For nearly a century, the U.S. mortgage system has excelled at one thing: mass‑producing a single, homogeneous product—the fully amortizing, fixed‑rate mortgage underwritten to a wage‑earner’s debt‑to‑income (DTI) ratio. That “post‑Depression” design stabilized a shaky market and became the world’s benchmark. But today’s households and neighborhoods don’t look like they did in 1938. We live with roommates, rent out accessory units, work from home, and stitch incomes from multiple sources. When the financing toolkit refuses to see those realities, it doesn’t just miss nuance—it warps incentives, funnels capital into narrow property types, and suppresses affordability.
The good news: we don’t have to force price controls or wait years for new supply. In a moment of regulatory openness, Fannie Mae and its younger cousin, Freddie Mac, can make targeted adjustments to their product menus that widen the doorway to ownership and increase rental supply—without compromising safety and soundness.
Where today’s rules block affordability
1) The system discounts household cash flow from boarders/roommates and ADUs
GSE rules do allow limited credit for boarder or accessory dwelling unit (ADU) income—but almost entirely by adding a slice of that income to the borrower’s denominator (qualifying income), rather than offsetting the numerator (the monthly payment). For example, Fannie Mae’s HomeReady® policy permits boarder income to contribute up to 30% of total qualifying income (with documentation), and ADU rent is recognized with a vacancy haircut, typically 75% of lease in illustrative scenarios. Both are added to income and then tested against a DTI cap, rather than netting them against the mortgage payment. Fannie Mae+2Fannie Mae+2
Freddie Mac likewise allows ADU income on a 1‑unit primary residence under defined conditions, again as qualifying income subject to Guide requirements—not as a direct payment offset. Freddie Mac+1
Why does this matter mechanically? Adding income is the weakest way to recognize rent, as it scales with the DTI cap. Suppose a borrower earns $6,000/month, carries $300/month in other debt, and considers taking in a boarder paying $800/month.
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Current “income‑add” method (assume a 40% total DTI cap): capacity increases by 0.40 × $800 = $320.
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“Payment‑offset” method with a prudent haircut (use 75% of rent, consistent with how the GSEs treat ADU rent in examples): capacity increases by $600.
That’s ~1.9× more effective at a 40% DTI—and 1.5× to 2.3× more effective across common DTI caps (50% down to 33%). In plain terms: the same boarder can safely support a meaningfully larger mortgage when their rent reduces the payment instead of being dribbled into income. (The 75% haircut mirrors how GSEs already account for vacancy/loss on ADU rents today.) Fannie Mae
Reducing net mortgage payment is vastly more effective for affordability
| DTI Cap |
Baseline (No Boarder) |
With Boarder – Income‑Add |
With Boarder – Payment‑Offset |
Increase vs Baseline – Income‑Add |
Increase vs Baseline – Payment‑Offset |
Offset vs Income‑Add – Effectiveness |
| 33% |
$1,680 |
$1,944 |
$2,280 |
$264 |
$600 |
2.27× |
| 38% |
$1,980 |
$2,284 |
$2,580 |
$304 |
$600 |
1.97× |
| 40% |
$2,100 |
$2,420 |
$2,700 |
$320 |
$600 |
1.88× |
| 45% |
$2,400 |
$2,760 |
$3,000 |
$360 |
$600 |
1.67× |
| 50% |
$2,700 |
$3,100 |
$3,300 |
$400 |
$600 |
1.50× |
Impact on affordability - Maximum Housing Prices increases dramatically
| DTI Cap |
No Boarder |
With Boarder – Income‑Add |
With Boarder – Payment‑Offset |
Extra Loan from Boarder – Income‑Add |
Extra Loan from Boarder – Payment‑Offset |
Effectiveness (Offset ÷ Income‑Add) |
| 33% |
$272,853 |
$315,729 |
$370,300 |
$42,877 |
$97,447 |
2.27× |
| 38% |
$321,576 |
$370,950 |
$419,024 |
$49,373 |
$97,447 |
1.97× |
| 40% |
$341,066 |
$393,038 |
$438,513 |
$51,972 |
$97,447 |
1.88× |
| 45% |
$389,789 |
$448,258 |
$487,237 |
$58,468 |
$97,447 |
1.67× |
| 50% |
$438,513 |
$503,478 |
$535,960 |
$64,965 |
$97,447 |
1.50× |
This is exactly how banks underwrite commercial and small‑income property: cash‑flow first. We already trust property‑level cash flows in DSCR underwriting for investments; we simply refuse to apply the same logic for owner‑occupied homes with modest, verifiable rent streams.
2) Property‑type rules nudge capital away from mixed‑use neighborhoods
Fannie’s mixed‑use eligibility keeps properties primarily residential, requires the borrower to be the owner‑operator of any business use, and values the property on its residential characteristics—effectively sidelining the commercial value that gives many “shopkeeper” buildings their economic logic. For condos and co‑ops, project rules generally cap non‑residential space at 35%. These constraints make above‑store apartments, live‑work buildings, and main‑street assets harder to finance in the single‑family channel, even when that is the natural housing near jobs and services. Selling Guide+2Selling Guide+2
The result: households often must finance a home and a nearby workspace separately (if at all). That fragments neighborhoods, lengthens commutes, and undercuts small‑business viability—the opposite of affordability.
Two product fixes that would move the needle fast
A) Net eligible rent against the payment (a “Residential Cash‑Flow” adjustment)
What to change: When a borrower can document stable boarder/roommate or ADU income under existing verification standards, allow that income—after standard haircuts (e.g., 25% for vacancy/loss)—to offset the monthly PITI before testing DTI.
Why it’s safe:
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The haircut already embeds a cushion (the same logic GSEs use for ADU rent in examples). Fannie Mae
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Require 12 months of history (or DU/LP‑approved alternatives such as verified recurring deposits), a simple lease, and market‑rent support (Form 1007 for rooms/ADUs as applicable). Selling Guide
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Cap the net‑rent offset to a prudent share of PITI (e.g., ≤60%) and require two months’ “vacancy reserves” when relying on offsets above a threshold.
What it unlocks:
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Using the $6,000 income / $800 boarder example: with today’s income‑add method, a borrower at a 40% cap qualifies for roughly $2,420 of PITI; netting rent would support about $2,700—a meaningful boost in competitive markets.
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Multiply that across millions of roommates and ADUs, and you expand ownership and add immediate rental capacity (the boarder’s room). No subsidies required.
B) A true “Shopkeeper” or “Mixed‑Use Lite” mortgage
What to change: Offer a standard single‑family execution for one‑ to two‑unit properties with a modest ground‑floor commercial space (or live‑work layout), without forcing the borrower to be the business operator or stripping out commercial value in the appraisal. Keep existing zoning‑compliance and habitability tests; set clear commercial‑area and use thresholds; let the business rent offset payment under the same cash‑flow rules.
Why it’s safe:
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Fannie already purchases mixed‑use that is primarily residential, with legal use and careful appraisal; condos and co‑ops already manage the share of commercial space at the project level. Adjusting eligibility and valuation to recognize modest commercial value—rather than pretend it doesn’t exist—aligns risk with reality. Selling Guide+2Selling Guide+2
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Freddie’s ADU and property‑eligibility framework shows how accessory income and use can be governed within the single‑family guide. A parallel “Mixed‑Use Lite” construct is administratively feasible. Freddie Mac+1
What it unlocks:
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Shopkeepers, restaurateurs, and repair tradespeople can live near work, eliminating a second lease and transportation costs.
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Neighborhoods gain housing above storefronts—the most walkable, employment‑adjacent stock we can add without new land.
Implementation playbook (no statute needed)
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Underwriting guides & AUS updates
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Add a “Net Eligible Rent” field in DU/LP that subtracts from PITI before DTI is calculated (with haircuts and caps).
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Maintain today’s boarder/ADU documentation standards (e.g., 9–12 months history for boarders; market‑rent support; primary‑residence restrictions; DU/LP documentation). Fannie Mae+1
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Data & model governance
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Begin with pilot pools (e.g., up to a defined UPB) to measure default/loss differences versus matched controls.
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Track payment‑shock, vacancy incidence, and prepay behavior for loans using payment offsets.
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Risk rails
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Haircut rents (e.g., 25%) and cap offsets as a share of PITI.
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Require two months of vacancy reserves for high‑offset cases.
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Apply modest loan‑level price adjustments (LLPAs) until performance data accumulate.
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Property eligibility tweaks
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Define “Mixed‑Use Lite”: e.g., max commercial area 20%–25% of gross building area; clearly permissible uses; residential safety/egress intact.
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Remove the owner‑operator requirement for the commercial space on one‑ to two‑unit properties when third‑party tenant leases and market‑rent support are provided; continue to require legal use/zoning. (Fannie’s current rule requires owner‑operator and “primarily residential” treatment; the tweak would let neighborhoods function as communities, not silos.) Selling Guide
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Appraisal modernization
Answering the usual objections
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“This just hides risk.” It doesn’t. Offsetting with a haircut is more conservative than pretending the rent doesn’t reduce the bill. It aligns with how GSEs already model ADU rent in examples (75% of lease) and with how income property is underwritten broadly. Fannie Mae
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“Fraud and roommate churn will spike.” That’s what documentation and reserves are for. The boarder policy already requires history and shared‑residency documentation. Extending those same standards to a payment‑offset approach, adding verified deposits/receipts and a small vacancy reserve, manages churn risk. Fannie Mae
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“Mixed‑use belongs in multifamily or commercial.” Not when we’re talking about one‑ and two‑unit shopkeeper buildings—ubiquitous in older urban grids. The GSEs already touch mixed‑use; aligning eligibility and valuation with real cash flows simply stops penalizing neighborhoods that co‑locate homes and jobs. Selling Guide+1
Why this beats heavy‑handed market interventions
Forcing nominal price cuts or waiting on large‑scale greenfield supply pushes the problem down the road. The two changes above:
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Increase demand‑side capacity where the household already has a paying roommate or ADU.
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Increase supply—immediately—by validating rooms/ADUs and shopkeeper apartments as part of the housing stock.
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Reduce commute costs and labor frictions by letting people live near their storefronts and workshops.
And because these are product changes, not subsidies, they scale through the secondary market with standard risk management and capital treatment.
A system built for the 1930s can be modernized in 2025
We don’t need to abandon the 30‑year fixed mortgage. We need to stop underwriting 21st‑century households as if every home is a suburban box occupied by a single wage earner. Let’s do what the GSEs already know how to do in other corners of their guides:
Fannie Mae and Freddie Mac can catalyze affordability right now by (1) netting prudent, documented rent against the payment and (2) offering a standardized “Mixed‑Use Lite / Shopkeeper” mortgage. Those two simple changes would widen access to ownership, expand rental options, and rebuild communities as places where living and working belong together.
Key sources: Fannie Mae’s Selling Guide and HomeReady materials on boarder and ADU income; Fannie Mae’s mixed‑use and project‑level rules; Freddie Mac’s ADU guidance and fact sheet. Freddie Mac+7Fannie Mae+7Selling Guide+7
Appendix: What the current rules say (select highlights)
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Boarder/roommate income: Fannie Mae (HomeReady) allows it to contribute up to 30% of qualifying income with documentation (e.g., 9 of the past 12 months and proof of shared residency). Fannie Mae
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ADU rental income: Fannie Mae explicitly allows ADU income on a 1‑unit primary with conditions, and illustrative scenarios apply a 25% vacancy haircut (75% of lease). Selling Guide+1
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Freddie Mac ADUs: Freddie permits ADU income to qualify on a 1‑unit primary residence subject to Guide 5306; see Freddie’s ADU fact sheet. Freddie Mac+1
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Mixed‑use eligibility: Fannie Mae requires mixed‑use properties to be primarily residential, with the borrower as owner‑operator of the business use, and appraisals to value the residential characteristics. Selling Guide+1
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Commercial share limits (condos/co‑ops): Projects are generally limited to ≤35% commercial/mixed‑use space. Selling Guide
These are solid, safety‑oriented rules. The proposal here keeps the guardrails—while finally aligning underwriting with how Americans actually live, earn, and house each other.
