sfrcalc.comHow to Underwrite a Single-Family Rental
"Underwriting" sounds like something a lender does, but the same idea applies before you ever apply for a loan: it's the process of testing whether a property's numbers support the price being asked, independent of how good the deal feels. A single-family rental underwriting comes down to five inputs — rent, operating expenses, financing, cash flow, and a couple of return metrics — laid out honestly enough that a bad deal can't hide inside an optimistic spreadsheet.
Rent
Start from actual comparable rents for the unit type, bedroom count, and condition — not the listing agent's rent estimate, and not what a property last rented for three years ago. Overestimating rent by even $100–150/month is the single most common way an underwriting turns out to be too optimistic, because every downstream number is built on it.
Operating expenses
Property taxes, insurance, a vacancy reserve, a maintenance and capital expenditure reserve, and property management if you won't self-manage. These don't show up on the listing — they have to be sourced or estimated separately, and skipping any one of them is the second most common way a deal looks better on paper than it performs.
Financing
Purchase price, down payment, interest rate, term, and closing costs — or, if paying cash, the full purchase amount and closing costs with no debt service. This is also where it's worth underwriting a rate a bit higher than your quote, since rates can move between an accepted offer and closing.
Cash flow
Rent minus operating expenses minus debt service. This is the number that actually pays you every month, and it's the one most easily distorted by an optimistic rent or a missing expense line further up.
Return metrics
Cap rate (net operating income ÷ purchase price) measures the property independent of financing; cash-on-cash return (annual cash flow ÷ cash actually invested) measures your return on the money you put in. They can move in opposite directions on the same deal, which is why underwriting a rental usually means looking at both, not just one.
A worked example
| Purchase price | $260,000 |
| Down payment (20%) + closing costs | $58,500 |
| Monthly market rent | $2,150 |
| Monthly operating expenses (taxes, insurance, vacancy, maintenance, capex) | $790 |
| Monthly debt service | $1,180 |
| Monthly cash flow | $180 |
| Cap rate | 6.3% |
| Cash-on-cash return | 3.7% |
None of these numbers are unusual for a fully underwritten deal — this is what an honestly modeled single-family rental often looks like once every expense line is actually counted, rather than a deal that's been rent-optimized and expense-trimmed to look better than it is.
Where a rushed underwriting breaks
- Rent from the listing, not the market. A listing's suggested rent is a selling tool, not a comp.
- No capex reserve. A roof or HVAC replacement doesn't show up monthly — until the year it does.
- Vacancy at zero. Even a well-managed rental turns over occasionally; a deal that only works at 100% occupancy isn't underwritten, it's hoped.
- One interest rate, no stress test. If the deal only cash flows at today's exact rate, a small move against you erases the margin.
- Self-management assumed, unpriced. If you're not confident you'll self-manage for the life of the loan, underwrite the property management fee in from the start.