Form 3115 lookback for prior-year MF.

Cost segregation isn't only for new acquisitions. §481(a) lets sponsors catch up missed depreciation on multifamily acquired 2+ years ago — all of it, in a single year, with no amended returns. The mechanic that most partnership CPAs underuse.

Catch-up adjustment without amended returns

§481(a) governs adjustments required when a taxpayer changes accounting methods. The rule: when the new method would have produced different results in prior years, the cumulative difference between methods enters income (or deduction) in the year of change. For depreciation method changes — including cost segregation reclassification — the rule means the cumulative missed accelerated depreciation lands in the current year as a single ordinary deduction.

Critically, the §481(a) mechanism is an automatic procedure for cost segregation changes under Rev. Proc. 2024-23 (formerly Rev. Proc. 2015-13 and predecessors). Automatic means: no IRS user fee, no advance consent, no waiting period. The taxpayer files Form 3115 with the current-year return and the change is effective for that year.

The K-1s issued for the catch-up year reflect each LP's share of the cumulative §481(a) adjustment. LPs do not refile prior-year personal returns. The catch-up sits on the K-1 they receive for the current year, alongside the corrected forward-going depreciation schedule.

DCN 244 vs DCN 7

Form 3115 requires a Designated Change Number (DCN) identifying which automatic procedure the filer is using. For cost segregation, two DCNs come up:

• DCN 244 — Change in method of accounting for depreciation under §168 (cost segregation study results). The cost-segregation-specific procedure. Signals to the IRS examiner what's being changed and incorporates the Audit Techniques Guide framework into the filing automatically.
• DCN 7 — Generic change in depreciation method. Broader catch-all that also accepts cost seg method changes but doesn't signal the specific framework.

DCN 244 is the cleaner choice. It connects the filing to the IRS's own cost segregation audit framework, which means a future examiner is reading the change in the context they're trained to evaluate. Some preparers default to DCN 7 because it's familiar; that works but creates avoidable friction at examination.

How the §481(a) adjustment is computed

The §481(a) adjustment equals depreciation that would have been claimed under the engineered allocation, from the original placed-in-service date through the year before the method change, minus depreciation actually claimed under the original method.

Worked example for a $18M Ventana-class MF acquired 3 years prior, where the original CPA ran straight-line 27.5-year only. Engineered cost seg in Year 4 reclassifies 16% of basis to accelerated buckets ($2.36M to 5/15-year). Under the engineered method with 100% bonus depreciation available at acquisition, Year-1 accelerated depreciation would have been ~$2.4M instead of ~$535K under straight-line. The Year-2 and Year-3 deductions would have been roughly the same under both methods (since accelerated property already fully bonused in Year 1). Net cumulative difference: roughly $1.05M of missed accelerated depreciation, captured as the §481(a) adjustment on the Year-4 partnership return.

The magnitude is sensitive to the bonus depreciation rate available during the missed years. Pre-OBBBA (2025), bonus rates were 100% (2017–2022), 80% (2023), 60% (2024). A property acquired during a partial-bonus year captures less catch-up than one acquired during 100%-bonus years. The engineered study models the year-specific bonus rate.

Three scenarios where the lookback doesn't pencil

• Pre-sale (within 12–18 months): The accelerated depreciation captured via §481(a) creates §1245/§1250 recapture at sale. If the disposition is imminent, the recapture math can neutralize or reverse the catch-up benefit. Model the recapture before filing.
• Pre-dissolution partnership: If the partnership is winding down within 1–2 years and LPs may not have suspended-loss capacity to absorb the catch-up before the partnership dissolves, the §481(a) flows to LPs who can't use it. Suspended losses do release on disposition of the partnership interest — but only for LPs who hold through dissolution.
• Restrictive §704(c) basis caps: If the partnership has built-in losses or §704(c) allocations that limit how depreciation flows to specific partners, the §481(a) catch-up may route in unexpected directions. Less common in single-property syndications; more common in fund-of-funds structures with rolling capital.

For each of these scenarios, the engineered study still happens — the model is built — but the Form 3115 filing pauses until the partnership's circumstances support the catch-up landing usefully.