The Form 3115 Look-Back: Claiming Missed Depreciation Without Amending Returns

If you have owned a commercial property for more than a year and never ran a cost segregation study, you have been leaving depreciation on the table. The good news: you do not have to amend a single prior return to get it back. One form, filed this year, can pull every dollar of that missed deduction into the present.

The problem: depreciation you already earned but never took

When you buy a commercial building, the IRS generally has you depreciate it straight-line over 39 years. That is the default, and it is conservative. It treats the entire structure, from the foundation to the carpet to the parking lot, as if every piece wears out at the same slow pace. It does not. A cost segregation study identifies the components that genuinely have shorter lives and reclassifies them into 5, 7, and 15-year property, which accelerates the deductions dramatically.

The trouble is that most owners never commission a study at purchase. So year after year, they claim the slow 39-year figure when they were entitled to far more. The depreciation does not vanish, but it sits unclaimed, and every year that passes adds to the gap between what was taken and what could have been.

What Form 3115 does: a one-time catch-up, no amended returns

This is where Form 3115, the Application for Change in Accounting Method, comes in. Rather than reopening and amending each prior return, you file Form 3115 and take a single catch-up adjustment under Section 481(a) of the Internal Revenue Code in the current tax year. That adjustment equals the difference between the depreciation you actually took, the straight-line 39-year amount, and what you could have taken had the components been correctly classified from the start.

In plain terms, the form bundles all of that missed prior-year depreciation into one deduction you claim now. There is no need to file amended returns for the years you under-depreciated. For this type of correction, the change is generally treated as an automatic change in accounting method, meaning it does not require the IRS to approve it before you file.

A simple illustrative example

Suppose you bought an office building several years ago and have been depreciating it straight-line over 39 years the entire time. A cost segregation study performed today determines that a meaningful share of the building basis should have been classified as shorter-life property all along. The catch-up is the gap between the two depreciation paths, added up across every year you owned the building.

• What you took: the slow 39-year straight-line amount, each year
• What you could have taken: the accelerated amount on the reclassified 5, 7, and 15-year components
• The Section 481(a) catch-up: the cumulative difference, claimed in one current-year deduction

Because the gap compounds with each year of ownership, an older property held many years often produces the largest catch-up. These figures are illustrative; your actual result depends on your property and an engineered study.

Why now: it pairs with permanent bonus depreciation

The timing matters. The One Big Beautiful Bill Act of 2025 made 100% bonus depreciation permanent for qualifying property acquired and placed in service after January 19, 2025. For the shorter-life components a study identifies, that means full first-year expensing rather than a slow write-off. Combined with the catch-up mechanism on older property, the deduction available this year can be substantial. The look-back recovers what you missed, and the current rules let the eligible portion land all at once.

What it requires: a study and your CPA

Two things make this work. First, an engineering-based cost segregation study, which identifies and documents the components, supplies the defensible numbers, and supports the Form 3115. Second, your CPA, who actually prepares and files the form alongside your return. The study is the evidence; the filing is the execution. You and your CPA file it, and a credible study is what stands behind the position if it is ever examined.

The next step

If you have owned a commercial building for more than a year and never ran a study, the missed depreciation is almost certainly there, waiting. The first step is simple and free: find out how much a study is likely to free up on your specific building before you spend a dollar.

Frequently asked questions

Do I have to amend my old tax returns?

No. That is the central advantage of this approach. You file a Form 3115 change in accounting method and take a one-time catch-up adjustment under Section 481(a) in the current tax year, rather than reopening and amending each prior return.

How far back can the catch-up go?

The Section 481(a) adjustment captures the entire difference between the depreciation you actually took and what you could have taken since the property was placed in service, bundled into the current year. Older properties held many years often produce the largest catch-up.

Is Form 3115 an automatic change?

For this type of correction, switching to a proper cost segregation classification is generally treated as an automatic change in accounting method, which does not require IRS pre-approval before filing. Your CPA confirms the correct procedure for your situation.

Can I do this on a property I bought years ago?

Yes. The look-back is designed for property you already own. If you have held a commercial building for more than a year and never had a study, you can capture the missed depreciation now, and longer holding periods often mean a larger one-time adjustment.

Sources

» Internal Revenue Service, Form 3115, Application for Change in Accounting Method, and instructions, irs.gov
» Internal Revenue Code Section 481(a), adjustments required by changes in method of accounting
» IRS Cost Segregation Audit Techniques Guide
» Internal Revenue Code Section 168(k), additional first-year depreciation