A cost segregation study is an engineering-based analysis that takes the lump sum you paid for a building and breaks it into component costs, then assigns each component the shortest depreciation life the tax code allows. By default, a residential rental depreciates over 27.5 years and a nonresidential building over 39 years under IRC section 168(c). A study carves out the parts that are really tangible personal property or land improvements and moves them into 5-, 7-, and 15-year MACRS classes.
Why shorter lives matter so much
Shorter lives would help on their own, but the real engine is bonus depreciation. Under IRC section 168(k), property with a recovery period of 20 years or less qualifies, and the OBBBA legislation of July 2025 made the rate a permanent 100% for qualified property acquired after January 19, 2025. Property acquired on or before that date keeps the old phase-down, 60% for 2024 and 40% for 2025 placements. So everything a study shifts into the 5-, 7-, or 15-year buckets can typically be deducted in full in year 1.
The method matters. The IRS Cost Segregation Audit Techniques Guide describes a quality study as one built on engineering takeoffs, construction documents, and component-level cost data, not rough percentages. The deliverable is a report your tax preparer uses to build the depreciation schedule and, on a look-back, to compute a Form 3115 catch-up.
A worked example
Say you buy a $750,000 single-family rental in March 2026. An appraisal supports allocating $150,000 to land, leaving $600,000 of depreciable basis. Without a study, your deduction is $600,000 spread over 27.5 years, about $17,000 in the first partial year under the mid-month convention.
A study reclassifies 25% of the basis: $95,000 of 5-year property (appliances, carpet, cabinetry, dedicated electrical) and $55,000 of 15-year land improvements (driveway, fencing, landscaping). Because the property was acquired after January 19, 2025, all $150,000 is eligible for 100% bonus depreciation. First-year deductions total roughly $163,000: the $150,000 bonus amount plus about $13,000 of regular depreciation on the remaining $450,000 of 27.5-year basis.
When it does not help
Land never depreciates, so a lot with a high land allocation shrinks the benefit. The deduction is also only as useful as your ability to absorb it: IRC section 469 treats rental losses as passive, so without real estate professional status, the short-term rental exception, or passive income to offset, a big first-year loss may simply sit suspended. On small properties the study fee can eat the benefit, which is why screening comes first. Amadae's tax platform includes a cost segregation screen that models the likely reclassification and first-year benefit before you spend anything on an engineering study, then coordinates the study and files the return that uses it.