How to Do a Cost Segregation Study: Steps, Filing, and Limits

To do a cost segregation study, you pull together the construction and acquisition records for a building, have an engineer inspect the property and identify components that qualify for shorter depreciation lives, allocate both direct and indirect costs to each component, and then report the results on your tax return, either directly in the first year the property is placed in service or through Form 3115 if you already own the building and want to claim missed depreciation as a catch-up. The point of the exercise is to move portions of a 27.5-year or 39-year write-off into 5, 7, and 15-year categories that qualify for 100 percent bonus depreciation on property acquired after January 19, 2025.¹

Step 1: Decide Whether the Property Justifies a Study

A professional study typically costs between $5,000 and $15,000, depending on building size, complexity, and the number of tenants. The math generally starts to work at a depreciable basis of roughly $500,000 or more. Below that, the accelerated deductions rarely cover the fee with enough left over to matter.

Timing changes the value too. A study delivers the most when it is done in the year the property is placed in service, because you capture the full benefit of bonus depreciation and the shorter recovery periods from day one. If you already own the building and never had a study done, you can still perform one retroactively and claim the cumulative missed depreciation in a single year through an accounting method change. Properties in the middle of major renovations or tenant improvements are also strong candidates, because the new construction costs can be broken out and reclassified immediately.

Two situations weaken the case. If you plan to sell within a year or two, accelerated depreciation lowers your basis and drives up the tax at sale. And if passive activity rules prevent you from using the deductions this year, the benefit is deferred rather than lost, but the study fee is not.

Step 2: Pull the Documentation

Before anyone visits the property, the study team needs a paper trail of what was built and what it cost. That typically means:

Architectural and engineering drawings, including blueprints, floor plans, and specifications showing the layout and materials.
Construction invoices and pay applications with contractor billing broken down by material, labor, and equipment for each phase.
Closing statements and appraisals. For acquired properties, the purchase price and its allocation between land, building, and other assets establishes the total depreciable basis.
Change orders documenting any modifications to the original scope, materials, or cost during construction.

For a building placed in service in the current year, this documentation flows directly into the depreciation schedules you file with the return. For a building already in service where you have been depreciating the whole structure over 27.5 or 39 years, the same records feed the catch-up calculation on Form 3115.²

Retention is not the usual three years. The IRS requires permanent records for depreciable property, covering the date and manner of acquisition, the cost basis, and all depreciation claimed or allowable.³ Keep the file for as long as you own the property and through the statute of limitations period after any sale.

Step 3: Inspect the Property in Person

A physical site visit is what separates a defensible study from a desk exercise. An engineer or qualified professional walks the property to verify that components identified in the construction documents actually exist, are installed, and are operational. The IRS Cost Segregation Audit Technique Guide looks for exactly this kind of physical inspection, and a desktop-only analysis is one of the easier things for an examiner to challenge.

During the walkthrough, the inspection team photographs and measures components that could qualify for shorter recovery periods. The focus is on functional distinctions: which electrical circuits serve dedicated equipment versus general building lighting, which plumbing is structural versus installed for a specific business use, which finishes are decorative rather than integral to the building envelope. The team also measures land improvements like parking areas, curbing, fencing, and signage.

Field notes tie each physical item back to the corresponding line in the construction records or blueprints. A dedicated HVAC unit for a commercial kitchen, for example, gets documented with its location, capacity, and the specific equipment it serves. Notes should also capture items that never appeared in the original blueprints, such as tenant improvements added after initial construction.

Step 4: Reclassify Components Into the Right Recovery Periods

A building is not one asset. It is hundreds of individual components, each assigned to a depreciation category based on its function and permanence. The structural shell, including load-bearing walls, the roof deck, and basic plumbing and electrical systems, depreciates over 27.5 years for residential rental property or 39 years for nonresidential real property under MACRS.⁴

Everything that is not part of that shell falls into shorter periods:

5-year property: carpeting, appliances, certain decorative finishes, and specialized electrical or plumbing systems that serve specific equipment rather than the building as a whole.⁴
7-year property: office furniture and fixtures like desks, file cabinets, and safes.⁵
15-year property: land improvements such as parking lots, sidewalks, fences, roads, and landscaping with a determinable useful life.⁴

The legal distinction rests on whether a component is Section 1245 property, meaning tangible personal property or certain non-structural assets eligible for faster recovery, or Section 1250 property, meaning the building and its structural components.⁶ A dedicated HVAC unit cooling a server room serves specific equipment rather than the building’s general climate, so it qualifies as personal property with a shorter life. The same building’s central air system is a structural component.

Qualified improvement property, meaning any improvement to the interior of an existing nonresidential building, also gets a 15-year recovery period and qualifies for bonus depreciation. This covers common tenant build-outs like interior walls, ceilings, lighting, and fire suppression systems installed after the building was first placed in service.⁷

Property with a recovery period of 20 years or less is eligible for 100 percent bonus depreciation when acquired after January 19, 2025, so anything reclassified into a 5, 7, or 15-year category can generally be written off in full in the year the property is placed in service.⁷ Both new and used property can qualify. For used property, the buyer cannot have previously used the asset, and it cannot come from a related party.

Step 5: Allocate Direct and Indirect Costs

With the site data in hand, the team assigns dollar values to each reclassified component. This is engineering-based cost estimation, not guesswork. Professionals use standardized cost manuals and historical construction data to calculate what each component cost to install, whether that is a specific light fixture, a drainage system, or decorative millwork.

The harder piece is indirect cost allocation. Every construction project carries soft costs that benefit the project as a whole rather than any single component: architectural fees, engineering services, permits, insurance during construction, and contractor overhead and profit. These get distributed proportionally. The calculation divides each component’s direct cost by the total direct cost of the building, then applies that ratio to the pool of indirect costs. A component representing 3 percent of direct costs picks up 3 percent of indirect costs.

The final output is a complete breakdown of the property’s depreciable basis across every recovery period. Every dollar must be accounted for. The reclassified amounts for 5, 7, and 15-year property, plus the remaining 27.5 or 39-year structural costs, need to reconcile exactly with the total depreciable basis. Overstating reclassified assets is the fastest way to draw audit attention, and a reputable firm will stay conservative on genuinely ambiguous components.

Step 6: Put the Results on the Tax Return

How you report the study depends on when the property was placed in service.

First-Year Property

For a building placed in service in the current tax year, you simply report the reclassified depreciation schedules on the return directly. No accounting method change is required because you are establishing the method for the first time. Bonus depreciation on the reclassified 5, 7, and 15-year property applies automatically without a separate election.

Existing Property: Form 3115

If the property was placed in service in a prior tax year and you have been depreciating it as a single asset over 27.5 or 39 years, claiming the reclassified depreciation requires IRS Form 3115. The form requests a change from an impermissible depreciation method (treating reclassifiable components as long-lived property) to a permissible one (depreciating them over their correct shorter lives).⁸ The details that matter for completing it:

Designated change number: use DCN 7, which covers changes from an impermissible to a permissible depreciation method under the automatic change procedures.⁸
Schedule E: all depreciation method changes require completing Schedule E, which asks for a description of the property, its placed-in-service date, the current method, and the proposed method.
Section 481(a) adjustment: this is the cumulative catch-up amount, the difference between what you actually deducted and what you should have deducted under the correct method. For cost segregation, the adjustment is almost always negative, meaning you under-depreciated, and a negative 481(a) adjustment is taken entirely in the year of change as a single deduction.⁸

The original Form 3115 is attached to your timely filed federal return for the year of change. Because this is an automatic change, you do not need IRS approval in advance. The catch-up deduction hits your return in that single year, which can produce a substantial reduction in taxable income for properties that have been in service for many years without a prior study.

Step 7: Keep the Final Report

The deliverable is a formal report documenting every step: the qualifications of the study team, the documentation reviewed, the site inspection findings, the cost allocation methodology, and the resulting depreciation schedules. This report is the primary defense in an audit and should be detailed enough that an IRS examiner can trace every reclassified dollar from the construction records through the engineering analysis to the return.

A well-prepared report includes asset-by-asset schedules showing each component, its recovery period, its allocated cost (direct and indirect), and the applicable depreciation method. Photographs from the site visit are typically included as exhibits. The methodology should be clearly described, and any judgment calls on borderline components should be documented with supporting reasoning.

Keep the report with the permanent tax records for the property. If you used Form 3115 to claim a catch-up deduction, the report substantiates both the reclassification and the Section 481(a) adjustment. For first-year filings, the report supports the accelerated deduction on the original return. Either way, it needs to be accessible for as long as you own the property and through the statute of limitations period after any disposition.

Two Limits Worth Knowing Before You Start

Passive Activity Rules Can Suspend the Deductions

Rental real estate is generally treated as a passive activity, and losses from passive activities can only offset passive income, not wages, business profits, or investment income.⁹ Three exceptions matter:

The $25,000 allowance lets an active participant in a rental deduct up to $25,000 in passive losses against nonpassive income, but it phases out by 50 cents for every dollar of modified AGI over $100,000 and disappears at $150,000.⁹ Most owners of properties large enough to justify a study have income above that ceiling.

Real estate professional status removes the automatic passive label. You must spend more than half your total personal services for the year in real property trades or businesses, log more than 750 hours in those activities, and materially participate in each rental activity, which usually means more than 500 hours per year in that specific activity.⁹ A W-2 employee with a rental on the side almost never qualifies.

The short-term rental exception applies where the average guest stay is seven days or less. Those properties are not treated as rental activities under the passive activity rules, so if you materially participate, the losses are nonpassive.

If none of these apply, the accelerated deductions still have value. They accumulate as suspended passive losses that can offset passive income from other sources or be deducted in full when the property is sold.

Recapture Hits When You Sell

Accelerated depreciation is not free. When you sell, part of the deduction comes back through recapture.

Section 1245 property (5 and 7-year assets): all depreciation claimed on personal property is recaptured as ordinary income at your regular tax rate.³
Unrecaptured Section 1250 gain (real property that stayed in the 27.5 or 39-year categories): taxed at a maximum rate of 25 percent when sold.¹⁰

Reclassifying components from Section 1250 to Section 1245 produces larger deductions sooner but can be taxed at a higher rate at sale. For properties held for many years, the time value of money still tends to favor the study. A 1031 exchange defers both the capital gain and the recapture by rolling into a replacement property, which is one reason cost segregation and 1031 exchanges are often paired.

1
Internal Revenue Service. Interim Guidance on Additional First Year Depreciation Deduction Under Section 168(k)
2
Internal Revenue Service. About Form 3115, Application for Change in Accounting Method
3
Internal Revenue Service. Publication 544 (2025), Sales and Other Dispositions of Assets
4
Internal Revenue Service. Publication 946 (2024), How To Depreciate Property
5
Internal Revenue Service. Publication 527 (2025), Residential Rental Property
6
Office of the Law Revision Counsel. 26 USC 1245 – Gain From Dispositions of Certain Depreciable Property
7
Office of the Law Revision Counsel. 26 USC 168 – Accelerated Cost Recovery System
8
Internal Revenue Service. Instructions for Form 3115 (Rev. December 2022)
9
Internal Revenue Service. Publication 925 (2025), Passive Activity and At-Risk Rules
10
Internal Revenue Service. Topic No. 409, Capital Gains and Losses