You bought a rental, filed your taxes, and only later found out about cost segregation and 100% bonus depreciation. Now you're staring at a return that took plain 27.5-year depreciation and wondering if that door is closed. It isn't. You can almost always go back and claim the accelerated depreciation you missed, and in many cases you don't even have to touch the old return to do it.
There are two routes, and picking the right one comes down to two questions: when did you acquire the property (which sets your bonus rate), and how many returns have you already filed with the slower depreciation (which decides whether you amend or file Form 3115). Get those two facts straight and the rest is arithmetic. Here's how the January 19, 2025 acquisition line works, what the catch-up is actually worth, and which path fits your situation.
First, the acquisition date sets your bonus rate
The One Big Beautiful Bill Act, signed July 4, 2025, made 100% bonus depreciation permanent, but only for qualified property acquired after January 19, 2025. That date is the hinge everything turns on. Property you acquired on or before it stays on the old phase-down schedule that Congress set years ago.
Because cost segregation reclassifies part of your building into 5-, 7-, and 15-year property, and bonus depreciation applies to property with a recovery period of 20 years or less, the bonus rate you can claim on that reclassified slice depends on the year the property was placed in service:
| Placed in service | Bonus depreciation rate |
|---|---|
| 2018 through 2022 | 100% |
| 2023 | 80% |
| 2024 | 60% |
| 2025, acquired on or before Jan 19 | 40% |
| 2025 or later, acquired after Jan 19, 2025 | 100% (permanent) |
The key thing most people get wrong: a look-back catch-up uses the bonus rate from the year the property was placed in service, not the year you file the paperwork. A rental you put in service in 2021 and never studied is still sitting on a 100% bonus opportunity today. One you placed in service in 2024 carries the 60% rate. Filing this year doesn't reset that rate up or down, so the "I missed it" panic is usually about timing, not eligibility.
The two ways to claim depreciation you already missed
Once you know your rate, the second question is procedural: do you amend the return you already filed, or file Form 3115 to change your accounting method going forward? The answer hinges on how long you've been using the wrong (slower) depreciation.
Amend the return: for a property you just placed in service
If you placed the property in service in the most recent tax year and have filed only that one return with straight-line depreciation, you generally correct it by amending that return. The IRS treats a depreciation method as "adopted" only after you've used it on two or more consecutively filed returns. Before that point, there's no accounting method to change, so an amended return (or a superseding return, if you're still inside the filing extension) is the clean route. You have three years from the date you filed the original return to amend it.
This is the common situation for a 2025 purchase: you filed your 2025 return without a study, realized the opportunity, and want to add the accelerated first-year deduction. Amending puts the bigger refund in the year the deduction belongs.
File Form 3115: for a property you've depreciated slowly for two or more years
If you've owned the rental for two or more years and taken the slower 27.5-year depreciation the whole time, that slow method has become your accounting method, and you generally cannot fix it by amending. Instead you file Form 3115, a change in accounting method, with your current-year return. This is the look-back study most owners end up using, and it has real advantages:
- You claim the entire catch-up as a single deduction on this year's return, through what's called a Section 481(a) adjustment. It equals all the depreciation you should have taken since the property went into service, minus what you actually took.
- You do not amend any prior returns, so it isn't capped by the three-year amendment window. A property placed in service in 2019 can still be caught up in full.
- It's an automatic change (no user fee) when done correctly, and the catch-up hits the current year, where you can plan around it.
Amend or Form 3115: how to choose
| Amended return | Form 3115 | |
|---|---|---|
| Best when | Only one return filed (just placed in service) | Slow depreciation used for 2+ years |
| Which year gets the deduction | The original year you amend | The current year (as a 481(a) catch-up) |
| Time limit | 3 years from filing the original return | No look-back window; any age property |
| Touches old returns | Yes | No |
There's one more angle worth weighing when both paths are technically open (for example, a property placed in service last year where you could still amend, but a method arguably hasn't locked in): which year do you want the deduction to land in? Amending a high-income prior year, or a year you qualified as a real estate professional or ran the property as a short-term rental with material participation, can pull the deduction against income taxed at a higher rate and generate a larger refund. If your income is steadier, taking the catch-up in the current year through Form 3115 is usually simpler and gets you to the same place. This is a conversation for your CPA, and it's part of the is-it-worth-it math before you order anything.
A worked example: catching up on a $400,000 rental
Take the single-family rental from our detailed case study: a $400,000 purchase, roughly $320,000 of building basis after the land carve-out. Say you placed it in service in 2022, took only standard 27.5-year depreciation, and never ran a study. A cost segregation analysis reclassifies about $46,000 of that basis into 5/7/15-year property.
Because the property was placed in service in 2022, that reclassified slice qualifies for 100% bonus at the 2022 rate. You already took a few years of straight-line depreciation on it (a small amount, since it was spread over 27.5 years), so the Section 481(a) catch-up is the 100% bonus amount minus what you already deducted, landing somewhere around $42,000 to $44,000 of net additional deduction on this year's return. At a 32% marginal rate, that's roughly $13,000 to $14,000 of tax pulled into the current year, with no amended return required.
Run the same property with a 2024 placed-in-service date and the reclassified slice carries the 60% bonus rate instead, so the first-year piece is smaller and the rest depreciates on its normal 5/7/15-year schedule. Same property, different year, materially different catch-up. That's why the acquisition and placed-in-service dates are the first thing to pin down.
Two things people get wrong
The acquisition date, not the closing you remember, controls the rate. For the January 19, 2025 line, "acquired" generally means when you entered a written binding contract to buy (or, for property you built, when construction began), not simply when you took title. A contract signed in December 2024 that closed in March 2025 is acquired before the line, so it's on the 40% phase-down for 2025, not the 100% permanent rate. If you're near the date, your CPA will want to see the contract.
A projection is not a filing. A free calculator or an instant estimate is a great way to size whether a catch-up is worth pursuing, but the number it produces is a projection. It cannot go on a return by itself. To actually claim the deduction, whether by amended return or Form 3115, your CPA needs the full report that breaks the building into its 5-, 7-, and 15-year components with an engineering-based analysis behind each number. That report is what supports Form 4562 and the Form 3115 Section 481(a) calculation if the IRS ever asks.
Frequently asked questions
Can I amend my tax return to claim bonus depreciation I missed?
Yes, if you've only filed one return with the property in service. An impermissible depreciation method is treated as adopted only after two or more consecutive returns, so a rental you just placed in service can be corrected by amending, within three years of filing the original return. If you've used the slower depreciation for two or more years, you generally can't amend; you file Form 3115 instead and take the catch-up on your current return.
Do I have to amend old returns to do a look-back cost segregation study?
Usually not. For a property you've owned and depreciated for two or more years, Form 3115 lets you claim all the missed depreciation as a single Section 481(a) adjustment on your current-year return, with no amended returns and no three-year time limit. That's why look-back studies are practical even on properties placed in service many years ago.
Does my property still qualify for 100% bonus depreciation in 2026?
It depends on when you acquired it. Property acquired after January 19, 2025 qualifies for the permanent 100% bonus under the One Big Beautiful Bill Act. Property acquired on or before that date follows the older phase-down: 40% for 2025 and 20% for 2026. For a look-back on an older property, the rate is set by the year it was placed in service, so a rental first rented out between 2018 and 2022 can still reach the full 100% on its reclassified components.
The bottom line
Missing cost segregation on a rental you already filed is a fixable mistake, not a lost one. Pin down two dates first: when you acquired the property, which sets your bonus rate, and how long you've been depreciating it, which decides between an amended return and Form 3115. Newer properties with one return filed get amended; anything you've held for two or more years gets caught up on the current return through Form 3115, with no old returns reopened and no look-back window to beat.
Want to know what the catch-up is worth on your property before you decide? RentalWriteOff runs a full engineering-based analysis for a flat $799, delivered in 2 business days, fully remote, with no site visit and audit support included. Start with a free instant estimate to size the missed deduction, then get your study when the math makes sense. The estimate is a projection to gauge the opportunity; the full report is what your CPA uses to claim it on Form 4562 or a Form 3115 catch-up.