Form 3115 for cost segregation lookback studies.

Why Form 3115 (not amended returns)

For cost segregation studies on prior-year properties, the IRS Cost Segregation ATG (IRS Pub 5653) explicitly directs taxpayers to use Form 3115 rather than amended returns. The reason is procedural — a depreciation method change is treated as an automatic accounting method change under §446, not as an error to correct. Form 3115 captures the entire cumulative shortfall in one §481(a) adjustment in the year of change. Amended returns are limited to three years and require year-by-year refiling, which is more burdensome and produces less benefit.

Filing the form — step by step

Step 1 — Confirm eligibility

Property must have been placed in service in a prior tax year. You can apply a cost segregation study retroactively to any prior tax year that's still within the §6511 refund window or going forward as a method change. Both the §481(a) catch-up adjustment AND the accelerated current-year depreciation are claimed via Form 3115.

Step 2 — Determine the change-number (automatic procedure)

Cost segregation method changes use automatic change-number #7 ('change from a permissible to another permissible method of accounting under §168 for tangible property') per Rev. Proc. 2019-08. Automatic means no IRS pre-approval, no $11,500 user fee, attach Form 3115 with the timely-filed return.

Step 3 — Order the engineered cost segregation study

Methodology must follow IRS Cost Segregation ATG (Pub 5653). The study produces the component-level analysis that supports your reclassification. Cost Seg Smart includes Form 3115 readiness — a one-page filing summary CPA-ready for attachment.

Step 4 — Compute the §481(a) adjustment

The §481(a) adjustment = (depreciation you should have claimed under the new method) minus (depreciation you actually claimed under the old method) for all prior years. The full negative §481(a) adjustment (i.e., catch-up deductions you missed) is deductible in the year of the change — no spreading over 4 years unless a positive §481(a) adjustment arises, which is rare for cost seg.

Step 5 — Complete Form 3115

Required schedules: Part I (general info + change-number 7), Part II (Schedule E for depreciation), Schedule E (the depreciation method change detail), and a §481(a) computation statement. Attach the engineered study report or a summary of its findings.

Step 6 — File two copies

Attach the original Form 3115 to the timely-filed return (or extension). Mail a duplicate to the IRS Ogden, UT service center within the same 30-day window. Per Form 3115 Instructions, the duplicate copy filing is mandatory for automatic procedures.

Step 7 — Report the §481(a) deduction

The §481(a) catch-up amount flows through to your Form 1040 (or 1120, 1065) as 'other deductions' on the relevant business schedule. Current-year accelerated depreciation flows through Form 4562 in the normal place.

Common filing mistakes

• Missing the duplicate-copy filing. Automatic procedures require BOTH attaching Form 3115 to the timely return AND mailing a duplicate to IRS Ogden within 30 days. Per Form 3115 Instructions, missing the duplicate doesn't void the change but does flag the return for examination.
• Wrong change-number. Cost seg uses automatic change #7 (depreciation method under §168). Some CPAs default to #88 or #107 — those are different changes and will trigger an IRS notice.
• Computing §481(a) on a single year. The §481(a) adjustment must include ALL prior years from placed-in-service date forward, not just one. Cost Seg Smart's Form 3115 readiness package includes the full multi-year computation table.
• Forgetting current-year depreciation on top of the catch-up. §481(a) captures prior-year shortfall; current-year accelerated depreciation still goes on Form 4562 in the normal place. Both deductions apply in the year of change.

Example §481(a) computation

Property placed in service 2022 for $1.2M ($960K depreciable basis after 20% land allocation). Original depreciation: straight-line 27.5-yr → $34,909/year = $104,727 cumulative through 2024. New method (post cost seg): 20% reclassified to 5-yr ($192K) → fully bonus-depreciated in 2022 under §168(k); 80% remains 27.5-yr → $27,927/year × 3 years = $83,781 cumulative. New cumulative depreciation: $192,000 + $83,781 = $275,781. §481(a) adjustment = $275,781 − $104,727 = $171,054 deduction in 2025 (year of change). On top: 2025 ongoing depreciation continues at the new $27,927/year rate.