If you forget to take depreciation, the IRS still treats you as if you had claimed it. Under the “allowed or allowable” rule, your property’s cost basis drops by the depreciation you were entitled to deduct each year, whether you actually put it on a return or not. That means the missed deduction quietly increases your taxable gain when you eventually sell. The deduction itself is not automatically lost, though: depending on how many years you skipped, you can recover it with an amended return (Form 1040-X) or a change in accounting method (Form 3115).
The Allowed or Allowable Rule
This is the rule that makes forgetting depreciation costly. Under 26 U.S.C. § 1016(a)(2), your adjusted basis in a depreciable asset must be reduced by the greater of the depreciation you actually claimed (“allowed”) or the amount you were legally entitled to claim (“allowable”). The IRS assumes you took the deduction whether you did or not. Where no depreciation method was ever adopted, the reduction defaults to the straight-line method.1Office of the Law Revision Counsel. 26 U.S. Code 1016 – Adjustments to Basis
Here is what that looks like in practice. Say you bought a rental property for $300,000 (excluding land) and placed it in service 10 years ago. Under the standard 27.5-year recovery period for residential rental property, your allowable depreciation is roughly $10,909 per year, or about $109,090 over the decade. Even if you never claimed a dollar of it, your adjusted basis drops to approximately $190,910. Sell for $350,000, and your taxable gain is calculated from that reduced basis, not from the $300,000 you paid.2Internal Revenue Service. Publication 551, Basis of Assets
Publication 551 states this directly: if you did not take a depreciation deduction, you must still reduce your basis by the full amount you could have taken.2Internal Revenue Service. Publication 551, Basis of Assets One boundary to check before you file anything: the asset actually has to qualify for depreciation in the first place. Land does not, and neither do inventory or property used purely for personal purposes.3Internal Revenue Service. Publication 946, How To Depreciate Property If you have been ignoring depreciation on land you allocated separately from a building, there is nothing to fix.
Fixing a Single Missed Year With Form 1040-X
If you forgot depreciation for just one tax year, the standard fix is an amended return. Recalculate the correct depreciation for that year on Form 1040-X, enter the adjustment, and claim the refund or reduced balance due. Most taxpayers can now file Form 1040-X electronically, which is faster than mailing paper.4Internal Revenue Service. Form 1040-X, Amended U.S. Individual Income Tax Return: Frequently Asked Questions
The deadline is strict. You must file the amended return within three years of your original filing date (including extensions) or within two years of the date you paid the tax, whichever is later.5Internal Revenue Service. Instructions for Form 1040-X (12/2025) If you filed early, the IRS treats your return as filed on the regular due date, typically April 15. Miss this window and the refund for that year is gone.
Expect the IRS to take 8 to 12 weeks to process the amendment, though some cases stretch to 16 weeks.6Internal Revenue Service. Where’s My Amended Return? Status is available online about three weeks after submission.
Fixing Multiple Missed Years With Form 3115
When depreciation was missed across two or more years, the IRS does not want a stack of amended returns. Instead, you file Form 3115, Application for Change in Accounting Method. The correction is treated as a switch from an impermissible depreciation method (taking zero) to a permissible one.7Internal Revenue Service. Form 3115, Application for Change in Accounting Method
Automatic Consent and DCN 7
Most depreciation corrections qualify for the automatic consent procedure under Revenue Procedure 2024-23, meaning no individual permission from the IRS is needed. On the form, enter Designated Change Number (DCN) 7, which covers changes from an impermissible to a permissible depreciation method. No user fee applies to automatic consent filings.8Internal Revenue Service. Instructions for Form 3115
File in two places. Attach the original to the timely filed federal income tax return for the year of the change. Then send a signed duplicate to the IRS National Office at Ogden, UT 84201, Attn: M/S 6111, no later than the date you file that return.9Internal Revenue Service. Where to File Form 3115 Faxing the duplicate is also accepted.
The Section 481(a) Catch-Up Deduction
This is the payoff. Form 3115 uses a Section 481(a) adjustment to total the depreciation you should have claimed in every prior year but didn’t. Because that adjustment is negative (it reduces income), you take the entire amount as a single deduction in the year of the change.10Internal Revenue Service. 4.11.6 Changes in Accounting Methods No need to amend each prior year individually.
For a rental property owner who skipped depreciation for a decade, this catch-up deduction can reach six figures in a single year. It is one of the largest one-year deductions available to individual taxpayers. Many people who missed depreciation by accident finish in a better position than if they had claimed it year by year, because the lump-sum deduction may drop them into a lower bracket for that tax year.
Unlike the 1040-X route, Form 3115 has no lookback cap. Because it changes an accounting method rather than claiming a specific-year refund, it can capture depreciation missed over any number of prior years. One caveat: if you are under IRS examination when you file, different spread periods can apply to positive adjustments. For a typical missed-depreciation scenario, though, the negative adjustment is deducted in full in one year.10Internal Revenue Service. 4.11.6 Changes in Accounting Methods
What It Costs If You Never Correct It
The financial damage from forgetting depreciation concentrates at sale, because the allowed-or-allowable rule calculates your gain from the reduced basis regardless of what you actually deducted. In effect, you pay tax on depreciation benefits you never received.
For equipment, machinery, vehicles, and other tangible personal property, Section 1245 recaptures the entire depreciation-related portion of the gain as ordinary income, at your marginal rate.11Office of the Law Revision Counsel. 26 U.S.C. 1245 – Gain From Dispositions of Certain Depreciable Property If you’re in the 32% bracket, every dollar of allowable depreciation is taxed at 32% whether you claimed it or not.
Real estate works differently. Section 1250 itself recaptures only depreciation that exceeded straight-line, which for most post-1986 property is zero because MACRS already requires straight-line depreciation on buildings.12Office of the Law Revision Counsel. 26 U.S.C. 1250 – Gain From Dispositions of Certain Depreciable Realty The remaining depreciation-related gain becomes “unrecaptured Section 1250 gain” under 26 U.S.C. § 1(h), taxed at a maximum 25%.13Office of the Law Revision Counsel. 26 U.S. Code 1 – Tax Imposed That rate applies to the full allowable amount, not just what you claimed. Correcting the error with Form 3115 before selling ensures you at least receive the deductions that offset the recapture tax.
What You Need to Prepare a Correction
Whether you file Form 1040-X or Form 3115, you need the same core information:
- The exact date the asset was placed in service, which starts the depreciation clock.
- Your original cost basis, including purchase price plus closing costs, sales tax, and installation or improvement costs.
- The correct recovery period and method under MACRS. Residential rental property uses 27.5-year straight-line, nonresidential real property uses 39 years, and most equipment falls into 5-year or 7-year categories under the 200% declining balance method.14Office of the Law Revision Counsel. 26 U.S.C. 168 – Accelerated Cost Recovery System
- Copies of prior returns showing what was previously reported on Form 4562, if anything.
For Form 3115 specifically, you also calculate the Section 481(a) adjustment: cumulative depreciation you should have claimed in all prior years minus what you actually claimed. That difference is the catch-up deduction on your current-year return.
Keep the underlying records until the statute of limitations expires for the year you dispose of the asset, not the year you placed it in service. Depreciation records must last long enough to calculate both the deductions and the gain or loss at sale.15Internal Revenue Service. How Long Should I Keep Records For a rental held 20 years, that means keeping purchase documents for more than two decades plus the standard three-year retention after disposal.