The Five Underwriting Mistakes First-Time Landlords Make

Most first deals don't go bad because the tenant trashed the place or the market crashed. They go bad because the numbers were wrong before the ink dried. Bad underwriting bakes in the loss on day one — you just don't feel it until month eight. Here are the five mistakes I see over and over, and the mechanics behind why each one hurts.

1. Treating Gross Rent Like Net Income

The most common rookie move is looking at the rent, subtracting the mortgage, and calling the difference "cash flow." That's not cash flow. That's the number before reality shows up.

Between the rent check and your pocket sit taxes, insurance, property management, vacancy, repairs, and capital reserves. A property renting for $1,800 with a $1,200 mortgage does not make you $600. After a realistic operating expense load, you might net $150 — or nothing.

As a rough working assumption for a standard single-family rental, expect operating expenses (excluding the mortgage) to eat 35% to 50% of gross rent. If your underwriting shows expenses at 15%, you've forgotten something. Run every deal through the analyzer with full expense lines before you decide anything.

2. Forgetting Capital Expenditures Entirely

Repairs and capex are not the same thing. A repair is a leaky faucet. Capex is the roof, the HVAC system, the water heater, the flooring between tenants. These don't hit every month, so beginners leave them off the spreadsheet entirely — which makes the deal look great right up until the furnace dies in January.

The fix is to reserve for them monthly even though you spend them in lumps. Think in terms of useful life. A roof that costs $10,000 and lasts 20 years is costing you roughly $500 a year, or about $40 a month, whether or not you write a check this year. Do that math for the roof, the HVAC, the water heater, and major systems, and set aside the total.

On an older property, a capex reserve of $150 to $250 a month per unit is not conservative — it's realistic. Skip it and your "cash flow" is really just a savings account you're going to empty in one bad quarter.

3. Trusting the Seller's Numbers

The pro forma a seller or listing agent hands you is a marketing document. It shows the property at its best possible day: full occupancy, below-market expenses, taxes at the current owner's assessment rather than what yours will be after the sale.

That last one bites people hard. In many areas, the property gets reassessed at your purchase price, so your tax bill can jump well above what the seller was paying. Underwrite taxes based on your likely reassessed value, not the current line item.

Same goes for rents. "Market rent" on a pro forma often means the top of the market. Pull actual comparable rents, and check current conditions in your market reports rather than taking a number on faith. Verify everything against real leases, real tax records, and real quotes.

4. Underestimating Vacancy and Turnover

Nobody rents a property 12 months a year, every year, forever. Tenants leave. Between them you have a gap for cleaning, painting, showing, and screening — and that gap costs you rent plus turn expenses.

Underwrite a vacancy factor even in a hot market. Five to eight percent of gross rent is a reasonable starting point; adjust up for older properties or softer areas. And remember turnover isn't just lost rent — it's paint, cleaning, small repairs, and possibly a leasing fee. A single turn can wipe out several months of profit if you didn't plan for it.

5. Ignoring Financing and Closing Costs in the Return Math

First-timers often anchor on the purchase price and forget the real all-in number. Closing costs, lender fees, inspection, initial repairs, and reserves can add several percent on top of the price. If your down payment is $50,000 but you actually need $65,000 to get keys-in-hand and safe, your return on invested capital is very different from what you calculated.

Worse, some people underwrite at a rate they can't actually get, or forget that a variable rate resets. Use the terms you're actually quoted, and stress-test the deal at a higher rate. If it only works at the best-case rate, it doesn't really work. Lean on calculators to model different rate and cost scenarios instead of guessing.

The Takeaway

Before you make an offer, rebuild the seller's pro forma from scratch using your own numbers: reassessed taxes, a real vacancy factor, an itemized capex reserve, your actual quoted rate, and your true all-in cash needed. If the deal still cash flows after all five corrections, buy it with confidence. If it only works on the seller's version, walk — the deal was never yours to begin with.