How 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 is 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 cannot hide inside an optimistic spreadsheet.
The five inputs
Rent
Start from actual comparable rents for the unit type, bedroom count and condition — not the listing agent's rent estimate, and not what the property last rented for three years ago. Overestimating rent by even $100 to $150 a month is the single most common way an underwriting turns out 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 will not self-manage. These do not 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 is worth underwriting a rate somewhat above your quote, since rates can move between an accepted offer and a closing.
Cash flow
Rent minus operating expenses minus debt service. This is the number that actually pays you every month, and the one most easily distorted by an optimistic rent or a missing expense line further up.
Return metrics
Cap rate — net operating income divided by purchase price — measures the property independent of financing. Cash-on-cash return — annual cash flow divided by the cash you actually invested — measures the return on the money you put in. They can move in opposite directions on the same deal, which is why underwriting a rental means looking at both. Our guide to cap rate versus cash-on-cash covers when they disagree and why.
Build it in this order
The sequence matters more than people expect, because working backwards from a desired answer is how optimistic assumptions get introduced without anyone deciding to introduce them. Establish the rent first, from comparables, before you know what the deal needs it to be. Then the operating expenses, which depend on the property and the jurisdiction rather than on your financing. Only then layer in the loan.
Doing it the other way round — starting from the payment and asking what rent would cover it — produces a number that is really a wish. The discipline is to let the first two steps produce whatever they produce, then find out whether the price works.
A worked example
These figures are illustrative — a teaching example, not market data for any particular city.
| 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% |
Reading it a line at a time: 20% of $260,000 is $52,000, so the loan is $208,000 and the remaining $6,500 of the cash requirement is closing costs. Rent of $2,150 less $790 of operating expenses leaves $1,360 a month of net operating income, which is $16,320 a year — and against the $260,000 price, that is the 6.3% cap rate. Subtract the $1,180 mortgage payment and $180 a month survives, or $2,164 a year, which on $58,500 invested is the 3.7% cash-on-cash return.
One detail worth naming, because the table does not: a $1,180 payment on a $208,000 thirty-year loan implies an interest rate of about 5.49%. None of these numbers are unusual for a fully underwritten deal, but the financing assumption is doing a great deal of work, which the next section makes concrete.
What happens when the rate moves
Hold everything else in that example fixed — same price, same rent, same expenses — and vary only the interest rate:
| 5.49% | $180/mo · 3.7% CoC |
| 6.00% | $113/mo · 2.3% CoC |
| 6.50% | $45/mo · 0.9% CoC |
| 7.00% | -$24/mo · -0.5% CoC |
| 7.50% | -$94/mo · -1.9% CoC |
The deal stops producing cash at about a 6.83% rate. A point and a half of rate is the difference between a modest income property and one you subsidise — and nothing about the house, the rent or the neighborhood changed. This is why underwriting at a rate above your quote is not pessimism but ordinary prudence: a rate lock can expire, an appraisal can come in low enough to change your pricing tier, and a deal with no margin at today's exact quote has no margin at all.
Operating expenses, line by line
The $790 in the example is not one number, and treating it as one is how it gets understated. Taxes and insurance are the two you can look up, and they are also the two most likely to change the moment you buy: many jurisdictions reassess on sale, and the previous owner's homestead exemption does not transfer to an investor.
The other three are reserves, which is to say money you set aside for costs that have not arrived yet. A vacancy allowance covers the weeks between tenants. Maintenance covers the steady trickle of small repairs. Capital expenditure covers the roof, the HVAC system, the water heater — items that cost thousands and arrive on nobody's schedule. Folding maintenance and capital expenditure into one line reliably understates the second, because small repairs are the ones you have recent memory of.
Property management is the line most often set to zero, on the reasoning that you will self-manage. That may be true now. The question is whether it will be true for the whole hold, through a job change or a move, and whether the property still works if you ever have to pay someone. A deal that only clears while you work for free is a deal with a hidden wage in it.
Our guide on where to find your numbers covers which public source owns each of these figures, and the real cost of vacancy covers why a turnover costs more than the empty weeks alone.
Where a rushed underwriting breaks
- Rent from the listing, not the market. A listing's suggested rent is a selling tool, not a comparable.
- No capital reserve. A roof or HVAC replacement does not 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 is not underwritten, it is hoped.
- One interest rate, no stress test. As the table above shows, a point of rate can erase the entire margin.
- Self-management assumed, unpriced. If you are not certain you will self-manage for the life of the loan, underwrite the management fee from the start.
- Taxes taken from the current bill. If the jurisdiction reassesses on sale, the number you inherit is not the number you will pay.
What underwriting cannot tell you
It is worth being clear about the limits. Underwriting measures one year of operating performance under a set of assumptions. It does not value appreciation, it does not count the principal your tenant retires on your behalf each month, and it says nothing about tax treatment, where depreciation can make the after-tax result meaningfully different from the before-tax one.
So a deal showing thin cash flow is not automatically a bad deal — it is a deal whose return is arriving in forms this calculation does not measure. What underwriting does tell you, reliably, is whether the property can pay its own bills without help from you. That is the question worth settling before the softer ones. Our note on realistic return targets covers how to weigh the rest.
Running it yourself
The single-family rental analyzer performs this whole sequence on a specific property, including the maximum offer that would produce a cap rate you name. Run the honest version first, then run it again at a rate half a point to a point above your quote, and treat the second result as the one you are actually underwriting.