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A building's cost is recovered slowly. The tax code's recovery table gives residential rental property 27.5 years and nonresidential real property 39 years. But not everything you pay for in a building is really "building." Some of it is equipment, finishes or site work, and the same table gives those much shorter lives: 5, 7 or 15 years. The IRS audit guide (ATG) puts the contrast this way. Section 1250 property is "non-residential real property (with a 39-year recovery period) or residential rental property (with a 27.5-year recovery period) that must use straight-line depreciation," while "Tangible personal property has a shorter recovery period (e.g., 5 or 7 years)." A cost segregation study finds the shorter-lived pieces, supports their cost with evidence, and moves them to the right class. Total depreciation over the life of the property does not change. What changes is when you get it. That is the time value of money. Why is this allowed? The ATG says: "In a landmark decision, the Tax Court ruled that, to the extent tangible personal property is included in an acquisition or in overall costs, it should be treated as such for depreciation purposes," citing Hospital Corporation of America, 109 T.C. 21 (1997). It adds that "The Service acquiesced to the use of ITC rules for distinguishing § 1245 property from § 1250 property." (ITC means investment tax credit.) We describe the case only in the ATG's words here. But it is contested ground. The ATG says "The allocation of building components to § 1245 property is often a contentious issue," and that "the determination of § 1245 property is factually intensive and must be supported by corroborating evidence." Example math, not a typical result. Assume $1,000,000 of depreciable building basis (land already excluded), nonresidential, acquired after January 19, 2025 and placed in service in January 2026. Assume the study supports $200,000 of it as 5-, 7- or 15-year property. That $200,000 is invented for the arithmetic. Without a study, year 1 is straight-line over 39 years with the mid-month convention, which means a January placed-in-service date gets 11.5 months of the first year: $1,000,000 ÷ 39 × 11.5/12 ≈ $24,573. With the study, and 100% bonus claimed (not elected out), year 1 is $200,000 + ($800,000 ÷ 39 × 11.5/12 ≈ $19,658) ≈ $219,658. Over the full life, both columns total $1,000,000. The study moved deductions earlier. It did not create new ones. Whether the earlier deduction is usable this year is a separate question (Section 3), and so is what happens at sale (Section 8).
Bonus depreciation is the first-year allowance in IRC §168(k). The 2025 law changed it. Notice 2026-11 says: "Section 70301 of the OBBBA … made several amendments to § 168(k) to provide taxpayers with a permanent 100 percent additional first year depreciation deduction for qualified property acquired and placed in service, and specified plants planted or grafted, after January 19, 2025." The law is Public Law 119-21 (July 4, 2025), "commonly known as the One, Big, Beautiful Bill Act (OBBBA)." Section 168(k)(1)(A) now reads "an allowance equal to 100 percent of the adjusted basis of the qualified property." Note the words "acquired and placed in service." The acquisition date matters, not just the date the property went into use. Under P.L. 119-21 §70301(c), "property shall not be treated as acquired after the date on which a written binding contract is entered into." A deal signed before January 20, 2025 can fall under the old phase-down even if the property is placed in service later. Notice 2026-11 restates prior law, for example "(i) 40 percent for qualified property placed in service during 2025." There is also a transition election to "deduct 40 percent (60 percent for certain property having longer production periods or certain aircraft), instead of 100 percent… during the first taxable year ending after January 19, 2025." The 2025 Form 4562 instructions say the same. For 2026 property under a pre-2025 contract, check the prior-law phase-down. Which property qualifies? "Tangible property depreciated under MACRS with a recovery period of 20 years or less," according to the 2025 Form 4562 instructions. That is why the study matters. The 27.5- and 39-year building shell gets no bonus. The 5-, 7- and 15-year components the study identifies can. Qualified improvement property (QIP) is also in play. Section 168(e)(6) defines it as "any improvement made by the taxpayer to an interior portion of a building which is nonresidential real property if such improvement is placed in service after the date such building was first placed in service," excluding "(i) the enlargement of the building, (ii) any elevator or escalator, or (iii) the internal structural framework of the building." QIP is listed as 15-year property in §168(e)(3)(E)(vii). Two short notes. Section 179 is a separate expensing rule: "For tax years beginning in 2025, the maximum section 179 expense deduction is $2,500,000," per the Form 4562 instructions. And the law added §168(n), a special allowance for "qualified production property" (the instructions say "placed in service after July 4, 2025"). The IRS issued interim guidance in Notice 2026-16. It is narrow, so just ask whether it applies. Two cautions. First, the ATG's bonus chapter predates the 2025 law (the guide is Publication 5653, dated 2-2025). For bonus depreciation, use Notice 2026-11, IRC §168(k) as amended and the 2025 Form 4562 instructions. Second, state conformity to federal bonus depreciation varies, so check the state.
Before anyone orders a study, sort the situation. The ATG separates new property from acquired property, and in practice there are four cases. New construction. Actual cost records exist, which supports the strongest approach (Section 4). Acquired buildings. There are no construction records, so the purchase price has to be allocated. "Land value is always determined first and is based on 'highest and best use'" (ATG pp. 36–38). The ATG also says acquired property allocations "should be performed by qualified appraisers," and cites AmeriSouth XXXII, Ltd. v. Commissioner, T.C. Memo. 2012-67. Reg. §1.167(a)-5 supplies the rule: "the basis for depreciation cannot exceed an amount which bears the same proportion to the lump sum as the value of the depreciable property at the time of acquisition bears to the value of the entire property at that time." Renovations and improvements. QIP (Section 2), the improvement rules (Section 8), and the chance to write off old components being removed (partial dispositions, Section 8). Look-back (property already in service). Catch-up depreciation through Form 3115, without amending returns, when the eligibility rules are met (Section 4, step 7). Residential rental property has its own rules. It is "a building with 80% or more of the gross rental income from such building is rental income derived from dwelling units." The ATG adds: "Per § 168(c), RRP is recovered over 27.5 years. A rental office/clubhouse building is non-residential real property… recovered over 39 years." The ATG's residential matrix says: "If the taxpayer's tax return position for these assets is consistent with the recommendations in Exhibit A, examiners should not make adjustments to categorization and recovery periods." That makes the matrix a practical review tool. Then comes a timing caveat: when the benefit actually shows up. This is not a passive-loss article, so keep it short. "the term 'passive activity' includes any rental activity" (§469(c)(2)). A real estate professional has to meet a test that includes performing "more than 750 hours of services" (§469(c)(7)(B)). There is a "$25,000 offset for rental real estate activities" for taxpayers who "actively participated" (§469(i)). And an activity is not treated as a rental activity if "The average period of customer use for such property is seven days or less" (Reg. §1.469-1T(e)(3)(ii)(A)), which matters for short-term rentals. Whether a larger deduction reduces this year's tax depends on these rules.
Here is the typical process, in order. Step 1: Engagement and scope. Define the property, the tax years, whether this is new, acquired, renovation or look-back, and who does what. Examiners ask for this first. IDR 1 (information document request 1) in the ATG's sample asks for the engagement letter "that shows the extent of the Cost Segregation engagement," plus a conference with the consultant. Step 2: Documents in. Gather the closing statement and purchase agreement (with the contract date, for the §168(k) acquisition test), land valuation or appraisal support, drawings, construction contracts, pay applications and change orders (the ATG names AIA G-701 to G-704 forms), and prior depreciation schedules. The ATG's rule: "Contemporaneous documentation is the most reliable and trustworthy." Step 3: Site visit. "A field inspection is recommended for all quality studies," with photographs. Step 4: Choose a method. The ATG describes six approaches. The detailed engineering approach from actual cost records is, "In general, … the most methodical and accurate approach." Detailed estimates use cost manuals such as "R. S. Means or Marshall Valuation Service." The residual approach "can also be less accurate," and for sampling, "a population less than 50 could limit the accuracy." A rule-of-thumb method gets a warning: "An examiner should view this approach with caution since it lacks sufficient documentation." Step 5: Cost allocation. The ATG lists the detailed-engineering steps: identify the project, substantiate the total cost, visit the site and photograph it, review drawings and pay requests, assign classes, do quantity take-offs, reconcile, allocate indirect costs, and group the assets. Indirect costs (fees, permits and similar) are allocated, and §263A is considered. Step 6: The report. A quality report contains an executive summary, a narrative, a schedule of assets, direct and indirect costs, property units, engineering procedures, assumptions, a certification and exhibits. It should also explain the background, methodology, asset details, class lives and rationale. Step 7: Implementation on the return. For a current-year study, the classes flow to Form 4562 and the fixed-asset ledger, and you reconcile to the depreciation schedules. A look-back is different. The ATG notes that "the use of a cost segregation study to reclassify property and/or reallocate costs requires the consent of the Commissioner." The automatic route is Rev. Proc. 2025-23 §6.01, "Impermissible to permissible method of accounting for depreciation or amortization." It applies where the taxpayer "used the impermissible method of accounting in at least two taxable years immediately preceding the year of change." Its designated change number is 7: "The designated automatic accounting method change number for a change under this section 6.01 is '7.'" Rev. Proc. 2025-23 "is effective for a Form 3115 filed on or after June 9, 2025, for a year of change ending on or after October 31, 2024." (Rev. Proc. 2026-32 modifies other parts of it.) The catch-up is a §481(a) adjustment. Under Rev. Proc. 2015-13, "the § 481(a) adjustment period is one taxable year (year of change) for a negative § 481(a) adjustment and four taxable years (year of change and next three taxable years) for a positive § 481(a) adjustment." A catch-up of missed depreciation is a negative adjustment, so it is taken in one year. For one-year-only situations, or whether bonus can be picked up on a look-back, ask the specialist and check Rev. Proc. 2025-23.
Start with what does not exist. "The Internal Revenue Service (Service) has not established any requirements or standards for the preparation of cost segregation studies," says the ATG, and "Currently, there is no standard format for either cost segregation or cost segregation reports." The ATG's quality list is the closest thing to a standard. The ATG says a study should "Classify assets…; Explain the rationale (including legal citations)…; Substantiate the cost basis of each asset and reconcile total allocated costs to total actual costs." It then lists "The 13 principal elements of a quality study." Use them as a reviewer's checklist: preparer expertise, methodology, documentation, interviews, nomenclature (the names given to assets), numbering, legal analysis, unit costs and take-offs, asset lists and groups, reconciliation, indirect costs, a §1245 listing, and related aspects. Your own duty matters too. Circular 230 §10.22(a) says: "A practitioner must exercise due diligence." Under §10.22(b), a practitioner is "presumed to have exercised due diligence… if the practitioner relies on the work product of another person and the practitioner used reasonable care in engaging, supervising, training, and evaluating the person." The AICPA's tax standards point the same way. SSTS 2.3.2 allows a member to rely in good faith on information from the taxpayer or third parties without verification, but "a member should not ignore the implications… and should make reasonable inquiries if the information furnished appears to be incorrect, incomplete, or inconsistent." And SSTS 1.4.4 says: "Use of a tool does not absolve the member of professional obligations." That matters when a "study" is really a software output. Seven questions to put to any study: Was there a site visit? Which method was used, and why? Does the asset schedule reconcile to total cost? How was land valued? Do the asset names match what is actually there? Are the placed-in-service dates right? Are any assets already on the books (double counts)? If you want a related look at reviewing work you did not produce yourself, see our article on whether AI can check a tax return.
Build the file to match the ATG's sample information document requests (Chapter 6.G). Here is what each one asks for, and what to keep. IDR 1 asks for the engagement letter "that shows the extent of the Cost Segregation engagement," and a conference with the consultant. Keep the signed engagement letter and scope, the consultant's contact, and a note of who prepared what. IDR 2 asks which properties were visited. Keep a site-visit log: date, who attended, photos. IDR 3 asks for drawings. Keep the architectural, MEP and site drawings used. IDR 4 asks for the complete study and workpapers. Keep the final report plus the workpapers: take-offs, unit-cost sources and allocations. IDR 5 asks for a "machine sensible copy of the data files." Keep native spreadsheets and asset files, not just a PDF. IDR 6 asks about specific assets, such as receptacles. Keep support for the judgment-heavy items: electrical, plumbing and finishes. IDR 7 asks for contracts and change orders. Keep the construction contracts, pay applications, change orders and invoices. Then add the CPA-side items. The purchase agreement and closing statement, with the contract date for the §168(k) acquisition test. Land allocation support (Reg. §1.167(a)-5 is the proportional-value rule, and an appraisal is preferred for acquired property). The reconciliation from the study to the fixed-asset ledger and Form 4562. For look-backs, the Form 3115 as filed and the §481(a) computation. Any bonus elections or election-outs, by class and year. Partial-disposition and improvement records afterward (Section 8). And reviewer notes showing the due diligence described in Section 5. Contemporaneous beats reconstructed. "Contemporaneous documentation is the most reliable and trustworthy," says the ATG, and it flags "requests for contemporaneous records" as an audit-potential situation.
The ATG lists situations with audit potential: mixed asset types, structural components classified as short-lived, "Minimal or no dollar amounts assigned to land," creative nomenclature, and requests for contemporaneous records. Its examples of creative names are "process piping" and "decorative placard," and it points to CSI MasterFormat for standard names. Common findings include "Double deductions for separately acquired assets," "Incorrect placed-in-service dates" and "Large look-back computations." Examiners may bring in specialists. On electrical and similar allocations, the IRS view is that they "should be based on usage or load studies." Marking up an acquired property's tax basis by a construction-cost model is "not an acceptable method," and the ATG lists reasons for error, including incorrect land fair market value. Now the penalties. Section 6662 imposes "an amount equal to 20 percent of the portion of the underpayment." For non-corporate taxpayers, a substantial understatement exists if it "exceeds the greater of— (i) 10 percent of the tax required to be shown on the return for the taxable year, or (ii) $5,000" (§6662(d)). But §6664 says: "No penalty shall be imposed under section 6662 or 6663… if it is shown that there was a reasonable cause for such portion and that the taxpayer acted in good faith." The regulation explains how that is judged. Reg. §1.6664-4 says: "Generally, the most important factor is the extent of the taxpayer's effort to assess the taxpayer's proper tax liability." It also warns: "Reliance on … the advice of a professional tax advisor or an appraiser does not necessarily demonstrate reasonable cause and good faith." And for written advice, Circular 230 §10.37(a)(2)(i) says to "Base the written advice on reasonable factual and legal assumptions."
The study is one moment. The records it creates matter until the property is sold. Improvements. Under Reg. §1.263(a)-3(d), "a taxpayer generally must capitalize the related amounts… paid to improve a unit of property." A unit of property "is improved if the amounts paid for activities performed after the property is placed in service by the taxpayer— (1) Are for a betterment… (2) Restore the unit of property… or (3) Adapt the unit of property to a new or different use." The IRS's guide for this area is Publication 5712 (8-2026). Partial dispositions. "A taxpayer may make an election under this paragraph (d)(2) to apply this section to a disposition of a portion of an asset" (Reg. §1.168(i)-8). It is generally made "by the due date, including extensions, of the original Federal tax return for the taxable year in which the portion of an asset is disp[osed]." In the regulation's Example 7, a retail building owner replaces 60% of the roof, and "D recognizes a loss upon this retirement." The study's component costs are what make this possible later. Rev. Proc. 2025-23 also has a separate automatic change for partial dispositions after an IRS adjustment (§6.10, change number 198). Recapture at sale. For §1245 property, gain up to "the lower of (i) the recomputed basis of the property, or (ii) the amount realized … exceeds the adjusted basis … shall be recognized as ordinary income," and "The amount of such gain shall be determined separately for each item of section 1245 property" (Reg. §1.1245-1). Building depreciation (§1250) feeds "unrecaptured section 1250 gain," taxed at up to "25 percent" (§1(h)(1)(E)). So the study changes the character of part of the gain at exit. Model the exit before you sell the client on year 1. Exit planning is a separate conversation. Who owns what. The owner or client owns the facts and documents: contract dates, the closing file, construction records, use of the property, hours (if §469 matters) and the holding plan. The CPA or tax preparer owns screening (Section 3), engaging and reviewing the specialist (Section 5), elections, Forms 4562 and 3115, the §481(a) adjustment, reconciliation, the defense file and client advice on recapture. The engineer or cost specialist owns the site visit, method, take-offs, cost allocation, the classification rationale, the report and workpapers. For acquired property, the appraiser owns the land versus building value.
Cost segregation does not create new deductions. It moves building cost into shorter-lived classes when the facts support it, so deductions arrive earlier. Since the 2025 law, property acquired and placed in service after January 19, 2025 can get a permanent 100% first-year allowance on qualifying components of 20 years or less. That makes classification worth more, and examiners scrutinize it more. The IRS's audit guide says there is no official standard for studies, but it describes what quality looks like: a site visit, a documented method, reconciliation to actual cost, sound land value, and honest names for assets. For the CPA, the work is to screen whether the client benefits this year, review the specialist with reasonable care, implement it correctly (including Form 3115 for look-backs), and keep a file that answers the examiner's first document requests.
In your own words: (1) Why doesn't a cost segregation study increase total depreciation, and what does it change? (2) A client bought a building in 2026 under a contract signed in December 2024. What question do you ask before assuming 100% bonus? (3) Name four things you'd want in the file before an exam, and what each one proves. Answer key. (1) Total cost recovered is the same. The study moves cost to shorter classes, so deductions come earlier. In the Section 1 example math, both columns total $1,000,000. (2) When was it acquired? A written binding contract fixes the acquisition date (P.L. 119-21 §70301(c)). 100% applies to property "acquired and placed in service… after January 19, 2025" (Notice 2026-11). Otherwise the prior-law percentage applies, for example "40 percent for qualified property placed in service during 2025." For 2026 property under a pre-2025 contract, check the prior-law phase-down. (3) Any four: the engagement letter (scope; IDR 1); the site-visit record (IDR 2); the complete study, workpapers and native files (IDR 4–5); contracts and change orders (IDR 7); land allocation support (Reg. §1.167(a)-5); the reconciliation to the ledger and Form 4562; the Form 3115 and §481(a) computation; reviewer notes (Circular 230 §10.22(b)).
Reading a good explanation feels like understanding it. Usually it isn't the same thing — and you don't find out which one you've got until someone asks you to explain it back.