Can I do a cost segregation study on a building I bought years ago?

Yes. Depreciating a building for two or more years under an incorrect classification establishes an accounting method, which is corrected by filing Form 3115 under the automatic change procedures (designated change number 7). The entire difference between depreciation claimed and depreciation allowable under the study is deducted as a Section 481(a) adjustment in the year of change — no amended returns.

Is a Section 481(a) catch-up deduction taken all in one year?

Yes, when it is taxpayer-favorable. A negative Section 481(a) adjustment — additional deductions — is taken entirely in the year of change. Only unfavorable (positive) adjustments are spread, over four years. For a look-back cost segregation study the adjustment is almost always negative, so the full catch-up lands in a single year.

When is Form 3115 due for a cost segregation catch-up?

The automatic change is filed in duplicate: the original attached to the timely filed return (including extensions) for the year of change, and a copy sent to the IRS in Ogden no later than the filing date. There is no user fee for automatic changes, and the change can be filed for the current year even after the year has closed for amendment purposes.

Look-back cost segregation: catching up years of depreciation on Form 3115

An owner who placed a building in service in, say, 2019 and depreciated everything over 39 years has not lost the cost segregation opportunity — and does not need to amend a single return to capture it. Misclassifying depreciable property for two or more consecutive years establishes an accounting method under Section 446, and the fix for a wrong method is a method change on Form 3115, not amendment. The change is automatic (no advance IRS consent, no user fee), and the entire shortfall in depreciation — every dollar the taxpayer could have deducted from the placed-in-service year through last year — arrives as a single Section 481(a) adjustment, deducted in full in the year of change.

This is why "we should have done this at closing" is rarely fatal. It is also why the timing question — study now or later — is a real planning choice rather than a use-it-or-lose-it deadline.

Why this is a method change, not an error

Depreciation lives under the accounting-method rules: using an impermissible recovery period or classification for the same asset on two or more consecutive returns is a method of accounting, changeable only with IRS consent under Section 446(e). Rev. Proc. 2015-13 supplies the general procedures, and the annually updated automatic-change list (Rev. Proc. 2024-23 and its successors) includes the change from an impermissible to a permissible method of depreciation — designated automatic change number 7 — which covers reclassifying building components based on a cost segregation study. Automatic status means no advance ruling and no fee; consent is granted by properly filing.

One boundary: if the building was placed in service last year and only one return has been filed, no method exists yet — the fix there is an amended return or, in some cases, treating the first-filed return itself as adopting the study. Two-plus returns filed on the wrong classification is the Form 3115 fact pattern. Our general primer on accounting method changes covers the framework beyond depreciation.

The Section 481(a) mechanics

The 481(a) adjustment equals the difference between depreciation actually claimed and depreciation that would have been allowable under the new method from the placed-in-service date through the end of the year preceding the year of change — including any bonus depreciation that applied at the original placed-in-service date. The bonus rate is the historical one: a building placed in service in 2023 catches up at 80 percent bonus on its short-life property, not today's restored 100 percent, because the acquisition predates the January 19, 2025 seam (see the bonus interaction).

Illustrative catch-up on a building placed in service in 2021, study filed with the 2026 return:

Item	Amount
Depreciation claimed 2021–2025 (39-year on full basis)	$310,000
Depreciation allowable 2021–2025 per study (incl. 100% bonus on reclassified property)	$1,090,000
Negative §481(a) adjustment deducted in 2026	$(780,000)

Favorable (negative) adjustments are taken entirely in the year of change; only unfavorable adjustments spread over four years. The catch-up is an ordinary deduction on the year-of-change return — which means it runs the same gauntlet as any other large depreciation deduction: Section 469 passive limits, Section 461(l), and basis. A suspended catch-up is no better than a suspended year-one deduction.

Filing mechanics

Duplicate filing. Original Form 3115 attached to the timely filed (including extensions) federal return for the year of change; a duplicate copy to the IRS in Ogden, Utah, filed no later than the return.
Year of change flexibility. The change is made for the current tax year — the look-back can be filed with the 2026 return even though the study covers 2019 forward.
Eligibility limits. The automatic procedures carry restrictions — for example, generally no automatic change for the final year of a trade or business, and special rules if the same item was changed within the past five years. An asset already disposed of cannot have its depreciation fixed through a current-year method change.
Audit protection. A properly filed change generally gives audit protection for prior years on the item changed — the examiner cannot reclassify those years back, which is a quiet but substantial benefit of doing the catch-up correctly.

For owners who skipped the study at acquisition, the look-back is usually the cheapest good decision available: the deductions were never lost, only parked. What was lost is the time value of the intervening years — which is the honest argument for not parking them longer than the client's tax posture requires.