You may be sitting on a pile of untapped tax losses. If you own real estate that went into service without a cost segregation study, there may be a large deduction sitting inside the building right now. In many cases you can still claim it, and the end of the year is the right time to find out how big it is.
The tool is a look-back cost segregation study paired with Form 3115. Here's how it works, and where it gets tricky.
What does a cost segregation study actually do?
Put simply, a cost segregation study is an engineering study that breaks a large real asset into its component pieces, each with its own depreciable life.
Without a study, the IRS sees your property as two things: land and building. Land never depreciates. The building depreciates on a straight line over 27.5 years for residential rental property and 39 years for commercial property.
A building is really a collection of components with very different useful lives. Carpet, cabinets, appliances, and certain dedicated electrical generally fall into 5- or 7-year property. Parking lots, sidewalks, landscaping, and other site improvements are 15-year property. A study pulls those components out of the building and puts each one on its correct, shorter schedule.
That does two things. It accelerates depreciation over shorter lives, and it makes those shorter-life components eligible for bonus depreciation, which can write off a large share of their cost in the first year.
Conventional wisdom says to order the study in the year the property is placed in service and take the bonus depreciation in year one. That's the best way to use these studies, because it captures the most time value of money.
Can you do cost segregation on a property you already own?
All hope is not lost. You can order a cost segregation study on a property that's been in service for years. The engineering is exactly the same.
To claim the benefit, you file Form 3115, Application for Change in Accounting Method. The IRS treats the way you depreciate an asset as a method of accounting, and moving from a building-only approach to properly classified components is a change the IRS generally allows under its automatic consent procedures. You don't amend prior returns, and you don't wait on the IRS to approve the change.
Mechanically, it works like this. You recalculate the cumulative depreciation you would have taken on the property to date if the study had been done in the year it was placed in service, including any bonus depreciation available in that year. Then you compare that figure to the depreciation you actually took. The difference is a Section 481(a) adjustment, and when the adjustment reduces your taxable income, you take the whole thing in the current year.
Plain English: years of missed deductions collapse into a single number on this year's return.
Why does waiting shrink the 481(a) catch-up?
Depreciable property is fully depreciated when all is said and done, so total depreciation over the life of the building is the same with or without a study. The only difference is timing.
Picture two lines on a chart of cumulative depreciation. The cost segregation line jumps early, because the short-life components and the bonus depreciation front-load the deductions. The straight-line version climbs slowly and steadily. The gap between those two lines is your 481(a) adjustment.
That gap usually peaks within the first few years. Once the short-life components would have been fully depreciated under the study, the straight-line method keeps grinding along and slowly closes the distance, by roughly 1/27.5 or 1/39 of the reclassified cost each year. Waiting also costs you the time value of deductions you could have been using. Either way, the sooner you look, the more the study is worth.
What bonus rate do you get on a look-back study?
This is where operators get the number wrong in their heads. The bonus rate is locked to when the property was acquired and placed in service. The year you file the 3115 has no bearing on it.
Property placed in service from late 2017 through 2022 generally got 100% bonus. Under the phase-down that followed, 2023 got 80% and 2024 got 60%, and property placed in service in 2025 but acquired before January 20, 2025 gets 40%. The 2025 tax law restored 100% bonus for property acquired after January 19, 2025. A look-back study on a 2023 acquisition uses 80%. That's still a very large number, just smaller than the headlines suggest.
Two more checks belong at the front of the process. First, pull the original return and confirm nobody elected out of bonus depreciation. That election is generally permanent, and a 3115 can't undo it. Second, if the property was placed in service last year and only one return has been filed, you may have a choice between a 3115 with this year's return and an amended return (an administrative adjustment request for most partnerships). Your CPA will know which path fits.
What happens at the state level?
Many states don't allow bonus depreciation, but most still recognize the shorter recovery lives for personal property and land improvements that the study produces. In those states you lose the bonus layer, but you keep the benefit of pulling components off the 27.5- or 39-year schedule.
The state catch-up is usually smaller than the federal one, but it's real. Each state has its own conformity rules and addbacks, and if the property throws income into more than one state, you may be running this analysis several times. I wrote about how far apart the federal and state numbers can land in One Deal. Two Bottom Lines.
Which properties should you check before year-end?
There's no better time to look at this than right now. Go through your portfolio with a few questions in hand.
Was the property placed in service in the last several years without a study? That's where the gap is widest. Is the depreciable building basis large enough to justify the cost of the study? Did anyone elect out of bonus on the original return? And the one that gets skipped most often: can the owners actually use the losses? A large 481(a) adjustment that lands on LPs with no passive income simply becomes a suspended loss. I break that down in Can K-1 Losses Offset W-2 Income?
Also consider how long you plan to hold. Accelerated depreciation comes back as recapture when you sell, much of it at ordinary rates. If an exit is on the calendar for next year, the math changes. More on that in Tax on Sale of Commercial Real Estate.
One timing note. The 3115 is filed with the tax return for the year of the change, so a study finished early next year can still produce a 2026 deduction. The planning, though, belongs before December 31. Knowing a large deduction is coming changes your estimated payments, whether it makes sense to pull a sale or other income into the same year, and which year you want the change to land in. Engineering firms also get busy in the first quarter.
Why do you need a professional for this?
The process sounds simple, and it's fairly complex in practice. The 3115 has to be attached to a timely filed return, including extensions, with a signed duplicate filed separately with the IRS. The 481(a) computation has to be supported, reported correctly, and carried through to the state returns. For a partnership, the partnership files the change, and the adjustment flows through to each partner's K-1, where it then runs into every partner's own limitations.
It may be that the tax losses you need for 2026 have been right under your nose this whole time. The building didn't change. The only thing missing was the study.
This is general education, not tax advice. Accounting method changes, bonus depreciation eligibility, and state conformity are fact-specific and depend on when and how each property was acquired, placed in service, and reported. Review any look-back study with your CPA before you count on the deduction. For the full framework on whether a study pencils, start with the cost segregation guide.