Most of the property owners we talk to did not skip cost segregation on purpose. They bought the building, their CPA set it up on a 39-year schedule, and nobody revisited it. Five or eight years later, the building is producing income and the depreciation is a rounding error.
The good news is that the tax code has a clean mechanism for fixing this, and it does not require amending a single prior return.
How the look-back works
A cost segregation study on an existing property does the same thing as a study on a new purchase: an engineering-based analysis breaks the building into components and assigns each the shortest recovery period the code allows. The difference is timing. Instead of applying the reclassification going forward only, the study calculates the depreciation you would have taken in every prior year if the components had been classified correctly from the start.
The gap between what you actually deducted and what you could have deducted is claimed in the current year as a Section 481(a) adjustment. You file Form 3115, Application for Change in Accounting Method, with your return. Because this is treated as an automatic method change, no IRS approval is required in advance and no amended returns are filed.
The result is a single, often large, deduction in the year of the study.
An example
A dental practice bought its building in 2019 for $1.8 million and has been depreciating it straight-line over 39 years. Assume $1.5 million of depreciable basis after land. Through 2025, roughly $270,000 of depreciation has been taken.
A study reclassifies 30 percent of the basis, or $450,000, into 5-, 7-, and 15-year property. Recomputing depreciation from 2019 forward on those components, and applying the bonus depreciation rules in effect for each year, produces a catch-up deduction in 2026 that can be several hundred thousand dollars. The exact figure depends on the component mix and the bonus percentages that applied when the property was placed in service.
That is a 2026 deduction on a 2019 purchase, with no amended returns.
Which bonus rules apply
Here is a detail worth understanding. For a look-back study, the bonus depreciation percentage is determined by when the property was originally placed in service, not when the study is done. A property placed in service in 2019 gets the 100 percent bonus that applied in 2019. A property placed in service in 2023 gets the 80 percent that applied then. The catch-up captures whatever you were entitled to at the time.
Improvements and renovations you made after January 19, 2025, qualify for the restored 100 percent bonus on their own.
Who should look at this
Owners of commercial or multifamily property who have never had a study. There is no minimum property size.
Owner-occupants such as medical, dental, veterinary, and professional practices that own their building.
Investors with multiple properties where the studies were never done because each one seemed too small on its own.
Anyone with a large tax year ahead who owns an under-depreciated building.
The passive loss question
If the property is a passive activity for you, the catch-up deduction may create a loss that is suspended rather than usable this year. Suspended losses are not lost. They carry forward and offset future passive income, and they are released in full when you dispose of the property. Real estate professionals and owner-occupants generally face fewer limits. Your CPA should model this before the study is ordered.