Short answer
A cost segregation study on a building you constructed and a study on a building you bought reach the same kind of result, a classified asset list reconciled to basis, by different roads. For new construction, the road is the actual cost records: contracts, pay applications, change orders and invoices, which the IRS's Audit Techniques Guide describes as the basis of its most accurate methodology. For an acquisition, there is usually one number, the price, and the components must be identified and priced by measurement and documented estimates, which the guide accepts when actual records are unavailable. The two roads differ in evidence, in how basis is built, in the dates that matter, and in where errors hide. This article walks both. It is general education, not tax advice.
Side by side
| New construction | Acquisition | |
|---|---|---|
| Basis | Land plus all capitalized construction costs: direct costs, and an allocable share of indirect costs under section 263A | Price plus settlement costs that enter basis, allocated between land and building by fair market value |
| Evidence | Schedule of values, pay applications, change orders, subcontractor and supplier invoices, drawings, permits | Closing statement, appraisal, assessor record, drawings if any, site visit, prior schedules |
| Costing method | Detailed engineering approach from actual cost records, with take-offs to split trade costs between classes | Engineering cost estimate: take-offs from drawings or the site, priced with documented unit costs; building as the residual |
| Indirect costs | Explicit: design fees, permits, general conditions, overhead and profit allocated across components | Embedded in the price; the estimate allocates them by a stated method |
| Acquisition date for bonus depreciation | When construction begins (self-constructed property rule) | The closing, or no later than the written binding contract date |
| Placed-in-service date | When the building is ready and available for its use, typically at completion and certificate of occupancy | When ready and available for rent after purchase |
| Where errors hide | Trade invoices that mix building and personal property work; indirect costs left unallocated; phased completion | Unsupported land split; unit costs from the wrong year or region; residual absorbing missing components |
New construction: working from records
The advantage of a construction study is that the money was actually spent on identifiable things. The general contractor's schedule of values and pay applications show cost by trade; change orders and supplier invoices fill in the detail. The practitioner's task is to map that record to tax classes, which still needs take-offs, because a trade is not a class. The electrical subcontractor's line covers the building's wiring, which is part of the structure, and the dedicated circuits serving equipment, which may be section 1245 property; separating them requires quantities from the drawings and unit costs from the records.
Basis for constructed property includes labor, materials, architect's fees and permit charges, and section 263A requires capitalizing direct costs and an allocable share of indirect costs. The study therefore starts from the owner's capitalized total, not the contractor's, allocates indirect costs across components by a stated method, and reconciles to that total. Owners who built often discover the study's total is larger than they expected because soft costs were capitalized that they had mentally expensed.
Dates on a construction project need care. For bonus depreciation, self-constructed property is treated as acquired when construction begins; a project begun before January 20, 2025 and completed after may fall under the earlier phase-down rather than the permanent 100 percent rule, while components acquired later under their own contracts may qualify separately. The placed-in-service date is completion and availability for use, and phased projects have phase-by-phase dates.
Acquisition: working from a price
A purchased building comes with a closing statement and, usually, no construction records. The study builds the components up: land is separated by fair market value at purchase, using the appraisal, the contract and market evidence, or the assessor's ratio when values are uncertain; components are identified from drawings if they exist and from a site visit; quantities are measured; unit costs for the location and the acquisition date are applied from a documented source; indirect costs are allocated by a stated method; and the building is what remains. The guide accepts this engineering cost estimate approach when records are unavailable, provided it is documented.
Because the building is a residual, the acquisition study's reliability depends on the components being complete and correctly priced, and the reconciliation is where a bad estimate shows: components priced at today's replacement cost on a building bought years ago can exceed the price, which is the study telling you it is wrong.
For bonus depreciation, the acquisition date is the closing or, for a written binding contract, no later than the contract date. A purchase closed in 2025 on a 2024 contract is the classic trap.
Renovation after acquisition: both roads at once
Many owners buy and then renovate. The purchase is an acquisition study; the renovation is a construction study with its own records, its own dates and its own bonus depreciation analysis. The two are reconciled separately, because they have separate bases, and the renovation may dispose of components the acquisition study identified, such as a roof or a kitchen. A practitioner who folds the renovation into the acquisition produces one wrong set of dates for at least half the assets.
What each study should show you
- Construction: the tie from the owner's capitalized total to the contractor's records, the take-offs that split trades between classes, the indirect cost allocation, and any estimated items where records were missing despite the project being new.
- Acquisition: the land split and its evidence, the site record, the take-offs, the unit-cost source and date, the residual building cost and a plausibility check on it, and the acquisition date evidence including any contract.
Common errors, by road
- Construction: reconciling to the contractor's total instead of the capitalized basis; treating a trade as a class; omitting owner-paid soft costs; using the start of construction as the placed-in-service date.
- Acquisition: land at a remembered percentage; unit costs from a cost service edition years off; a residual that hides missing components; the contract date ignored for bonus depreciation.
What people ask on Reddit and other forums
Builders and buyers ask different questions in the "cost segregation reddit" threads on r/realestateinvesting and r/Construction. The searches that lead people to these threads are usually phrased "cost segregation new construction", "cost segregation on a purchased building" and "construction cost records depreciation".
"I built it. Do I even need a study, since I have every invoice?" You have the evidence; the study is the classification and the reconciliation. Invoices by trade are not asset classes, and the indirect costs still have to be allocated.
"I bought it. The seller has no records. Is a study still possible?" Yes. The guide accepts an engineering cost estimate from take-offs and documented unit costs when records are unavailable, with the building as a disclosed residual.
"We broke ground in late 2024 and finished in 2025. 100 percent bonus?" Self-constructed property is treated as acquired when construction begins, so the earlier phase-down may apply to that property. Components acquired later under separate contracts are judged on their own. Ask your adviser.
"Which is more accurate?" Records beat estimates, which is why the guide ranks the detailed engineering approach from actual cost records first. A well-documented estimate is still acceptable.
Questions people also ask
Is cost segregation different for new construction?
The result is the same kind of asset list, but the evidence is actual cost records rather than estimates, the basis includes capitalized indirect costs, and the acquisition date for bonus depreciation is when construction began.
Can a study be done on a purchased building with no records?
Yes, by an engineering cost estimate: take-offs, documented unit costs for the location and date, indirect cost allocation and the building as the residual, all disclosed.
Does the placed-in-service date differ?
For construction it is completion and availability for use; for a purchase it is availability for rent after closing. Renovations have their own dates in both cases.
What is the biggest error on each side?
Construction: reconciling to the wrong total. Acquisition: an unsupported land split.
Should I get the study before or after construction ends?
The study is prepared from the final records after completion, but engaging the practitioner during construction lets them ask for the records in the form they need.
Sources
- https://www.irs.gov/pub/irs-pdf/p5653.pdf — IRS Publication 5653, Cost Segregation Audit Techniques Guide (Rev. 2-2025): Chapter 3, methodologies; Chapter 4, principal elements.
- https://www.irs.gov/publications/p551 — IRS Publication 551 (Rev. December 2024), Basis of Assets: cost basis; constructing assets; allocating the basis.
- https://www.law.cornell.edu/uscode/text/26/263A — 26 U.S.C. §263A.
- https://www.irs.gov/publications/p946 — IRS Publication 946 (2025), How To Depreciate Property: Chapter 1, placed in service.
- https://www.law.cornell.edu/uscode/text/26/168 — 26 U.S.C. §168(k)(2)(E), acquisition date rules for self-constructed property and binding contracts.
- https://www.irs.gov/newsroom/treasury-irs-issue-guidance-on-the-additional-first-year-depreciation-deduction-amended-as-part-of-the-one-big-beautiful-bill — IR-2026-06 (January 14, 2026), bonus depreciation guidance.