One of the simplest underwriting checks is also one of the most revealing:
Can the residents actually afford the rent?
Affordability is not a social commentary or a macro talking point (at least that is NOT my intent with this brief article). It is a cash-flow durability test. When affordability is stretched, collections weaken, turnover rises, rent growth slows, and valuations become fragile. When affordability is ignored altogether, investors often discover the problem only after distributions fall short.
If you want to underwrite apartments with clarity rather than optimism, affordability needs to be part of your deal vetting process.
What Apartment Affordability Really Measures
At its core, affordability answers one question: What percentage of a household’s income goes toward rent?
The long-standing benchmark has been: Rent should not exceed ~30% of gross household income.
Above this level, households have less room to absorb:
- Job disruptions
- Rising insurance, food, or transportation costs
- Unexpected expenses
Speaking of, if you want to have a cap that is closer aligned to reality and factor in other fixed charges such as utilities, food, and insurance, a 20-25% mark is more appropriate to apply.
From an underwriting lens, rent above this threshold translates directly into higher income volatility at the property level.
How to Calculate Apartment Affordability (Plain English)
You do not need a complex model to do this well. You need accurate inputs and discipline.
- Start with median household income (MHI)
Use submarket-level data, not just metro averages. - Convert income to monthly terms
Annual MHI ÷ 12 = Monthly household income - Apply the affordability threshold
Monthly income × 30% (or 20-25% for a more realistic calculation) = Affordable monthly rent
Example:
- Median household income: $60,000
- Monthly income: $5,000
- Affordable rent: ~$1,500 (or $1,000-1,250)
Now compare that figure to:
- In-place rents
- Market rents
- Pro forma rents after renovations
If your underwriting assumes rents meaningfully above that level, you are relying on tenant stretch, not fundamentals.
Why Affordability Matters in Underwriting
Affordability is about sustainability, not whether rents can be pushed in the short term.
Deals that rely on renters spending 35–40%+ of income on housing often require:
- Aggressive rent growth assumptions
- Flawless execution
- Stable or improving economic conditions
- Unique positioning to justify the premium charged
- Specific clientele/ tenant demographic
That combination leaves little margin for error.
By contrast, deals with affordability cushion tend to show:
- More resilient cash flow
- Lower delinquency risk
- Greater flexibility during market slowdowns
This becomes especially important in Class B and Class C assets, where tenants are more sensitive to economic pressure.
The Rent vs. Own Gap — and Why Underwriters Should Care
Affordability does not exist in a vacuum. One of the most important contextual signals today is the rent vs. own gap.
Historically the monthly cost difference between renting and owning averaged around $400. However, today that gap is closer to $1,200 per month on average (as usual that amount varies by market and sub-market). This matters … but not in the simplistic way it is often discussed.
How the Rent vs. Own Gap Impacts Deal Underwriting
When the rent-to-own gap is wide:
- Many households are locked out of homeownership
- Renting remains the default choice for longer
- Rental demand is structurally supported
That is generally supportive for apartments. However, there is an important second layer.
A wide gap does not mean renters suddenly have more disposable income. It often means:
- They are renting longer than planned
- They are more payment-sensitive
- They have less financial flexibility
From an underwriting perspective, this calls for discipline, not rent exuberance.
Now consider the opposite scenario.
When the rent-to-own gap is too small (under ~$400):
- The financial barrier to homeownership shrinks
- Households with stable incomes are more likely to buy
- Renters are less likely to stay renters for extended periods
In those markets, underwriting aggressive long-term rent growth, especially at the mid-to-upper end of the renter pool, becomes riskier. Demand may still exist, but tenant churn and competition from ownership increase.
Connecting Affordability to Risk Management
Affordability should directly inform several underwriting decisions:
- Rent growth assumptions
Markets with stretched affordability warrant conservative growth, particularly in Year 1. - Reserve levels
Payment sensitivity increases collection risk. Thin reserves amplify downside exposure. - Asset class selection
Class B with affordability cushion often outperforms “aspirational” deals that rely on rapid rent jumps. - Exit assumptions
Buyers and lenders increasingly scrutinize rent sustainability, not just trailing NOI.
Affordability does not eliminate risk. It helps you see it sooner.
Affordability Red Flags to Watch For
Use this as a quick underwriting filter:
- Pro forma rents exceed 30–35% of median household income
- Rent growth assumptions outpace local income growth
- Renovation premiums assume tenants can “stretch” indefinitely
- Markets where the rent-to-own gap is shrinking, yet long holds are assumed
- No contingency plan if rent growth slows or collections soften
If multiple red flags show up at once, the deal may still work — but only with stronger buffers and more conservative expectations.
The Bottom Line
Apartment affordability is one of the clearest lenses through which to view real operating risk.
In a market where:
- Household budgets are under pressure, and
- The rent-to-own gap is historically wide in some areas and narrowing in others,
Affordability is no longer optional in underwriting.
It is not about predicting the future perfectly.
It is about avoiding deals that only work if everything goes right.
That is how you protect the downside – and build durable returns.
Vessi Kapoulian
Breaking down multifamily underwriting one step at a time to create educated and empowered investors