What Is Cost Segregation? A Plain-English Guide
Cost segregation is an IRS-approved tax strategy that reclassifies parts of your building from slow depreciation (27.5 or 39 years) to fast depreciation (5, 7, or 15 years). With 100% bonus depreciation now permanently available, most of the reclassified amount is deductible in Year 1. For a typical rental property, this can mean $20,000 to $100,000 or more in immediate tax savings.
I have reviewed dozens of these studies and talked to CPAs who file hundreds of them. Here is how it actually works, stripped of the sales language that most providers use to make it sound more complicated than it is.
The Basic Concept
When you buy a rental or commercial property, the IRS lets you depreciate the building (not the land) over its "useful life." For residential rental property, that is 27.5 years. For commercial property, 39 years. Without cost segregation, your entire building depreciates at that single slow rate.
But your building is not one asset. It is hundreds of individual components -- appliances, carpeting, cabinetry, landscaping, paving, plumbing fixtures, electrical wiring, and much more. Many of these components wear out far faster than the building structure itself. The IRS recognizes this and allows you to depreciate them on shorter schedules.
A cost segregation study is the engineering analysis that identifies these components, assigns a cost to each one using standardized construction cost data, and reclassifies them into the correct MACRS depreciation class. The result: a significant portion of your building's cost basis moves from 27.5 or 39 years to 5, 7, or 15 years.
What Gets Reclassified
Here is what a typical cost segregation study identifies, organized by MACRS class:
| MACRS Class | Recovery Period | Common Components |
|---|---|---|
| 5-Year Property | 5 years | Appliances, carpeting, cabinetry, decorative fixtures, window treatments, specialty lighting, security systems |
| 7-Year Property | 7 years | Furniture, office equipment, certain specialty fixtures, some specialized electrical |
| 15-Year Property | 15 years | Landscaping, paving, parking lots, fencing, sidewalks, site drainage, retaining walls, exterior signage |
| 27.5 or 39-Year | 27.5 / 39 years | Structural frame, roof structure, foundation, load-bearing walls (stays the same -- not reclassified) |
For a typical residential rental property, 20% to 30% of the building's cost basis gets reclassified to shorter-lived classes. For furnished short-term rentals, that figure can reach 30% to 35% because of all the personal property (furniture, electronics, linens, decor). For commercial properties with significant site work, the 15-year category alone can represent 10% to 18% of the total basis.
How the Study Works
A cost segregation study is fundamentally an engineering exercise, not an accounting exercise. The firm performing the study does the following:
1. Identifies the building's components. Using construction drawings, county assessor data, satellite imagery, and (sometimes) a physical inspection, the firm catalogs every component of the building and its site improvements.
2. Assigns costs using standardized data. Each component is priced using recognized construction cost databases, primarily RSMeans. This is the same data that contractors, insurers, and appraisers use. The costs are adjusted for geographic location, building age, and quality.
3. Classifies each component under MACRS. Every component is assigned to its correct depreciation class (5, 7, 15, 27.5, or 39 years) based on IRS rules and the guidance in the IRS Cost Segregation Audit Techniques Guide.
4. Produces a depreciation schedule. The final deliverable is a detailed report showing every reclassified component, its cost, and its MACRS class. This report is what your CPA uses to claim the accelerated depreciation on your tax return.
Who Does the Study
Cost segregation studies are performed by specialized firms, not by your CPA. The firms typically employ engineers, construction professionals, or tax specialists who understand both building systems and IRS classification rules. In my review of 26 providers, I found four tiers of firms ranging from large national engineering practices to lean online platforms.
Your CPA's role is to take the completed study and apply it to your tax return. Some CPAs will recommend a specific cost segregation firm, but be aware that they often add their own markup. Always get a direct quote from the provider as well.
How Your CPA Uses It
Once you have the completed cost segregation study, your CPA applies it in one of two ways:
Current-year properties (Form 4562). If you purchased the property in the current tax year, the reclassified depreciation is simply reported on Form 4562 (Depreciation and Amortization). This is the straightforward case.
Prior-year properties (Form 3115). If you have owned the property for one or more years and never did a cost segregation study, your CPA files Form 3115 (Application for Change in Accounting Method) to claim the "catch-up" depreciation. This is called a lookback study, and it lets you take all the missed accelerated depreciation in a single year without amending prior returns. This is one of the most powerful aspects of cost segregation -- you do not have to do it in the year of purchase.
Bonus Depreciation: The Multiplier
Bonus depreciation is what makes cost segregation so powerful right now. Under the One Big Beautiful Bill Act (signed July 2025), 100% bonus depreciation is permanently available for 2025 and all future years. This means that every dollar reclassified to 5-year, 7-year, or 15-year property can be deducted in full in Year 1.
Without bonus depreciation, a $50,000 component classified as 5-year property would be depreciated over five years (roughly $10,000 per year). With 100% bonus depreciation, the entire $50,000 is deductible immediately. This is the difference between a useful tax strategy and a transformative one.
Is It an Aggressive Tax Strategy?
No. This is one of the most common misconceptions I encountered in my research, and it is important to address directly.
Cost segregation is explicitly supported by the IRS. The IRS published a comprehensive Cost Segregation Audit Techniques Guide -- a 200+ page document that provides detailed instructions for how these studies should be conducted and how IRS agents should review them. The IRS does not publish audit guides for strategies it considers illegitimate.
Cost segregation has been upheld repeatedly in Tax Court, including the landmark Hospital Corporation of America v. Commissioner case. The legal foundation is solid and decades old.
What the IRS does scrutinize is the quality of the study itself. A properly documented engineering-based study with component-level detail is defensible. A template-based study that applies generic percentages without property-specific analysis may not be. This is why the choice of provider matters -- not because the strategy is risky, but because the documentation needs to be thorough.
See our provider rankings to find firms with strong IRS compliance track records.