David's situation
David Martinez is 52, a partner at a consulting firm, makes $480K. He bought a 4-bedroom STR in East Nashville in March 2019 for $750,000. He's been renting it on Airbnb since then. Average stay is around 5 nights. He manages the property himself with help from a remote co-host. He satisfies the STR loophole (avg stay <7 days + material participation).
He never did a cost segregation study. His CPA never raised it. He took straight-line depreciation on Schedule E — $24,545/year on the depreciable basis — for six years. He had a friend mention cost seg at a dinner party last month; David asked his CPA, his CPA said "we can do that retroactively, it's called Form 3115 lookback," and that's the conversation that brought David here.
What Form 3115 actually is
IRS Form 3115 is the "Application for Change in Accounting Method." When you switch from straight-line depreciation to the cost-seg accelerated method on an existing asset, you're changing your accounting method for that asset's depreciation. Form 3115 captures the change.
The mechanism: a cost segregation study, dated retroactively to David's original placement-in-service date (March 2019), reclassifies the components that should have depreciated faster. The difference between (a) the depreciation he ACTUALLY took straight-line over 2019-2025 and (b) the depreciation he WOULD HAVE taken under cost seg is the catch-up amount. That amount is called the §481(a) adjustment. It lands on Form 4562 in the current tax year as a single deduction.
This is explicitly permitted under Rev. Proc. 2015-13 and successor procedures. It is an automatic accounting method change — no IRS approval required. No amended returns. No prior-year audit risk for the change itself.
The math, worked
| Line | Amount | Source |
|---|---|---|
| Purchase price | $750,000 | closing |
| Land allocation (24%) | −$180,000 | assessor |
| Depreciable basis | $570,000 | computed |
| 5/15-yr reclass at original date (~26%) | $148,200 | 2019 study |
| Less: depreciation already taken on those components, straight-line 2019-2025 | −$32,604 | SL @ 27.5/15yr |
| Plus: bonus depreciation that would have applied (60% in 2019, 100% in 2020-2022, 80% in 2023, 60% in 2024) | +$67,432 | §168(k) sched |
| §481(a) net catch-up adjustment | $94,228 | computed |
| Year-1 federal tax savings @ 35% | $32,980 | Form 4562 |
Study fee: $1,895 at Cost Seg Smart (lookback studies cost slightly more than fresh-acquisition studies because the engineer must back-cast the depreciation schedule). ROI: 17×. Net benefit: $31,085 federal — landing on David's 2026 return.
Plus all forward-year benefit. From 2026 onward, David depreciates the reclassified components on the accelerated 5/15-year schedule for whatever time remains on those lives. The lookback captures the past years; the cost-seg method continues prospectively.
The bonus depreciation history matters
The §481(a) calculation depends on what bonus depreciation rate would have applied in the placement year. Bonus rates have moved around:
- 2017 (after Sept 27) - 2022: 100% bonus
- 2023: 80% bonus
- 2024: 60% bonus
- 2025+: 100% (permanent under OBBBA)
For property placed in service before 2025, the §481(a) adjustment uses the bonus rate from the original placement year. David's 2019 acquisition gets 60% bonus on the §481(a) reclassified components (the 2019 rate). For property placed in service in 2025 or later, it's 100% under OBBBA.
This is why lookback math depends heavily on placement year. Properties bought 2017-2022 get the most generous catch-up. 2023-2024 acquisitions are slightly less. 2025+ properties don't need a lookback if cost seg is done in year one.
The 10-year wall
Lookback math gets weaker on properties owned 10+ years, for two reasons:
- 5- and 7-year property is fully depreciated. If David had bought the property in 2014, the 5-year personal property would already be fully depreciated under straight-line by 2024. There's nothing left to "accelerate" via cost seg. Only the 15-year site improvements still have remaining life.
- The §481(a) adjustment shrinks proportionally. The longer you've held, the more depreciation you've already taken — leaving less of a gap for the catch-up to capture.
The sweet spot for lookback studies is years 2-7 of ownership. Beyond 7, the math degrades. Beyond 12, it's usually not worth the study fee.
What about §469 — does the suspended-loss problem apply?
Yes. The catch-up deduction creates a loss in the year it lands. That loss is subject to the same passive activity rules as any other rental loss. For David, the STR loophole applies (his property satisfies the 7-day test and he materially participates), so the $94K §481(a) loss is non-passive and offsets his $480K W-2 directly.
If David instead had a long-term rental and didn't qualify under REPS, the §481(a) catch-up would still happen — but it would be a suspended passive loss. Useful eventually (when he sells, or has passive income), but not a current-year refund. See the Long-Term Rental W-2 persona for that scenario.
The decision tree
When the lookback fails
- Owned 12+ years. Most accelerated benefit already claimed via straight-line. §481(a) adjustment too small to justify the study fee.
- Selling this year. Recapture on the same components you just reclassified eats the deduction.
- Can't use the loss. No REPS, no STR loophole, no passive income. The §481(a) loss gets suspended, possibly indefinitely. Still legitimate to do, but the cash benefit is deferred.
- Property under $250K. Even with 6-7 years of catch-up, the absolute dollar yield doesn't clear the study fee economics.
- Already filed for the current year. Form 3115 must accompany the return. If you've already filed, you can amend within the statute, but it adds complexity.
What David should actually do
David's window is exactly right: 6 years of ownership, $750K basis, STR-qualified, 35% bracket. Order the lookback study immediately. Receive it within 2 weeks. Hand it plus Form 3115 to his CPA before the 2026 filing deadline. Capture roughly $33K of federal tax savings on this year's return, plus accelerated depreciation forward into 2027-2034.
If he was undecided, the 30-second decision tool walks the same logic with his actual inputs.
Run a lookback study
Cost Seg Smart handles Form 3115 lookbacks as a standard add-on. We deliver the study; your CPA files Form 3115 with your return. The §481(a) adjustment lands on Form 4562 as a single current-year deduction.
Disclosure. This page is operated by Cost Seg Smart LLC. The "order a study" CTA routes to costsegsmart.com, the same operator. Numbers in the worked example are modeled from Cost Seg Smart's 2026 benchmarks dataset (n=260 studies). Your actual study will differ. Nothing on this page is tax, legal, or financial advice — consult a qualified CPA.