You bought the building in 2019, nobody mentioned cost segregation, and six years of accelerated depreciation went unclaimed. You have not lost it, and no deadline has passed. A look back cost segregation study reconstructs the asset detail as of the original purchase date, and every dollar you should have deducted since then lands on your current return in one number. No amended returns.
The short answer
- There is no statute-of-limitations cutoff on the catch-up. A study can be performed on property placed in service in any prior year, and the missed depreciation is still recoverable.
- You do not amend prior returns. The correction is made prospectively on Form 3115, Application for Change in Accounting Method, filed as an automatic change.
- The entire catch-up hits one year. A negative §481(a) adjustment, meaning one that decreases income, is taken 100% in the year of change rather than spread over four years.
- Your bonus depreciation percentage is frozen at the original placed-in-service date, not the study date. A 2021 building gets 2021's 100% bonus; a 2024 building gets 60%.
- If only one return has been filed on the property, amend instead. A method is not adopted until used on two consecutively filed returns.
How Far Back Can a Look-Back Study Actually Go?
To the original placed-in-service date, whenever that was. This surprises people, because almost every other tax correction is boxed in by three years.
The catch-up is not a refund claim, which is why the usual limitation period does not apply. You are not asking the IRS to reopen 2019. You are telling the IRS that going forward you will compute depreciation correctly, and the cumulative difference between what you claimed and what you were entitled to claim gets trued up in the current year.
The real constraint is economic, not legal. A building bought in 2008 has already run most of its short-life assets through their recovery periods, so the catch-up is thin.
Why There Are No Amended Returns
Depreciating a building on a single 39-year or 27.5-year line, when part of the basis is properly 5-, 7-, or 15-year property, is treated as an impermissible method of accounting for depreciation. Correcting a method is not the same as correcting an error on a return.
The vehicle is the Form 3115 catch-up process, filed with your timely filed return for the year of change, including extensions. The number it carries is the §481(a) adjustment: the cumulative depreciation shortfall measured as of the first day of the year of change.
The taxpayer-favorable part is the timing rule. A negative §481(a) adjustment is deducted entirely in the year of change. A positive one, where you over-depreciated, is generally spread over four years. Cost segregation catch-ups are essentially always negative, so essentially always fully deductible now.
The Nuance Most Owners Get Wrong
The most common mistake in look-back planning is assuming that because the study is done in 2026, the property gets 2026's 100% bonus. It does not. The bonus percentage is fixed by the property's original placed-in-service date.
| Original placed in service | Bonus rate applied to reclassified 5-, 7-, and 15-year property |
|---|---|
| 2021 acquisition | 100% |
| 2023 acquisition | 80% |
| 2024 acquisition | 60% |
| Acquired and placed in service after January 19, 2025 | 100% |
That last row is the One Big Beautiful Bill Act, signed July 4, 2025, which restored 100% bonus permanently for property acquired and placed in service after January 19, 2025. Anything acquired on or before that date still lives under the old phase-down. Our guide to bonus depreciation in 2026 has the full mechanics.
The 20% or 40% that bonus does not cover is not lost. It depreciates over the asset's normal 5-, 7-, or 15-year life instead of landing in year one.
Worked Example: A 2019 Apartment Building Studied in 2026
Example: a $3.2M apartment building purchased November 2019, studied in 2026 Land allocated at $600,000, leaving $2,600,000 of depreciable basis on a 27.5-year residential rental schedule. The engineering study reclassifies 24% of basis: $390,000 to 5-year property (appliances, carpeting, cabinetry, decorative lighting) and $234,000 to 15-year land improvements (paving, site lighting, landscaping). Because the property was placed in service in 2019, that $624,000 carries 100% bonus.
| Through December 31, 2025 | Depreciation taken | Depreciation allowable |
|---|---|---|
| 5- and 15-year property (100% bonus in 2019) | $0 | $624,000 |
| Building structure | $579,091 | $440,109 |
| Cumulative total | $579,091 | $1,064,109 |
| §481(a) catch-up deducted in 2026 | $485,018 |
At a combined 35% marginal rate, that is roughly $170,000 of cash tax reduction in a single year, on a building you already owned. Your result depends on your tax rate and whether you can use passive losses.
When a Look-Back Study Doesn't Work
Two situations, both worth checking before you spend anything.
You have filed only one return on the property. A method of accounting is adopted once it has been used on two consecutively filed returns. Buy in 2025, file one return, and there is no adopted method to change. The correct path is an amended return, not Form 3115.
You cannot use the loss. This is the bigger trap. A $485,000 catch-up is worth nothing this year if the property is passive to you, you have no other passive income, and you do not qualify under the real estate professional or short-term rental rules. The deduction is not destroyed; it suspends and carries forward under §469. But a deduction you use in 2031 is worth materially less than one you use in 2026, and a very large catch-up can also trip the excess business loss limitation. Read the passive activity loss rules before you order anything, and think about when to order a study relative to your income picture.
Frequently Asked Questions
Will a look-back study trigger an audit?
Filing Form 3115 is a routine compliance event, and a properly filed automatic change carries audit protection for the method itself. What draws scrutiny is a weak study behind the number: no engineering basis, no reconciliation of allocated costs to actual cost, no support for the land allocation.
How far back is too far back?
There is no legal cutoff, only an economic one. Older properties have already recovered part of their short-life basis, so the catch-up shrinks. As a rough screen, properties placed in service within the last 10 to 12 years with at least $500,000 of depreciable basis are worth modeling.
Do I get the current 100% bonus on my 2023 building?
No. A 2023 acquisition is locked at 80%. The remaining 20% of reclassified basis depreciates over the asset's normal recovery period. Only property acquired and placed in service after January 19, 2025 gets the restored permanent 100% rate.
Can I do this on a property I already sold?
No. A method change applies going forward on property you still own and depreciate. Once sold, the depreciation history is settled in the gain calculation.
Find Out in 24 Hours Whether a Study Pays for Itself
Not every property justifies a study, and we will tell you when yours doesn't. Send Precision Cost Segregation the address, purchase price, closing date, and your rough tax rate, and we will come back with a modeled reclassification range and an estimated first-year benefit at no cost.
This article is provided by Precision Cost Segregation for general educational purposes and does not constitute tax, legal, or accounting advice. Tax outcomes depend on your specific facts, your tax rate, your passive activity position, and your entity structure. Figures shown are illustrative. Consult your CPA or tax advisor before acting, and engage a qualified professional to perform any cost segregation study. Information is current as of publication.
This article is general information, not tax or legal advice. Depreciation outcomes depend on your facts, elections, and current law — consult your CPA before acting. © 2026 Precision Cost Segregation.