Don't Underwrite the Rent You Advertise — Underwrite the Rent You'll Collect

Every rental analysis starts with a number, and it's tempting to start with the wrong one. Take the asking rent — say $2,200 a month — multiply by twelve, and treat $26,400 as the property's income. That figure has a name: gross potential rent (GPR), your assumed market rent times twelve across all units. But GPR is the modeled potential at your assumed market rent — not the revenue forecast. It's the income the property would produce if every unit were full every day of the year, every tenant paid in full and on time, and you never gave a dollar of concession. Using GPR as if it were expected collected rent removes the normal friction between potential revenue and realized revenue.

What you want instead is effective gross income (EGI) — the underwritten top-line forecast after your vacancy and credit assumptions and other income. Getting from GPR to EGI means subtracting what you don't expect to collect and adding the smaller income streams you do. Before the arithmetic, though, one distinction does most of the work.

Physical vs. economic occupancy. Physical occupancy measures time: occupied unit-time ÷ available unit-time — is the unit occupied? Economic occupancy measures dollars: base rent actually collected ÷ GPR. They answer different questions, so they can diverge. A unit can be physically occupied and still not collecting full modeled rent — a tenant on a below-market lease, a concession like a free month, or rent that's owed but not paid — which pushes economic occupancy below physical. (The reverse is possible too: because GPR is built on an assumed market rent, a lease signed above that assumption can push economic occupancy slightly above physical.) In ordinary underwriting the concessions-and-collection-loss direction dominates, so economic occupancy is usually the lower of the two — but it isn't guaranteed by the definition. Either way, the dollar version is the one that underwrites a deal: a property can be "95% occupied" by the door count and materially different by the dollar.

A related discipline: keep delinquency and bad debt — billed rent you never collect — tracked separately from vacancy. They land in the same place (less cash), but they're different problems with different fixes (leasing and pricing on one side, screening and collections on the other), and blending them hides which one is hurting you.

Now the build, and it's short. Effective gross income = gross potential rent − vacancy and credit loss + other income. Vacancy and credit loss is your allowance for empty time plus uncollected rent — a percentage of GPR you set from the market and the property's evidence, not a hopeful round number. Other income is the smaller, real money a rental throws off: late fees, pet rent, parking, laundry. Add it back because it's income you expect to bank. What's left, EGI, is the underwritten top line — and it's the number every downstream metric should be built on. (After you own the property, you compare the actual collected revenue against this forecast; at underwriting, it's an estimate, per P05.)

Watch it work on our canonical rental. Market rent is $2,200/month, so GPR = $26,400. Apply the canonical 6% vacancy-and-credit-loss assumption — that's −$1,584 — and add $300 of other income for the year. EGI = $25,116. That $1,284 gap between the asking-rent headline ($26,400) and the underwritten top line ($25,116) isn't pessimism; it's the difference between the year you model at full potential and the year you actually underwrite. And here's the part that matters downstream: the revenue line is overstated by only about 5% in this example, but that error doesn't stay 5% as it flows down — because many costs are fixed, the distortion amplifies. Hold the canonical operating expenses at $9,600: real modeled NOI is $25,116 − $9,600 = $15,516, but plugging in GPR instead gives $26,400 − $9,600 = $16,800 — about 8.3% too high, not 5%. And when cash flow sits near zero (as it does here), the percentage distortion in cash-on-cash can be far larger still. A small overstatement at the top becomes a big one at the bottom.

Two boundaries so this node stays in its lane. First, this is a revenue step only — expenses, the management allowance, the CapEx reserve, and the returns come in their own guides; effective rent is the top of the funnel that feeds them. Second, asking rent still has a job — as a screen and a ceiling. It tells you the property's upper bound and lets you compare listings quickly. It just isn't the number you underwrite. Use it to decide what's worth a closer look; use effective gross income to decide whether the closer look pencils.

So the plain-English version: gross potential rent is the modeled year at full potential; effective gross income is the year you actually underwrite. Subtract a vacancy-and-credit-loss allowance drawn from the property's evidence, add the other income you expect to collect, and underwrite that. It's the least glamorous number in the analysis, and getting it honest is what keeps every metric built on top of it honest.

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The bottom line

Gross potential rent is the modeled year at full potential; effective gross income is the year you underwrite. The honest top line is GPR minus a vacancy-and-credit-loss allowance drawn from the property's evidence plus the other income you expect to collect — and because every return metric is built on it, an inflated top line inflates the whole deal, by more than the top-line error once fixed costs are in the mix. Underwrite what you expect to collect, not what you'll advertise.

About the author

Matt Nunn is the founder of Builders Finance. He has spent two decades working with the financial side of real estate businesses, and started Builders Finance to give short-term-rental operators the financial systems, frameworks, and plain-language education that most hosting advice skips over. Builders Finance publishes educational content for STR owners; it is not legal, tax, or investment advice, treatment depends on your facts and circumstances, and it is not a substitute for guidance from your own qualified professionals.

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This resource provides general educational information and is not individualized investment advice. Underwriting assumptions are property- and market-specific; verify rents, vacancy, and other income for your own deal.

Primary sources / provenance: BFC Deal Analysis P05 (estimate the year you'll actually have) and the Phase-2 LTR metric definitions (GPR, physical vs. economic occupancy, EGI, delinquency). Market inputs (rents, vacancy, other income) are property-specific and verified per deal, not evergreen.