Cost Segregation After the Big Beautiful Bill: What 100% Bonus Depreciation Means for Your Rental

A cost segregation study reclassifies components of a building from the default 27.5-year (residential) or 39-year (commercial) recovery period into shorter asset classes: 5-year, 7-year, and 15-year property. With 100% bonus depreciation now permanently restored under IRC 168(k) by the One Big Beautiful Bill Act (signed July 4, 2025), those reclassified components can be expensed entirely in the year the property is placed in service. On a $500,000 residential rental, a study that reclassifies 30% of the basis generates roughly $150,000 in first-year depreciation deductions that would otherwise trickle out over decades.

What does a cost segregation study reclassify?

A residential rental property depreciates over 27.5 years under IRC 168(c), which includes the building structure and its structural components. A cost seg study separates the building into its constituent parts and identifies those that qualify as personal property (5-year or 7-year recovery) or land improvements (15-year recovery) rather than structural components of the building.

Components typically reclassified to 5-year property include appliances (refrigerator, stove, dishwasher, washer, dryer), carpeting, vinyl flooring, window treatments, certain cabinetry, decorative lighting fixtures, and specialized electrical circuits (for appliances rather than general building power). Components reclassified to 7-year property include furniture, outdoor furniture, and certain fixtures. Land improvements on a 15-year schedule include paving (driveways, parking areas), landscaping, fencing, retaining walls, sidewalks, outdoor lighting, and irrigation systems.

The percentage reclassified depends on the property type and construction. A typical single-family residential rental yields 15-30% in reclassified components. A furnished short-term rental can reach 25-40% because of the furniture, appliances, and finishing that an STR requires. A commercial property (39-year default) often yields 20-40% because of more extensive HVAC, specialized electrical, and tenant improvements.

The remaining basis stays on the 27.5-year schedule and continues to depreciate normally. The cost seg study does not change the total depreciation, it changes the timing: more in the early years, less later.

How does 100% bonus depreciation work now?

IRC 168(k) allows a 100% first-year deduction for qualifying property with a recovery period of 20 years or less. The One Big Beautiful Bill Act made this permanent for property acquired and placed in service after January 19, 2025. Under the prior TCJA phase-down schedule, the rate had already dropped to 80% for property placed in service in 2023, 60% in 2024, and 40% in 2025 before the Act intervened.

The practical effect: every dollar of basis that a cost seg study moves from the 27.5-year building class into 5-year, 7-year, or 15-year property is now eligible for 100% expensing in the year the property is placed in service. On a $600,000 building (after land allocation), a study that reclassifies 30% moves $180,000 into bonus-eligible classes. First-year depreciation: $180,000 in bonus plus roughly $15,270 in regular depreciation on the remaining $420,000 (at 27.5 years), totaling approximately $195,270 in year-one depreciation deductions.

Bonus depreciation is available to both new and used property (the TCJA expanded it to used property in 2017, and the 2025 Act maintained that expansion). So a cost seg study on a property you purchased from a previous owner is fully eligible. The property must be placed in service in the tax year you claim the deduction, meaning it must be ready and available for its intended use.

When does a cost seg study pay for itself?

The study costs money (engineering fees), and the benefit is a timing shift in depreciation (not additional depreciation). The break-even depends on the depreciable basis, the percentage reclassified, and your marginal tax rate.

At a 37% federal marginal rate plus 3.8% NIIT plus state tax (call it 45% combined), every $100,000 of accelerated depreciation saves roughly $45,000 in tax in year one. A $5,000 cost seg study that reclassifies $150,000 saves roughly $67,500 in first-year tax. The study pays for itself 13 times over.

The practical floor is a depreciable basis of roughly $300,000 to $400,000 for a residential property. Below $300,000, the dollar amount reclassified (even at 30%) may not justify the study fee. Above $400,000, the math is almost always favorable if the owner’s marginal rate is 24% or higher.

What is the CPA’s role vs the engineer’s?

The cost seg study itself is performed by an engineering firm, not a CPA. The engineer inspects the property (or reviews plans and photos for a desktop study), identifies and values each component, and produces a report that maps each component to its proper asset class and recovery period. The American Society of Cost Segregation Professionals (ASCSP) credential indicates the firm specializes in this work.

The CPA’s role is on the tax return side: reviewing the study for reasonableness, booking the reclassified depreciation on the return (or amending prior returns via Form 3115), and ensuring the bonus depreciation election is properly made. The CPA also advises on whether the study makes sense given the client’s overall tax picture: if the client has no income to offset (because they are already in a low bracket or have passive loss carryforwards), the timing benefit is smaller.

There are two types of studies: site-visit and desktop. A site-visit study costs $5,000 to $10,000+ for residential and $10,000 to $25,000+ for commercial. The engineer physically inspects the property. A desktop study costs $1,500 to $4,000 and relies on photographs, floor plans, and comparable data. The IRS has not formally rejected desktop studies, but the Audit Techniques Guide for Cost Segregation (2022 revision) emphasizes site-specific analysis, and a site-visit study is stronger in an exam.

Can I do a cost seg on a property I already own?

Yes, using a “lookback” or retroactive cost seg study filed with Form 3115, Application for Change in Accounting Method. The change is filed under the automatic consent procedures (currently DCN 7 for depreciation method changes), meaning you do not need IRS approval in advance. You file the Form 3115 with the return for the year of change, and a section 481(a) adjustment catches up all the depreciation you missed in prior years, taken as a single deduction in the current year.

The catch-up can be substantial. If you purchased a rental property five years ago for $500,000 (depreciable basis) and have been depreciating it straight-line at 27.5 years, you have taken roughly $90,900 in depreciation over five years. A cost seg study reclassifies $150,000 to bonus-eligible property. The 481(a) adjustment gives you the full $150,000 minus whatever depreciation was already taken on those components under the straight-line method. The net catch-up, filed with the current-year return, is a single-year deduction.

This is the most overlooked application of cost segregation. Investors who bought properties before 2025 and never ordered a study can still capture the benefit retroactively. The study fee is the same; the return preparation adds the Form 3115 filing.

What are the audit risks?

The IRS published an Audit Techniques Guide for Cost Segregation in 2004 (revised 2022) that examiners use to evaluate studies. The primary targets are land allocation (understating land value to inflate depreciable basis), component classification (aggressive reclassification of structural components as personal property), and study quality (desktop studies with generic percentages rather than property-specific analysis).

The strongest defense is a site-visit study from a credentialed firm that documents each component with photographs, measurements, and specific cost estimates tied to the property. Generic “percentage of construction cost” allocations without property-specific support are the weakest.

Land allocation is the most common audit adjustment. The IRS compares the taxpayer’s land allocation to county assessor records, comparable sales, and appraisals. If the assessor values the land at 40% of total value and the taxpayer allocates 15%, the discrepancy invites examination. Use a defensible allocation method (comparable sales, county assessment ratio, or an independent appraisal) and document the basis for it.

The recapture on sale is not an audit risk, but it is a planning consideration. Everything you expense through bonus depreciation reduces your basis and increases your gain on sale. The gain on reclassified personal property is recaptured as ordinary income under IRC 1245. The gain on the building is taxed at the unrecaptured section 1250 rate (maximum 25%) to the extent of depreciation taken. A 1031 exchange defers the recapture into the replacement property.

What should I do next?

If you own a rental property with $400,000 or more in depreciable basis and your marginal tax rate is 24% or higher, a cost seg study is almost certainly worth the fee. If you own an STR and meet the material participation requirements, the combination of cost seg and the STR loophole is the most powerful depreciation strategy available to individual real estate investors.

• The STR loophole, how STR losses offset W-2 income when both conditions are met
• Real Estate Professional Status, the alternative path for deducting rental losses against active income
• Fix-and-flip tax: dealer classification, why cost seg does not help flippers (dealer property is inventory, not depreciable)
• Real estate wholesaling tax, the dealer classification for contract assignments
• Schedule E vs Schedule C for rentals, how the classification affects where the depreciation deduction lands
• Self-directed IRA real estate: UBIT and UDFI, how cost seg works inside an IRA (accelerates depreciation but the IRA pays tax on debt-financed income)

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.