The smallest line in the budget is the one that sinks the deal. Not taxes, not insurance — reserves. The money set aside every year for the roof, the boiler, the hallway carpet nobody thinks about until it needs replacing.
Lenders treat replacement reserves as a fact, not a judgment call. Fannie Mae's multifamily guide requires a minimum of $300 per unit per year set aside for replacements on a standard property. Not $300 if you feel like it. $300 minimum, funded during the loan term, whether the escrow is collecting it or not.
On underwriting, the engineer's property condition assessment usually lands in the same place: somewhere around $250 to $300 per unit per year on an average building, often more on an older one. I've seen investors pencil $150 per unit into the pro forma to make the numbers work. The lender then underwrites $300, the net cash flow drops, and the loan amount drops with it.
Here's the part that actually matters for ownership, beyond the loan: the lender's number is a floor, not a plan. An older walk-up with a twenty-year-old roof and original windows doesn't care that $300 is the agency minimum. Its real capital needs over the next decade might be double that. The engineer's remaining-useful-life table tells you this in plain arithmetic — roof has eight years left, costs $180,000 to replace, that's $22,500 a year you owe yourself starting now.
The mistake I watch most often is treating reserves as an afterthought line at the bottom of the budget. Buyers model rent growth to the second decimal place and then shrug at capex with a round number copied from the last deal. When the boiler dies in February, there is no second decimal place. There is an emergency replacement, a financing scramble, and a building full of cold tenants who will remember the experience when renewal season comes.
So I do the arithmetic the other way now. List every major system, write down its remaining life and its replacement cost, and divide. If the building's honest annual number is $600 per unit, I underwrite $600 — not the lender's $300, not the seller's $150. The deal either absorbs the real number or it doesn't deserve to close.
A fully funded reserve account is also the cheapest financing you'll ever arrange. You are borrowing from yourself at zero percent, and the interest rate never moves. The buildings that run on thin reserves always end up paying more — in emergency pricing, in deferred everything else, in tenants who leave. Size it like a lender, then add what the building is actually telling you.
The test I run before closing: take the annual reserve number I believe in, multiply by ten, and compare it to the engineer's ten-year capital forecast. If they're close, I'm being honest. If there's a gap, the gap has an owner — and it's me.
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