How to File Form 3115 to Catch Up Missed Depreciation From a Prior-Year Cost Segregation Study

What Form 3115 actually does

Form 3115 is the official IRS form for requesting a change in accounting method. Depreciation method is an accounting method, and treating a building component as 39-year property when it should have been 5-year property is an impermissible depreciation method that can be corrected by a Form 3115 method change. The same is true for missing a 15-year QIP classification, missing a Section 168(e)(3)(E)(iii) 15-year recovery on a retail motor fuels outlet, or omitting any depreciation deduction on assets placed in service in prior years.

The form has two basic flavors: automatic consent and non-automatic consent. Automatic consent under Rev. Proc. 2022-14 is what cost seg look-back changes use. The IRS does not need to approve the change in advance. The taxpayer files Form 3115 and the change is treated as approved on filing. Non-automatic consent requires advance written IRS approval and is rare for cost seg purposes.

Designated Change Number (DCN) 7 is the specific category for changing from an impermissible to a permissible depreciation method or recovery period. It is the DCN used for look-back cost segregation studies. The DCN appears on Form 3115 Part I and tells the IRS which automatic-consent procedure governs the filing.

Why no amended returns are required

Most investors expect that capturing missed depreciation from prior years requires filing amended returns for each affected year. That intuition is wrong for depreciation method changes. Under Treas. Reg. 1.446-1(e)(2)(ii), a change in depreciation method is treated as a change in accounting method, and the cumulative effect of the change is captured in a Section 481(a) adjustment in the year of change.

The Section 481(a) adjustment computes the difference between (a) the depreciation that would have been taken under the new method from inception, and (b) the depreciation that was actually taken under the old method. The difference, if negative (more depreciation would have been taken), is the catch-up deduction posted on the current return.

The practical result is dramatic. A taxpayer who has owned a $1M rental for five years and never performed cost segregation can typically claim $100K to $200K of catch-up depreciation in a single tax year by filing a look-back study with Form 3115. No need to reopen the 2021, 2022, 2023, or 2024 returns. No statute-of-limitations issues. The IRS treats the change as a current-year event.

Who qualifies to use DCN 7

DCN 7 under Rev. Proc. 2022-14 (the current consolidated automatic-consent procedure) applies to taxpayers who placed property in service in a prior year and want to change the method or recovery period for that property to a permissible method. The taxpayer must have used the impermissible method for at least two consecutive tax years prior to the year of change.

There is no minimum holding period beyond the two-year rule. There is no statutory deadline. A property placed in service in 2010 and depreciated as 39-year property through 2025 can have a Form 3115 filed with the 2026 return that catches up 16 years of accelerated depreciation in one year. The earlier the look-back happens, the larger the catch-up.

The taxpayer must still own the property at the time of the Form 3115 filing. If the property has been sold, the corrective opportunity is to amend the return for the year of sale (subject to statute of limitations) rather than use Form 3115. Form 3115 is forward-looking from the filing year. It does not apply to dispositions completed in prior years.

Filing mechanics: original plus Ogden duplicate

Form 3115 is filed twice. The original (or photocopy of the original) attaches to the timely-filed (with extensions) federal tax return for the year of change. A signed duplicate is mailed separately to the IRS service center in Ogden, Utah under Rev. Proc. 2015-13.

The duplicate filing is critical. The IRS treats the change as not having been validly requested if only one copy is filed. WeCostSeg prepares both copies when bundled with a look-back study. The Ogden mailing address is published in the most recent revision of Form 3115 instructions. Verify against the latest IRS form before mailing.

Timing matters. The Form 3115 must be filed with the timely-filed return for the year of change. An extension extends the filing window. Filing after the extended due date misses the year and pushes the change to the next year, which is usually fine but reduces the catch-up amount because another year of impermissible depreciation accumulates.

• Form 3115 itself (the IRS form, four pages as of 2026).
• Statement attachment showing the Section 481(a) adjustment computation.
• Description of the change in accounting method (typically references DCN 7 and the cost segregation study).
• Cost segregation study report (often attached as an exhibit or referenced as available on request).
• Statement of taxpayer representations required by Rev. Proc. 2022-14.
• Signed original (or photocopy) attached to the timely-filed federal return.
• Signed duplicate mailed to IRS Ogden, Utah service center.

The Section 481(a) catch-up computation

The Section 481(a) adjustment is the cumulative difference between depreciation under the new method and depreciation under the old method, computed from inception of the property's service life through the year before the change. Take what should have been taken cumulatively, subtract what was actually taken cumulatively, and the difference is the adjustment.

For a negative 481(a) adjustment (more depreciation under the new method, meaning a deduction to the taxpayer), the entire adjustment is deducted in the year of change. There is no four-year spread or other recovery period limitation under DCN 7. The taxpayer takes the full deduction in one year, subject to passive activity loss limitations and other normal income limitations.

For a positive 481(a) adjustment (less depreciation under the new method, meaning income to the taxpayer), the adjustment is generally spread over four years under Section 481(a) and Section 481(b). This direction is uncommon for cost seg look-backs but can occur if the taxpayer was previously taking more depreciation than was permissible.

When the look-back makes sense and when it does not

Look-back studies make sense for properties owned for one or more tax years where cost segregation was not performed at acquisition. The catch-up deduction is real and immediate. On a $1M depreciable basis residential rental owned for 5 years without cost seg, the typical Section 481(a) catch-up runs $100K to $200K, deducted entirely in the year of change. The fee for the look-back study plus Form 3115 preparation is typically $2,495 to $2,995, the same as a fresh study (WeCostSeg includes Form 3115 prep at no extra cost when bundled with a look-back).

Look-back studies make less sense when (1) the property's depreciable basis is too small to justify the study fee against the catch-up, (2) the taxpayer cannot use the catch-up against current-year income (passive losses are suspended without REPS or the STR loophole), or (3) the property is held in a low-marginal-rate entity where the deduction value is small.

Look-back studies are particularly powerful when paired with a current-year REPS qualification or a new STR loophole qualification. The catch-up deduction is treated like any other current-year rental loss for passive activity purposes. A REPS-qualifying taxpayer or STR loophole operator can use the entire catch-up against W-2 and other active income in the year of change.

Common pitfalls and how to avoid them

First common pitfall: filing only the original Form 3115 and not the Ogden duplicate. The IRS treats the change as invalid. Always file both copies, both signed.

Second: trying to file Form 3115 for a year where the taxpayer has already filed the return without it. The form must accompany the timely-filed return for the year of change. If the year of change return has been filed, the change is for the next year, not the current year.

Third: confusing DCN 7 (the impermissible-to-permissible category) with other DCNs. The cost seg look-back is always DCN 7 unless there is an unusual fact pattern. Always verify the DCN in the most recent revision of Rev. Proc. 2022-14 (or its successor).

Fourth: forgetting that the new depreciation method continues going forward. The Form 3115 changes the method permanently. The property continues to depreciate on the new schedule for all future years. The 481(a) catch-up is a one-time event. The future-year depreciation is the recurring benefit.

Form 3115 service from WeCostSeg

Every WeCostSeg look-back study includes Form 3115 preparation at no extra cost. Our deliverable includes the completed Form 3115 (both the original for return attachment and the duplicate for Ogden mailing), the Section 481(a) computation statement, the supporting cost segregation report, and a one-page filing instruction sheet for your CPA.

The three-touch CPA Coordination Protocol applies. We send a preliminary analysis to your CPA on intake, share the draft Form 3115 five business days before final delivery, and coordinate the filing timing with the current-year return. Most CPAs are comfortable with Form 3115 once they have the prepared form in hand. The form is technical but not difficult to file when prepared by the engineer.