Look-back cost segregation

Form 3115 for Real Estate Cost Segregation

If your property has been depreciating on a straight 27.5- or 39-year schedule for a few years and a cost segregation study reclassifies part of the basis into 5-, 7- or 15-year property, the “missed” depreciation from the earlier years does not vanish. IRS Form 3115 — Application for Change in Accounting Method — pulls it forward as a single deduction in the year of the change. No amended returns.

Why Form 3115 exists

Depreciation method is an accounting method in the tax sense. Once a depreciation method has been used on two consecutive returns, it is locked in — the taxpayer cannot simply start using a different one without IRS consent. Form 3115 is the IRS's mechanism for that consent.

Two consent paths. Depending on the change, the taxpayer either requests advance consent (Form 3115 filed and approved in advance) or uses automatic consent (Form 3115 filed with the return in the year of the change, no pre-approval needed). For a cost-seg look-back study, the change qualifies as an automatic method change under Rev. Proc. 2015-13 and the current designated automatic method change list, where the depreciation method change lives as DCN 7.

No amended returns. This is the payoff. Because it is an accounting method change and not an error correction, the taxpayer does not amend the earlier years. The missed depreciation from every look-back year flows to a single line item — the §481(a) adjustment — deducted in the year of the change.

The DCN 7 automatic method change

DCN 7 is the designated change number for a change in method of depreciation under §§ 167, 168 or the former §168 for tangible property placed in service in a year prior to the year of the change.

In plain language: if your property was on the wrong recovery period for prior years, DCN 7 is the change that puts it on the right one going forward and recovers the difference back to day one via §481(a).

What qualifies. The taxpayer must have used the impermissible method for at least two consecutive prior tax years, still own the property in the year of the change, and be within the automatic change scope (no §280F(b)(3) recapture triggered, no other carve-outs that would push the change to advance consent).

What is filed. One Form 3115 per return, filed by the return due date (including extensions) for the year of the change, with a statement attachment covering the DCN 7 change, the §481(a) amount, and the depreciation method details. A duplicate copy goes to the Ogden, UT IRS service center.

The §481(a) catch-up — a worked example

The §481(a) adjustment is the difference between what depreciation would have been under the new (correct) method from placed-in-service date and what was actually taken under the old (impermissible) method through the end of the year before the change.

Property. $10,000,000 in improvements (land excluded), commercial strip retail, placed in service 2018-01-01.

Old method. Straight 39-year, half-year convention, no cost seg. Annual depreciation: $10,000,000 ÷ 39 = $256,410. Over six years (2018-2023): $1,538,461 actually deducted.

New method after a 2024 cost seg. Study reclassifies $1,500,000 into 5-year, $1,500,000 into 15-year, and $7,000,000 remains 39-year.

Correct cumulative depreciation had cost seg been done at PIS (through 2023, ignoring bonus for illustration):

BucketBasisCalculationCumulative
5-year$1,500,000Fully depreciated by 2022 (5 years + half-year convention)$1,500,000
15-year$1,500,0006 × ($1,500,000 ÷ 15)≈ $600,000
39-year$7,000,0006 × ($7,000,000 ÷ 39)≈ $1,076,923
Total correct cumulative$3,176,923

§481(a) adjustment: $3,176,923 − $1,538,461 = $1,638,462

The taxpayer deducts $1,638,462 in tax year 2024, in addition to the correct 2024 depreciation for each bucket going forward.

Bonus depreciation, when it applies, changes the numbers significantly in favor of the taxpayer — every study we run reports the §481(a) adjustment per component with the applicable bonus rate for the acquisition date, per §168(k) and IRS Notice 2026-11. Property acquired after 2025-01-19 carries permanent 100% bonus on the short-life slice under the One Big Beautiful Bill Act (OBBBA, 2025); earlier property is on the TCJA phase-down (80% in 2023, 60% in 2024, 40% in 2025). Full mechanic in our Cost Segregation Playbook.

Where Form 3115 does not help

An honest list. These situations Form 3115 will not fix:

  • Property already sold. DCN 7 requires the taxpayer to still own the property in the year of the change. A property disposed of before the year of change goes on an amended return for the year of disposition instead — subject to the ordinary statute of limitations.
  • Only one prior year on the old method. The two-year rule under Rev. Proc. 2015-13 is a floor. A property in its first year of depreciation does not need Form 3115 — pick the correct method on the original return.
  • A change in use rather than method. Converting a residence to a rental, for example, is a change in use governed by §168(i)(5) and Treas. Reg. §1.168(i)-4, not a method change under DCN 7.
  • §754 / §743(b) partnership adjustments to prior years. When a partnership had a §754 election in place but failed to compute or allocate a §743(b) adjustment for a prior year's partner transfer, a Form 3115 method change is the wrong instrument — a different Rev. Proc. path applies (typically Rev. Proc. 2020-23 for the late §754 election itself). Details in our §754 partner-basis primer.
  • A property acquired via inheritance (§1014 step-up). The step-up creates new depreciable basis but the property is “used-property from a related person” for §168(k) purposes (Treas. Reg. §1.168(k)-2(b)(3)(iv)(C)) — no bonus depreciation. The Form 3115 mechanic works if the property was mis-depreciated, but the §481(a) upside is smaller.

When a study includes Form 3115 support

Cost segregation studies fall into two buckets by timing:

  • First-year (concurrent) studies. The study is completed and applied on the property's first return with cost segregation. There is no prior wrong method to change from, so no Form 3115 is needed — the depreciation schedule is set correctly on the return.
  • Look-back studies. The property has been depreciating on a straight schedule for two or more prior tax years. The look-back study reclassifies the historic basis, and Form 3115 is the vehicle that puts the missed depreciation on this year's return.

Every look-back study we deliver ships with:

  • DCN 7 identification on the workpaper cover.
  • §481(a) adjustment calculation, per component and totals.
  • Method-change year designation.
  • A statement narrative the CPA can paste into the Form 3115 attachment.
  • A Form 4562 crosswalk so the CPA's filing software receives the correct line-by-line entries.

The CPA files the actual Form 3115 with the client's return. We prepare the workpaper.

Inside a Qualified Opportunity Fund

In an Opportunity Zone, the timing of the Form 3115 changes the exit math

For a property held inside a Qualified Opportunity Fund (QOF), a look-back cost seg + Form 3115 filed inside the 10-year hold window generates a positive §481(a) deduction in the year of the change — which offsets ordinary income for the taxpayer in that year, before the QOF's own gain-exclusion mechanic kicks in at year 10. Because the §1400Z-2(c) FMV basis election at year 10 eliminates the recapture on the accelerated depreciation, the Form 3115 catch-up + accelerated depreciation combination inside a QOF is a permanent benefit, not a timing one.

More on the OZ × cost seg mechanic: Opportunity Zone Cost Segregation and the §754 OZ-fund companion post.

Frequently Asked Questions

Do I have to amend my prior returns to claim the missed depreciation?

No. Form 3115 under Rev. Proc. 2015-13 (DCN 7) is designed exactly for this — the missed depreciation from every prior year is deducted in one line on this year's return, as the §481(a) adjustment. No amended returns are filed.

Is there a limit to how many years back a look-back study can reach?

Not by the DCN 7 procedure itself. If the property was placed in service in 2010 and has been on straight 39-year the whole time, a 2024 look-back study can compute the correct depreciation back to 2010 and put the entire difference on the 2024 return via §481(a). Depreciation method changes are one of the few tax-return items that are not bounded by the ordinary three-year statute of limitations.

Can I use Form 3115 on a property I already sold?

No. DCN 7 requires that the taxpayer still owns the property in the year of change. A cost seg look-back on a sold property is normally handled by an amended return for the year of disposition (subject to the three-year statute of limitations for that year).

What if I'm using bonus depreciation — does the §481(a) capture the missed bonus too?

Yes. The §481(a) adjustment is computed under the correct method from the property's placed-in-service date, which includes any bonus depreciation the property was entitled to but did not receive. For property acquired after 2025-01-19, that means the short-life slice carries 100% bonus under OBBBA per IRS Notice 2026-11; for earlier property, the TCJA phase-down rate for the placed-in-service year applies.

Is the §481(a) deduction spread over multiple years?

No, not for a cost seg look-back under DCN 7. The full positive adjustment is deducted in the year of the change. The four-year spread that applies to certain other §481(a) adjustments — generally in cases where the IRS initiates the change — does not apply here.

Does a Form 3115 filing draw IRS attention?

DCN 7 automatic method changes are routine — the IRS receives thousands per year. A properly prepared filing under Rev. Proc. 2015-13 with the standard DCN 7 attachment and the workpaper support we deliver is not an audit trigger by itself. Every classification in the underlying study is citation-backed to the IRS Cost Segregation Audit Techniques Guide, which is what a reviewing agent expects to see.

Look-back study? We ship the Form 3115 workpaper with it.

Every look-back cost segregation study we deliver carries the DCN 7 identification, the §481(a) calculation, the statement narrative, and a Form 4562 crosswalk — so your CPA files the Form 3115 confidently.

Estimate a look-back studyCPA partner program

This page is educational and reflects current law and IRS guidance as of 2026-09-14. It is not tax advice. Every claim above carries its statute or regulation citation. A completed engineering-based cost segregation study is required for any tax-filing decision, and a licensed tax professional must sign the Form 3115.

Disclaimer: The information provided on this platform is for general informational purposes only and does not constitute tax, financial, legal, or investment advice. Cost segregation studies and depreciation benefits vary based on property type, ownership structure, and applicable federal and state tax law. Results are estimates only. You should consult a qualified tax professional, CPA, or attorney before making any tax-related decisions. ClickDrag Finance does not guarantee specific tax outcomes.