right and remove the “poison pills.” Permit ADUs and small-plexes without owner-occupancy mandates, excess parking, or discretionary review — the steps that most reduce lender-perceived risk.

• Align building codes with the financing threshold. Allow three-to-four-unit structures to be reviewed under the residential rather than commercial code where safe, relieving fire-suppression and separation costs that kill feasibility.

• Reduce upfront cost and friction. Make impact and utility fees progressive or waivable, streamline permitting with a single point of contact, and offer property-tax relief tied to affordability.

• Convene the market and close the awareness gap. Bring lenders, appraisers, developers, and builders together; publish owner guidebooks and pre-approved plans; and educate appraisers on ADU valuation.

For federal agencies and the secondary market

• Finance new units on projected rent. Extend speculative new-construction lending based on documented market-rent potential, beyond today’s limited 203(k) allowance.

• Reach four-unit and cooperative structures. Extend ADU-income treatment to four-unit primary residences and develop whole-co-op and share-loan products for neighborhood-scale cooperatives.

• Fix the appraisal lid and deepen the secondary market. Move three-to-four-unit appraisal toward full per-unit valuation, and standardize missing middle loan products so capital can be recycled at scale.

  
  

Conclusion

Missing middle housing has always made sense on the merits: it adds attainable homes where infrastructure already exists, creates income and wealth for ordinary owners, and diversifies neighborhoods without large-scale redevelopment. What held it back was not a shortage of good ideas but a financing system that did not see the housing type clearly — that ignored the income these units produce and offered no path to finance new ones.

That is changing. The secondary market and FHA have done much of what advocates long asked: ADU rental income now helps borrowers qualify, appraisers must now measure it, and these units are explicitly eligible across the major lending channels. State legalization is removing the zoning and approval barriers on the other side of the equation. The remaining work — speculative construction lending, four-unit and cooperative finance, appraisal practice, and a deeper secondary market — is now a defined list rather than a general void.

The opportunity for public and civic leaders, lenders, and developers is to connect the capital that finally exists to the owners and small builders who need it: through education, product development, fee and process relief, and the convening that brings lenders, appraisers, and builders to the same table. The missing middle has been missing largely because it was unfinanceable. It is becoming financeable — and that changes what is possible.

This white paper draws on FHFA, FHA/HUD, Fannie Mae, and Freddie Mac guidance, and on research on state ADU legalization, current to approximately mid-2026. Loan limits, agency rules, and the status of state legislation should be re-verified at the time of use, as this is a rapidly evolving area.