Underwriting is the discipline of proving a deal works on paper — with pessimistic-honest numbers — before your earnest money is at risk. Most bad purchases weren't bad markets; they were optimistic spreadsheets. The listing's pro-forma is marketing. Your underwriting is the truth you're willing to bet on.
Build the income line honestly
Start from actual collected rents (the T12), not asking rents or the broker's "market rent potential." Apply a vacancy allowance of 5–8% even if the building is full today. If you believe rents can rise, underwrite the purchase at current rents and treat the upside as margin, not as the reason the deal works — a deal that needs the upside is a speculation wearing a spreadsheet.
Build the expense line pessimistically
Listings systematically understate expenses. Rebuild them yourself: taxes at YOUR purchase price (reassessment), a real insurance quote, maintenance from age and condition (not last year's suspiciously quiet number), management at 8–10% even if self-managing, utilities from actual bills, and a capital reserve ($250–$400/unit/year). On older small residential, total operating expenses below 35% of gross rent deserve suspicion; 35–50% is the normal band.
Stress-test, then decide
Compute cap rate, DSCR, and cash-on-cash at your numbers. Then break the deal on purpose: rents 10% lower, one extra vacancy month per unit, expenses 15% higher, and — if the loan isn't fixed — rate up 1%. A good deal bends (thinner returns); a bad one snaps (negative cash flow, DSCR under 1.0). Decide your walk-away price from the stressed case, and let that number, not the negotiation, make the decision.
Worked example
A fourplex lists at $480,000 with a pro-forma claiming $52,000 income and $14,000 expenses. Your underwriting: actual rents $47,600, minus 6% vacancy → $44,700; rebuilt expenses $19,800 (reassessed taxes +$2,100, real maintenance, management) → NOI $24,900, a 5.2% cap at asking. Stressed, DSCR drops to 1.04 — too tight. Your walk-away is $415,000, where stressed DSCR holds 1.2. The seller declines; you pass. That IS underwriting working.
When this rule of thumb breaks
Uniform pessimism has a cost: underwrite harshly enough and you'll never buy anything — the goal is honest, not maximally grim; every assumption should have a reason. Value-add deals legitimately price on the post-renovation numbers, but then the rehab budget and timeline need the same stress treatment, plus a contingency. And spreadsheet-perfect deals still fail on what underwriting can't see — title, condition, neighborhood trajectory — so the numbers earn you the right to spend on inspections, not to skip them.