How to File Form 3115 Step by Step (DCN 7 Look-Back)
File Form 3115 in duplicate under Designated Change Number 7 per Rev. Proc. 2022-14. Attach the original to the timely-filed federal return for the year of change. Mail a signed duplicate to the IRS Ogden, Utah service center under Rev. Proc. 2015-13. The Section 481(a) catch-up is deducted entirely in the year of change. No amended returns required.
Step 1: Confirm DCN 7 applies
Designated Change Number 7 under Rev. Proc. 2022-14 covers a change from impermissible to permissible depreciation method or recovery period. A look-back cost segregation study fits this category. The taxpayer must have used the impermissible method (typically straight-line 27.5 or 39 year on basis that should have been reclassified) for at least two consecutive tax years before the year of change.
Cost segregation work happens before Form 3115 preparation. The engineer signs the study identifying the components that should have been on 5-year, 7-year, or 15-year schedules. The Form 3115 captures the cumulative effect of correcting the prior method.
Step 2: Compute the Section 481(a) adjustment
The Section 481(a) adjustment is the cumulative difference between depreciation under the new method and depreciation under the old method, 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, the difference is the adjustment.
For a $1M residential rental placed in service in 2021 with no cost seg, accumulated straight-line through year-end 2025 is roughly $138K. After a 2026 look-back study reclassifying $200K to 5-year and $64K to 15-year, the cumulative depreciation under the corrected method would have been roughly $310K. The Section 481(a) catch-up is $172K, deducted entirely on the 2026 return.
Step 3: Complete Form 3115
Form 3115 has four pages plus attachments. The key fields: filer name and TIN, DCN 7 in Part I, signed declaration in Part II, automatic-consent procedure cited (Rev. Proc. 2022-14), and the Section 481(a) computation as an attachment.
The methodology disclosure attachment cites IRS Publication 5653 and the detailed engineering approach. The supporting cost segregation report is attached as an exhibit or referenced as available on request. WeCostSeg prepares both copies (the original and the Ogden duplicate) when bundled with a look-back study at no extra cost.
Step 4: File the original with the current return
Attach the signed Form 3115 to the timely-filed (with extensions) federal return for the year of change. The form must accompany the return. Filing after the extended due date pushes the change to the next year.
The deduction posts on the current return. Passive activity loss limitations under IRC Section 469 apply to the catch-up if the taxpayer is not REPS-qualified and does not qualify for the STR loophole. The loss suspends if unused.
Step 5: Mail the signed duplicate to Ogden
Under Rev. Proc. 2015-13, a signed duplicate of Form 3115 must be mailed to the IRS service center in Ogden, Utah. The current mailing address is published in the most recent revision of Form 3115 instructions. Verify against the latest IRS form before mailing.
Without the duplicate filing, the IRS treats the change as not validly requested. WeCostSeg ships both copies in the look-back study deliverable, with a one-page filing instruction sheet for the CPA.
Frequently asked questions
- How does WeCostSeg coordinate with my CPA?
- Every engagement follows the three-touch CPA Coordination Protocol. We send a preliminary analysis to your CPA on intake, share the draft report five business days before final delivery, and coordinate Form 3115 filing timing when a Section 481(a) adjustment applies. Your CPA never pays a fee.
- Does this analysis assume 100% bonus depreciation under OBBBA?
- Yes for property acquired with a binding contract on or after January 20, 2025 under Public Law 119-21. Property under a binding contract on or before January 19, 2025 stays on the legacy phase-down: 40% bonus in 2025, 20% in 2026, 0% in 2027 and after.
- Is the five-year audit defense included?
- Yes. Every WeCostSeg engagement includes five years of written audit defense at no extra cost. The defense aligns to the 13 Principal Elements of a Quality Cost Segregation Study under IRS Publication 5653 Chapter 4.
- Can I get a free preliminary analysis?
- Yes. Submit your property details via the free proposal form or WhatsApp. Our engineer returns a written estimate of your first-year deduction within four business hours during US Eastern hours. No payment, no contract.
- What if I miss the duplicate filing?
- The IRS treats the method change as invalid. Re-file with both copies in the next eligible year. Better to confirm both filings are mailed before the original return is submitted.
- Can my CPA file Form 3115 without my engineer?
- Yes, but the supporting cost segregation report is the substance behind the method change. The form references it. WeCostSeg includes both Form 3115 preparation and the supporting study report in the look-back engagement.
Zawwad Ul Sami, Founder
Zawwad Ul Sami is the founder of WeCostSeg, a founder-led cost segregation firm serving real estate investors across the US. He focuses on strategy, pricing, and the firm's overall direction.