The IRS default is that residential rental property depreciates over 27.5 years and commercial buildings over 39 years. That is slow. Painfully slow. A cost segregation study breaks the building down into its parts and lets the parts that should depreciate faster, do.
The math, in round numbers
Buy a $1,000,000 rental property. Without cost seg, you depreciate ~$36,000 a year for 27.5 years. With a typical study identifying ~25% of the building as short-life property and current bonus depreciation rules, you can pull six figures of depreciation into year one alone. At a 37% marginal tax rate, that translates into real cash you keep.
The Real Estate Professional pairing
A cost seg by itself only helps you to the extent the resulting loss is deductible. For most rental owners the loss is passive: it just sits and waits. Pair the cost seg with Real Estate Professional status or the short-term rental loophole, and the loss flows against your W-2 or business income immediately. That is where the headline numbers come from.
Already own the property? You haven’t missed the window.
If you bought a building three years ago and never did a cost seg, you can still do one now. The IRS lets you catch up all the missed depreciation in a single year via a Form 3115 (Change in Accounting Method). No amending prior returns, no penalty for the late discovery. The catch-up amount is a § 481(a) adjustment, taken in the year of the change.
What the study actually involves
A real cost seg is an engineering exercise. The firm reviews architectural drawings, walks the property, identifies and prices each component, and produces a detailed report tying every allocation back to either invoices or accepted engineering methods. The IRS publishes its own Audit Techniques Guide and reads the report against that guide. A “back of the envelope” cost seg from a tax software bolt-on is what gets disallowed in audit. A defensible engineering report by a qualified firm is what holds.
The often-overlooked bonus: partial asset dispositions
Once you have the building broken into components, you can write off old pieces when you replace them. Replace the roof in year four? With a cost seg on file you can deduct what was left on the old roof’s basis. Without the study, the old roof and the new roof are both depreciated, doubling up. See the partial asset dispositions article for how that works.
When it doesn’t make sense
If your basis is under about $500,000 the study fee starts eating the benefit. If you plan to sell the property in three or four years, much of the accelerated depreciation gets clawed back as recapture (see depreciation recapture). And if you cannot use the resulting loss (it is passive and trapped), the study is moving the deduction in a direction that does not help you. Run the numbers before you commission the study, not after.