How to Think About Unit Mix and AMI in Early Feasibility
Unit mix and AMI targeting are two of the most consequential decisions in affordable housing development — and two of the decisions most often made too late in the process. By the time many teams formalize their unit mix, design assumptions have been made, financing structures have been sketched, and changing course is expensive.
The better approach is to treat unit mix and AMI targeting as feasibility inputs that need to be in the frame from the very beginning of site evaluation. Here's what that looks like in practice.
Why unit mix is a financial question first
The instinct in early feasibility is to think about unit mix in terms of what the site can physically accommodate — how many one-bedrooms, two-bedrooms, and three-bedrooms fit in the building footprint given assumed unit sizes. That's a necessary question, but it's not the first one.
Unit mix is a financial question first because different unit types produce different rents, occupy different amounts of square footage, and carry different market demand profiles. The mix you choose directly shapes the deal's revenue potential, its cost structure, and its relationship to the program's requirements.
Start with the revenue: what combination of unit types and sizes produces a total rent roll that supports the capital structure the deal needs? Then work backward to whether that combination is physically achievable on the site. Not the other way around.
The AMI dimension
AMI targeting — the income levels you're restricting units to — interacts with unit mix in ways that aren't always intuitive.
Higher AMI targeting (60% AMI) produces higher restricted rents, which supports more debt service and reduces reliance on soft subsidy. Lower AMI targeting (30% or 40% AMI) produces lower rents but often earns additional QAP scoring points and better serves households in the most acute need. The choice is both a financial decision and a mission decision, and the right answer depends on the specific market, the available subsidy stack, and the organization's priorities.
The key things to understand early:
Rent limits vary by unit size and AMI band. A two-bedroom unit at 60% AMI will carry a different rent limit than a one-bedroom at 50% AMI. The specific rent limits for a given market area are published annually by HUD and should be a first-pass input in feasibility, not a late-stage discovery.
Mixed AMI structures require careful modeling. Many deals target a mix of income levels — some units at 30% AMI, others at 50% or 60% — to satisfy QAP requirements or maximize competitive scoring. These structures require modeling the revenue at each AMI band separately, then summing to a total rent roll. Getting this wrong at the screening stage produces a feasibility picture that doesn't hold up in underwriting.
Operating subsidy changes the calculus for deep affordability. Units at 30% AMI are often only truly viable if they carry project-based rental assistance — a HAP contract through HUD or a state equivalent — that supplements tenant rents up to a market or reasonable rent level. Without that operating subsidy, the rents produced at 30% AMI in most markets are too low to cover operating expenses, let alone debt service. Know whether you have a path to operating subsidy before you build a deal around deep affordability targets.
The family/senior decision
The population your project targets — families with children versus seniors — has implications beyond unit mix. It affects:
Unit size distribution. Senior housing is typically weighted toward studios and one-bedrooms. Family housing is typically weighted toward two- and three-bedroom units. Three-bedroom units are larger, more expensive per square foot to build, and generate more rent per unit — but the demand and operating dynamics for large family units are different from smaller units.
Service requirements. Senior housing increasingly involves supportive service components — coordination with healthcare, transportation, social programming — that add operational complexity and sometimes cost. Some QAPs require or reward service commitments for senior or supportive populations.
QAP alignment. Some states distinguish between family and senior set-asides in their LIHTC allocation. Knowing which category a deal falls into — and whether that category is competitive in the current QAP cycle — is part of the early feasibility assessment.
A practical approach to early-stage unit mix thinking
For a first-pass feasibility assessment, here's a workable approach:
Start with the site's likely density ceiling (from the zoning check) and an assumed average unit size. That gives you a rough gross building area and a rough unit count.
Apply a simple unit size distribution based on the target population — something like 20% studios, 50% one-bedrooms, 30% two-bedrooms for a typical mixed-income senior project; 10% one-bedrooms, 60% two-bedrooms, 30% three-bedrooms for a typical family project. These are illustrative starting points, not prescriptions.
Calculate the rent roll using the HUD income limits for the target market at the AMI bands you're considering. Run this at a few different AMI mixes to understand the revenue range.
Check whether that revenue range is compatible with the capital structure the deal needs. If the rent roll doesn't support the debt service plus operating costs under any plausible unit mix, you have a structural feasibility problem that no redesign will solve.
This isn't full underwriting. It's a rapid sanity check that tells you whether the unit mix question is worth solving — or whether the deal has a more fundamental problem.
What teams get wrong
The most common mistake in early-stage unit mix thinking is optimizing for the wrong objective. Teams that optimize purely for rental revenue may produce a unit mix that doesn't serve the target population or doesn't score well in the QAP. Teams that optimize purely for QAP scoring may produce a unit mix that creates operating problems or market absorption risk.
The better framing is to treat unit mix and AMI targeting as a set of trade-offs that need to be navigated explicitly, with visibility into all the relevant dimensions — financial, competitive, mission, and market — from the beginning of the evaluation.
That requires more nuance than a simple formula. But it also means you're making the decision with the right information rather than discovering the trade-offs after design has locked in assumptions that are expensive to change.
Alpha Deal helps teams model unit mix and AMI scenarios during early feasibility — surfacing the revenue and capital stack implications of different targeting decisions before underwriting begins.