Cost segregation basics

What is cost segregation in plain English?

Cost segregation is a tax-accounting method that breaks a building's purchase price into shorter-lived components — carpet, appliances, fences, landscaping, decorative lighting — instead of depreciating the whole thing over 27.5 years (residential) or 39 years (commercial). Components reclassified into 5-year, 7-year, or 15-year MACRS classes are deducted faster, which usually pulls a large deduction into the year of purchase when paired with bonus depreciation under IRC §168(k). The IRS has accepted this approach since the Hospital Corporation of America v. Commissioner decision (109 T.C. 21, 1997).

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Is cost segregation legitimate?

Yes. Cost segregation is an IRS-recognized method for classifying building components into their proper MACRS recovery periods. The IRS publishes a Cost Segregation Audit Techniques Guide (last revised June 2022) that walks examiners through how a study should be performed and documented. The legal basis is HCA v. Commissioner, which held that components meeting the Whiteco factors are §1245 personal property, not §1250 real property. Legitimacy depends on documentation quality — a properly engineered study following the ATG's "Principal Elements of a Quality Cost Segregation Study" is durable; a back-of-envelope reclassification is not.

Is cost segregation worth it for a small rental?

For a single-family rental under roughly $300,000 of depreciable basis, the math often does not work: a typical engineering study costs $3,000–$8,000, and IRS ATG Chapter 7.2 characterizes 5-year personal property in single-family and small multifamily as roughly 5–15% of basis — not the 20–30% engineering-firm marketing pages quote (that figure combines 5-year and 15-year and is selection-biased). Owners commonly evaluate further with a CPA when estimated Year-1 federal tax savings exceed the study's cost by a comfortable margin and material participation rules let the loss actually offset other income.

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How much does a cost segregation study cost?

Engineering-based studies typically run $3,000–$8,000 for a single-family or small multifamily rental, $5,000–$15,000 for mid-size commercial, and $15,000+ for large or unusual properties. Pricing reflects the site visit, engineering analysis, asset photography, Whiteco-factor documentation, and a written report sized for IRS examination. "DIY" reclassifications without engineering support are not recognized as a quality study by the IRS ATG and carry materially more audit risk. The IRS ATG Chapter 4 lists 13 elements an examiner expects to see in a defensible study.

Do I need an engineering firm or can I do it myself?

The IRS ATG Chapter 4 identifies six methodologies and describes the "Detailed Engineering Approach from Actual Cost Records" and "Detailed Engineering Cost Estimate Approach" as the most accurate and best-supported. The ATG explicitly criticizes "rule-of-thumb" approaches that lack engineering support. A property owner is legally permitted to prepare their own classification, but on examination the burden of proof is on the taxpayer and a study that does not contain the 13 ATG-required elements (property descriptions, photos, Whiteco analysis, statement of qualifications, etc.) is unlikely to survive scrutiny.

What's the difference between cost segregation and bonus depreciation?

They stack — they are not alternatives. Cost segregation reclassifies parts of a building from 27.5-year or 39-year MACRS into 5-, 7-, or 15-year classes. Bonus depreciation under IRC §168(k) then lets you deduct a percentage of those shorter-lived assets in Year 1. Without cost segregation, bonus depreciation has very little to bite on (a building shell isn't bonus-eligible). Without bonus, cost segregation still helps via accelerated MACRS, just less dramatically. After OBBBA (Public Law 119-21, signed July 4, 2025), bonus is back to 100% permanently for property acquired after January 19, 2025.

Can I do cost segregation on a property I bought years ago?

Yes — this is called a "look-back" study. Under Rev. Proc. 2024-23, you can change your accounting method to cost segregation by filing Form 3115 with DCN 7, claiming the entire missed depreciation as a §481(a) catch-up deduction in the year of change. There is no need to amend prior returns. The look-back captures depreciation you should have taken plus the bonus depreciation rate that was in effect in the original placed-in-service year (e.g., 100% for 2017–2022 acquisitions, 80% for 2023, 60% for 2024).

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Will cost segregation trigger an IRS audit?

There is no public data showing cost segregation as a standalone audit trigger. The IRS ATG itself states that cost segregation is an accepted method when properly documented. What examiners scrutinize, when an audit happens for other reasons, is the quality of the study: presence of the 13 ATG elements, Whiteco-factor analysis for each reclassified asset, qualifications of the preparer, and consistency with the placed-in-service date. A study that lacks engineering support, photographs, or a Whiteco analysis is what creates exposure — not the act of doing cost segregation.

What does the IRS Cost Segregation Audit Techniques Guide actually say?

The ATG is a 200+ page internal training document (last revised June 2022) used by IRS examiners. It states that cost segregation is an accepted method, defines what makes a "quality" study (Chapter 4: 13 principal elements plus 6 acceptable methodologies), walks through asset-class case law including HCA, Whiteco, and Scott Paper, and gives industry-specific guidance in Chapter 7 (residential rental, restaurants, retail, hotels, etc.). It is not law — it is examiner guidance — but courts and the IRS treat its 13-element framework as the de facto quality bar.