Cost segregation is easy to picture for a straight rental: buy the property, run a study, pull a chunk of the building into faster depreciation, deduct it. It gets murkier the moment you also use the place yourself — the beach condo you block off for two weeks in July, the cabin the family takes at Christmas. Does a study still pay when the property is partly a rental and partly yours? Usually yes — but how much it pays, and whether you can use the deduction this year, hinges almost entirely on how many nights you personally stay.
Does cost segregation still work if you use the place yourself?
Mechanically, nothing about the study changes. A cost segregation study still identifies the carpet, cabinetry, appliances, decorative lighting, land improvements, and other components that belong in 5-, 7-, and 15-year buckets instead of the 27.5-year schedule residential rental property is otherwise stuck on. Those components are still bonus-eligible, and with 100% bonus depreciation restored for property acquired after January 19, 2025, the reclassified slice is fully deductible in year one. What personal use changes is not the study — it’s how much of the resulting deduction you’re allowed to take, and in which year you get to take it.
The rule that decides everything: the 14-day / 10% test
The tax code cares about one line, drawn by Section 280A. Your property is treated as a residence — not a pure rental — if your personal use during the year exceeds the greater of 14 days, or 10% of the days it was rented at a fair market rate. Rent the place 200 days and your personal-use ceiling is 20 days; stay 21 and you’ve tipped over.
At or below the threshold. The property is treated as a rental. Your rental deductions — including the accelerated depreciation a study produces — can exceed rental income and throw off a loss, subject to the usual passive activity rules. This is the version where cost segregation does what the headlines promise.
Above the threshold. The property is a residence that you happen to rent, and Section 280A caps your rental deductions at your rental income for the year. You can’t use them to create a loss. The disallowed piece doesn’t disappear — it carries forward to future years — but the large first-year write-off you were counting on gets deferred.
What a vacation rental with light personal use looks like: a $600K example
Take a coastal short-term rental with $600,000 of depreciable basis after backing out the land, placed in service in 2026. You rent it 200 nights at market rate and personally stay 10 nights. Ten is under the 20-day ceiling (10% of 200 rental days), so the property is treated as a rental, not a residence.
A study on a furnished STR reclassifies aggressively — call it 25% of basis, or $150,000, into 5- and 15-year property. Because there’s some personal use, you allocate depreciation to the rental side by days: 200 rental nights out of 210 total nights used is about 95%. So about 95% of the $150,000 — roughly $142,500 — is deductible against the rental in year one, all of it up front with 100% bonus. At a 32% marginal federal rate, that works out to about $45,600 in year-one federal tax savings — before any state benefit, and against a single flat study fee. Whether you can use that loss against your W-2 or other income is a separate question that turns on material participation, which we walk through in the documentation to keep for a short-term rental.
What happens when personal use crosses the line?
Now run the same condo, but you stay 30 nights and rent it 150. Thirty is more than the greater of 14 days or 15 (10% of 150), so it’s a residence. The study reclassifies the same components, still bonus-eligible on the roughly 83% business-use portion — but Section 280A now caps your deduction at that year’s rental income. If the property netted, say, $18,000 of income before depreciation, that’s roughly where your deduction stops for the year. The rest of the accelerated depreciation carries forward. You didn’t lose it, but the whole point of a study — a large deduction now — is blunted. This is the single most common way a vacation-home cost seg disappoints an owner who didn’t count nights first.
How personal-use days quietly shrink your deduction
Even below the threshold, personal use isn’t free. Depreciation on a mixed-use dwelling is allocated between rental and personal days, so every personal night trims the deductible slice a little. Ten nights out of 210 is a rounding error; 14 nights out of 60 is not. If your plan is to rent lightly and use the place often, the math tilts fast — you’re allocating away more basis and marching toward the 280A cap at once. The owners who get the most from a study run the property as a genuine business and stay rarely. For the components that drive most of the reclassification in the first place, see the five short-life assets STR owners almost always under-claim.
So is a study worth it on a place you use?
Often, yes — if you keep personal nights under the line and the property has enough basis to reclassify. The study is the same audit-ready deliverable regardless of how you use the property; what varies is your ability to use the deduction, which is a function of nights and income you largely control. Run the day count before the tax year closes, not after — near the threshold, a handful of nights is the difference between a current-year loss and a carryforward.
You can sketch the reclassification with our savings estimate, then tell us about the property — how you rent it and how much you use it — and we’ll give you a candid read on whether a study pencils this year or is better timed for next.