Two different deadlines get tangled in this question, and separating them removes most of the December panic. The tax year of the deduction is fixed by the placed-in-service date. The study itself is documentation, and documentation only needs to exist before the return that relies on it is filed.
Deadline 1: placed in service by December 31
Under IRS Publication 946, depreciation begins when property is placed in service, meaning ready and available for its intended use, not when a study is performed. A rental that is in service on December 28 starts depreciating in that year, period. The same date controls the bonus depreciation rate under IRC section 168(k): under OBBBA, qualified property acquired after January 19, 2025 gets 100% bonus permanently, while property acquired on or before that date keeps the phase-down, 80% for 2023 placements, 60% for 2024, and 40% for 2025.
Deadline 2: the study before the return
The engineering report has to be in hand before the return claiming its allocations is prepared and filed. For a calendar-year individual that means April 15, or October 15 on extension. Filing an extension is the standard play for late-year purchases: it converts a 6-week scramble into a comfortable spring engagement. Note that an extension extends time to file, not time to pay, so estimate the tax with the expected deduction in mind and pay accordingly.
A worked example
An investor closes a $850,000 beach rental on December 19, 2025, having gone under contract in November 2025, and places it in service December 29 with a live listing and a ready certificate of occupancy. Acquired after January 19, 2025, the property's short-life components qualify for 100% bonus under OBBBA. She extends her 2025 return, signs the study engagement in January, receives the report in March, and files in April with the full first-year deduction. Had she instead waited and placed the property in service on January 5, 2026, the entire deduction would shift to tax year 2026, a 12-month delay caused by 7 calendar days.
If the return is already filed
Missing the filing deadline does not forfeit anything. Once depreciation has been reported on at least the prior return using the old method, the fix is a change in accounting method: Form 3115 filed with a current-year return under the automatic change procedures in the Form 3115 instructions, with the entire shortfall captured as a section 481(a) catch-up deduction in the year of change. No amended returns are involved.
Where Amadae fits
Amadae's cost segregation screen can run the December math in minutes, before you close, so you know whether racing to place the property in service is worth it. Amadae does not perform the engineering study; it models the benefit, coordinates timing with the study firm, prepares the Form 3115 when a look-back applies, and files the return on whichever deadline you land.