Written for CPA firms and tax advisors. If you're a property owner, the client-facing version of this conversation is our look-back study guide for rental owners.
The look-back is the highest-value cost segregation conversation most firms can have with a landlord client, and Form 3115 is what makes it work. A client who has been depreciating a rental on the 27.5-year schedule for several years can capture every dollar of missed acceleration in the current year, in a single deduction, without amending a single prior return.
The mechanism is a change in accounting method. You are moving the client from the undifferentiated 27.5-year treatment to the correct component-level method, and the cumulative difference lands as a Section 481(a) adjustment on the year-of-change return. Automatic consent applies, so there is no user fee and no waiting on the IRS to approve anything. What follows is how the filing actually goes together, plus the two places firms get the timing wrong.
Why a method change, not an amended return
Depreciation is a method of accounting. Once a taxpayer has used a given method on two consecutive filed returns, it is an established method, and correcting it is a Section 446 method change on Form 3115, not an amendment. The impermissible-to-permissible depreciation change is an automatic change (designated change number 7), which means no advance consent and no IRS user fee.
Here the method-change route is the taxpayer-favorable one. Instead of reopening prior years, some of which may sit outside the statute of limitations, the client takes the entire catch-up now. Because the client is owed more depreciation, the Section 481(a) adjustment is negative, and a negative adjustment is deductible in full in the year of change. (A positive 481(a) adjustment would spread over four years, but a cost segregation catch-up is almost always negative.)
The two-year trap: amend or 3115
A method is not "established" until it has been used on two filed returns. That single rule decides which correction route applies, and it is the detail firms most often miss when they promise a client a look-back:
- Placed in service this tax year. No Form 3115. Apply the study's schedule on the current return; the depreciation is correct from the start.
- Placed in service last year, one return filed. The method is not yet established. The correction is an amended return for that year, not a method change.
- Placed in service two or more years ago. The method is established. Form 3115 with the Section 481(a) catch-up is the route, and it is the one that produces the large single-year deduction.
Screen for this before you scope the engagement. A very recent acquisition that looks like a look-back candidate may actually be an amendment, which is a different filing and a different client conversation.
What the 481(a) adjustment captures
Take a residential rental placed in service in 2022: a $400,000 purchase, where an engineering-based analysis reclassifies $100,000 of basis into 5-, 7-, and 15-year property. Under the undifferentiated schedule, the client claimed roughly $14,000 of straight-line depreciation on that $100,000 across 2022 through 2025. Under the correct method, that basis was eligible for 100% bonus depreciation in 2022, its placed-in-service year, so the cumulative depreciation that should have been claimed is essentially the full $100,000. The Section 481(a) adjustment is the difference, roughly $86,000, deducted in full on the year-of-change return.
One point drives the whole number: the bonus rate the catch-up captures is the rate in effect for the year the property was placed in service, not the year you file the 3115. A property placed in service in 2022 catches up at 100%; 2023 at 80%; 2024 at 60%; property acquired on or before January 19, 2025 sits on the phase-down, while property acquired after that date is back to 100% permanent under the One Big Beautiful Bill Act. The bonus depreciation rules by acquisition date are worth checking before you size any catch-up.
The filing, step by step
- Confirm the method is established. Two or more filed returns under the old treatment, with the current year as the year of change.
- Get the study first. Form 3115 reports the reclassification; it does not perform it. The 481(a) figure comes from the study's reclassified asset schedule, so the analysis has to be in hand before the form is meaningful.
- Compute the Section 481(a) adjustment. Cumulative depreciation that should have been claimed under the correct method, less what was actually claimed, measured through the beginning of the year of change.
- Complete Form 3115 for change number 7. The 481(a) amount carries to Part IV; the depreciation schedules, the method description, and the property detail go in the attachments.
- File in duplicate. The original attaches to the timely filed return (including extensions) for the year of change, and a signed copy goes to the IRS separately within the window the form instructions set.
- Report the deduction. The negative 481(a) adjustment flows to the return as a current-year deduction, and the reclassified assets carry forward on Form 4562.
The caveat that belongs before the study, not after
The catch-up is only as useful as the client's ability to use the loss. A large 481(a) deduction on a residential rental is a passive loss under Section 469 unless the client clears a bar: real estate professional status, or short-term-rental activity that is non-passive because the average guest stay and participation tests are met. Absent that, the deduction does not disappear; it carries forward as a suspended passive loss, released against passive income or on disposition. That is still real value, but it changes the timing story, so it belongs in the conversation before the study is ordered. See the material participation tests for STR clients for what to verify first.
Recapture is the other back-end item. The reclassified 5- and 7-year assets are Section 1245 property, and the acceleration you create comes back as ordinary recapture on sale. For a client who plans to sell within a few years, size that back-end cost against the front-end timing benefit before recommending the look-back at all.
The practical takeaway
Screen the book for clients holding rentals placed in service two or more years ago at meaningful basis: those are the look-back candidates, and many of them are carrying acceleration they never claimed. The study drives the number, Form 3115 delivers it, and the whole benefit lands in one year with no amended returns.
For CPA firms and advisors: RentalWriteOff prepares the engineering-based analysis and reclassified asset schedule for your independent review. White-label reports can be delivered under your firm's brand with audit support included; your firm remains responsible for Form 3115 preparation, professional advice, and filing decisions. See the partner programs or more resources for tax professionals.