Cost segregation is an engineering-based study that reclassifies specific components of a commercial or residential rental building out of its 39-year or 27.5-year depreciation schedule and into 5-, 7-, or 15-year categories, making them eligible for bonus depreciation years sooner.
In plain terms
A building isn’t one asset to the tax code — it’s a bundle of them. The structural shell depreciates over 39 years (commercial) or 27.5 years (residential), but carpeting, certain wall finishes, specialty electrical and plumbing serving specific equipment, and site work like parking lots and landscaping belong to much shorter categories. A cost segregation study walks through the building, component by component, and sorts out everything that qualifies for the shorter life — then those components become eligible for accelerated and bonus depreciation immediately, instead of trickling out over decades.
How it works
The analysis sits on top of the Modified Accelerated Cost Recovery System (MACRS), the depreciation framework set out in IRC §168. An engineering firm or specialized accounting team inspects the property, documents construction costs component by component, and assigns each one to its correct MACRS class — typically 5-year (certain equipment and decorative elements), 7-year, 15-year (land improvements such as paving, fencing, and landscaping), or the building shell’s own 27.5-/39-year life. The IRS’s own Cost Segregation Audit Techniques Guide is explicit that this classification is “factually intensive” with no bright-line percentage the agency will simply accept — it has to be substantiated asset by asset.
Once a component lands in a 5-, 7-, or 15-year class, it becomes eligible for the bonus depreciation the building shell itself can never claim. A study can be done in the year a property is acquired or built, or retroactively on a property already owned, using a Form 3115 accounting method change to catch up the missed deductions in a single year rather than amending every prior return.
The numbers
- Recovery periods: 39 years (nonresidential real property) and 27.5 years (residential rental property) for the building shell, versus 5, 7, or 15 years for reclassified components — IRC §168, MACRS.
- Bonus depreciation rate: a permanent 100% first-year deduction for qualified property acquired after January 19, 2025, under the One Big Beautiful Bill Act — confirmed on the IRS’s bonus depreciation guidance page, with no scheduled phase-down.
- A sourced result: a 312-unit apartment complex facing an estimated $9 million federal tax liability cut that bill by roughly $2.6 million through a cost segregation study that reclassified about 22% of the property’s costs into faster-depreciating categories.
- Retroactive catch-up: a study on an already-owned property uses IRS Form 3115 to claim missed depreciation in the current year rather than filing amended returns for every prior year.
What people get wrong
That a cost segregation study reliably moves a fixed share — “20% to 40%” is the number repeated everywhere — of a building’s cost into short-life categories. The IRS’s own audit guide rejects that framing outright: there’s no bright-line percentage, and the actual share depends entirely on the property type, its construction, and how rigorously each component is documented. The second, more consequential misconception is that cost segregation stopped mattering once bonus depreciation went to 100%. It didn’t — the 100% rate only applies to property that already qualifies for a short recovery period. A component still sitting in the 39-year bucket because nobody bothered to reclassify it gets no bonus treatment at all, bonus rate or not. The study is what makes an asset eligible in the first place; the depreciation rate is a separate question that only matters afterward.
Related
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See also: Depreciation recapture · 1031 exchange · Real estate professional status