Cost Segregation vs Bonus Depreciation: How They Work Together
Cost segregation decides which parts of a building are short-life property; bonus depreciation decides how much of that property you deduct at once. One classifies, the other accelerates.
People often ask whether they should do cost segregation or take bonus depreciation, as if they were alternatives. They are not. Cost segregation decides which parts of a building are short-life property. Bonus depreciation decides how much of that short-life property you deduct in the first year. One classifies; the other accelerates. Each is far less useful without the other.
What bonus depreciation does on its own
Bonus depreciation under §168(k) lets you deduct a percentage of the cost of qualifying property in the year it is placed in service. Qualifying property has a recovery period of 20 years or less, which includes 5- and 7-year personal property, 15-year land improvements and qualified improvement property (15-year, straight line). It does not include a 27.5-year residential rental building or a 39-year nonresidential building.
So if you buy a building and put the whole cost on the return as one building asset, bonus depreciation does nothing for it, whatever the rate.
What cost segregation does on its own
A study identifies the parts of the building cost that are not structural: carpet, dedicated equipment wiring, millwork, parking lots, landscaping, site lighting. Those parts move from the 39- or 27.5-year schedule to 5-, 7- or 15-year schedules.
Even without bonus, that speeds things up. The first-year MACRS rate for 5-year property under the half-year convention is 20%, against at most 2.461% for a 39-year building placed in service in January (IRS Pub 946 Table A-7a), and less the later in the year it is placed in service.
Together
With both, the components a study identifies can be deducted at the bonus rate in year one. In 2026 the rate depends on when you acquired the property:
| Acquired under a written binding contract | Placed in service | Bonus rate |
|---|---|---|
| After January 19, 2025 | Any year | 100% |
| On or before January 19, 2025 | 2025 | 40% |
| On or before January 19, 2025 | 2026 | 20% |
| On or before January 19, 2025 | 2027 or later | 0% |
At 100%, everything the study moves is deducted at once. At 20%, a fifth is deducted at once and the rest follows regular MACRS. For an illustrative $1.9 million office basis, that difference is roughly $300,000 to $550,000 of added first-year deduction under a 2026 contract, against roughly $90,000 to $170,000 under a late-2024 contract (our calculator, published benchmark ranges; not a forecast).
Choices that change the math
- Electing out. You can elect out of bonus for a whole class of property for a year. The study’s classification still speeds depreciation through the shorter recovery periods.
- The 40% transition election. For the first tax year ending after January 19, 2025, you can elect 40% (60% for certain long-production-period property) instead of 100%.
- The mid-quarter convention. If more than 40% of the year’s non-real property is placed in service in the last quarter, every such asset that year uses the mid-quarter convention. A big fourth-quarter study can flip it, which matters only for the portion bonus does not cover.
- State tax. Many states decouple from bonus depreciation. The classification still applies for state purposes; the bonus generally does not.
Bottom line
If you own a building with meaningful basis, bonus depreciation is the reason a study pays off quickly, and the study is the reason bonus has anything to work on. Read the 2026 bonus depreciation guide for the acquisition-date rules, including self-constructed property.
Sources
- IRS Notice 2026-11: https://www.irs.gov/pub/irs-drop/n-26-11.pdf
- 26 U.S.C. §168 (Cornell LII): https://www.law.cornell.edu/uscode/text/26/168
- IRS Pub 946, How To Depreciate Property: https://www.irs.gov/publications/p946