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Section 179 expensing, bonus depreciation, and cost segregation are three different tools, and most owners use two of them on the same building. Cost segregation is the engineering study that identifies which parts of a building qualify as 5-, 7-, or 15-year property. Bonus depreciation, which is 100 percent for qualifying property acquired and placed in service after January 19, 2025, is the rule that lets you deduct that reclassified property in year one. Section 179 is a separate election that can, in limited cases, reach building components that bonus depreciation cannot, such as a new roof, HVAC system, fire protection, alarm, or security system on a nonresidential building. For most real estate owners, cost segregation plus bonus depreciation does the heavy lifting, and Section 179 is a precision tool used alongside them.
Two investors closed on properties in the same week of March 2026. Sofia bought a small medical office building in Cedar Rapids, Iowa for $3,000,000. Nora bought a 12-unit apartment building in Davenport, Iowa for the same price. Both CPAs said "you should look at depreciation," and each went home, read about Section 179, bonus depreciation, and cost segregation, and came away confused. The three terms get used interchangeably online. They do three different jobs.
Sofia's nonresidential building got a study and bonus depreciation in year one, and Section 179 entered her picture six months later when she replaced the roof. Nora's residential building never had Section 179 available. Her real question turned out to be whether she could use the deduction at all, and the answer depended on how she and her husband file.
Under the Modified Accelerated Cost Recovery System, residential rental property is depreciated over 27.5 years and nonresidential real property over 39 years. Those long recovery periods apply to the building structure. A building, though, is a collection of components, and the tax code assigns different recovery periods to different kinds of property.
A cost segregation study is a fully engineered and accounted analysis that identifies and values the components that qualify for shorter recovery periods. Personal property such as certain specialty electrical, dedicated equipment connections, removable floor coverings, and decorative lighting can be examined as 5- or 7-year property. Land improvements such as paving, fencing, landscaping, site lighting, and drainage are generally examined as 15-year property. The courts have developed tests, including the Whiteco factors, that help distinguish personal property from structural components; read our article on the Whiteco 6-factor test and our explanation of Section 1245 versus Section 1250 property if you want to see how they work.
The study does not decide how quickly you deduct the reclassified property. That is the job of bonus depreciation and, in some cases, Section 179.
How the study is built matters as much as what it finds. The IRS Audit Techniques Guide describes six common approaches. A fully engineered and accounted study uses the first two: every component is identified, measured and priced from actual cost records or documented engineering estimates reconciled to the actual cost, and the result is a component schedule your CPA imports and an examiner can trace line by line. It is signed by credentialed professionals and defended for as long as you own the building. An estimated study applies percentages to the purchase price and produces a number with nothing behind it. Both give you a deduction. The engineered one is the one that holds up when the IRS asks how you got it.
Bonus depreciation, Section 168(k), allows an immediate deduction for a percentage of the cost of qualifying property in the year it is placed in service. Qualifying property generally means property with a recovery period of 20 years or less, which covers the 5-, 7-, and 15-year property a cost segregation study identifies.
The One Big Beautiful Bill Act, signed July 4, 2025, restored 100 percent bonus depreciation on a permanent basis for qualified property acquired and placed in service after January 19, 2025. Property acquired before that date, including property under a binding written contract before January 20, 2025, stays at the lower phase-down rate that applied when it was acquired, whatever the closing date. We cover that in our article on IRS Notice 2026-11 and the acquisition date rules, and our explanation of 100 percent bonus depreciation under the One Big Beautiful Bill Act walks through the legislation.
Four features matter for the comparison with Section 179:
That last point is where Section 179 becomes relevant.
Section 179 lets a taxpayer elect to deduct the cost of certain qualifying property in the year it is placed in service rather than depreciating it over time. It was written as a small business incentive, and its rules show it.
For tax year 2026, the maximum Section 179 deduction is $2,560,000. That amount phases out dollar for dollar when the total cost of Section 179 property placed in service during the year exceeds $4,090,000, and it is eliminated entirely at $6,650,000. The amounts are indexed each year. Bonus depreciation has no such limit.
This is the rule that surprises people most. A Section 179 deduction cannot exceed the taxable income from your active trades or businesses for the year, computed before the Section 179 deduction. It cannot create or increase a net loss. Any amount you cannot deduct carries forward. If a business has $90,000 of net income this year, it cannot use a $400,000 Section 179 election to generate a $310,000 loss. It deducts $90,000 and carries $310,000 forward. Bonus depreciation can produce the loss, which then runs through the passive activity, at-risk, basis, and excess business loss rules.
Section 179 applies mainly to tangible personal property used in a trade or business, such as equipment, machinery, and certain vehicles, plus a narrow category of real property improvements. Since the 2017 tax law, qualifying real property includes qualified improvement property, an interior improvement to an existing nonresidential building, and specified improvements made to nonresidential real property after the building was first placed in service: roofs, heating, ventilation, and air conditioning property, fire protection and alarm systems, and security systems. Enlargements of a building, elevators and escalators, and the internal structural framework do not qualify. Land and the building itself are never Section 179 property.
The qualifying real property categories are limited to nonresidential real property, so Section 179 is generally unavailable for residential rental property. It also generally excludes property used predominantly to furnish lodging, with an exception for hotels, motels, and similar establishments where more than half of the living accommodations are used by transient guests. In practice, short-term rental owners and residential landlords get their accelerated depreciation through cost segregation and bonus depreciation.
If your question is "what in my building is shorter-lived property?" the answer is cost segregation. If your question is "how fast can I deduct it?" the answer is usually bonus depreciation. If your question is "can I also expense a new roof or HVAC system on my nonresidential building?" the answer may be Section 179.
The numbers are illustrative. Sofia purchases the medical office building for $3,000,000 and places it in service in March 2026. Of that, $400,000 is allocated to land, which is never depreciable, leaving a depreciable basis of $2,600,000. Without a study, that basis is depreciated over 39 years. The IRS mid-month table gives March a first-year rate of 2.033 percent, so her first-year deduction is $52,858, and $66,667 a year after that.
Suppose a cost segregation study determines that 24 percent of the depreciable basis, or $624,000, consists of 5-, 7-, and 15-year property. With 100 percent bonus depreciation, Sofia deducts that $624,000 in year one, with no proration. The remaining $1,976,000 stays on the 39-year schedule at the March rate: $40,172 in 2026. Her first-year depreciation is $664,172 instead of $52,858, a difference of $611,314. At a 37 percent federal rate that is $226,186 of federal tax she does not pay in 2026.
Now add the roof. Later in the year Sofia replaces the roof for $180,000 and upgrades the building HVAC for $120,000. These are 39-year improvements, so bonus depreciation does not apply to them. If Sofia makes a Section 179 election for those improvements, and her medical practice and other active businesses have enough taxable income, she can deduct the full $300,000 in the same year. If she does not make the election, those costs are depreciated over 39 years. If she replaced an old roof, she may also be able to write off the remaining basis of the roof that was removed, which we explain in our article on partial asset disposition. Her CPA must also confirm that the work is a capital improvement and not a deductible repair under the tangible property regulations.
Nora buys a 12-unit apartment building for $3,000,000 with a depreciable basis of $2,600,000 after allocating land, placed in service in March 2026. Residential rental property is depreciated over 27.5 years, so without a study she would deduct $74,854 in the first year at the March mid-month rate of 2.879 percent, and $94,545 a year after that.
For multifamily property, a study often finds a meaningful share of the depreciable basis in appliances, removable flooring, specialty electrical, and exterior land improvements such as paving, fencing, and landscaping. Suppose it identifies 22 percent, or $572,000, as 5-, 7-, and 15-year property. With 100 percent bonus depreciation, that amount is deductible in year one. Section 179 is generally out of the picture because the building and its improvements are residential rental property. We explain the multifamily methodology in our guide to cost segregation for multifamily and apartment buildings.
Whether Nora can use a $572,000 deduction against her other income is the real question. If she has no other qualifying status, rental losses are passive and can only offset passive income, with the unused loss carried forward until she has passive income or sells the property in a fully taxable sale. If she or her husband qualifies as a real estate professional and materially participates in the rental activity, the loss may be nonpassive and can offset other income, subject to the excess business loss limit under Section 461(l). Nora and her husband file jointly, and the 2026 joint cap is $512,000, adjusted annually. If the building throws off a $572,000 loss and nothing else offsets it, $512,000 is usable in 2026 and the remaining $60,000 carries forward as a net operating loss.
Owen and Jill own a lakefront short-term rental outside Osage Beach, Missouri, purchased for $1,200,000 in May 2026 with a depreciable basis of $960,000. The average guest stay is four days, so the activity is evaluated under the material participation tests instead of the rental rules, and because Owen materially participates and keeps a contemporaneous log of his hours, the loss is nonpassive. A study might identify a significant share of the basis in separately depreciable furnishings, flooring, specialty lighting, a dock, landscaping, and site improvements. With 100 percent bonus depreciation, that amount is deductible in year one and can offset other income, subject to the excess business loss limit.
There is no Section 179 election here. Short-term rental property furnishes lodging, so the deduction comes from the study, bonus depreciation, and documented hours.
Income limit. If the business does not have enough taxable income to absorb a Section 179 deduction, the excess carries forward. Bonus depreciation can generate a loss, so it may fit better when you expect a loss year anyway, but only if you can use that loss under the passive, at-risk, basis, and excess business loss rules.
Flexibility. Section 179 is an asset-by-asset election, and you choose the amount. Electing out of bonus depreciation applies to a whole class of property. An owner who wants to save deductions for a higher-income year cares about that control.
Recapture. Both Section 179 deductions and bonus depreciation on personal property are generally recaptured as ordinary income when the property is sold at a gain. The benefit is timing. A 1031 exchange can defer both the gain and the recapture, subject to the replacement property having enough Section 1245 property to absorb it.
Ordering. When both apply to the same property, the Section 179 deduction is taken first, and bonus depreciation then applies to the remaining basis. Your CPA coordinates this on Form 4562.
Accelerated depreciation creates a deduction on paper. Whether it reduces your tax this year depends on a series of limitations, applied in order.
Basis and at-risk limits. You generally cannot deduct losses beyond your tax basis or the amount you have at risk in the activity. Financing structure matters.
Passive activity rules. Rental activities are passive unless an exception applies. A limited exception allows some taxpayers to deduct up to $25,000 of rental losses against other income, but it phases out between $100,000 and $150,000 of modified adjusted gross income. Real estate professionals who materially participate and qualifying short-term rental owners can treat losses as nonpassive. Suspended losses come loose when the property is sold in a fully taxable sale.
Excess business loss limit. Under Section 461(l), noncorporate taxpayers cannot deduct business losses beyond a threshold in a given year. For tax years beginning in 2026, that threshold is $256,000 for single filers and $512,000 for joint filers, adjusted annually, and the One Big Beautiful Bill Act made the limit permanent. Disallowed excess is carried forward as a net operating loss. Our article on the 2026 excess business loss limit shows how this affects planning.
Assuming Section 179 applies to the whole building. Only certain improvements to nonresidential property qualify. Land and the building itself do not.
Assuming bonus depreciation applies to a new roof. A roof on a nonresidential building is 39-year property, outside the 20-year limit for bonus depreciation. That is the gap Section 179 can fill when it applies.
Skipping the study because Section 179 seems simpler. Section 179 cannot reclassify a building. Only a cost segregation study identifies the 5-, 7-, and 15-year components inside the structure.
Ignoring the placed-in-service date and the acquisition date. Both can change the available bonus percentage, and placed in service means ready and available for use, which is often a different day from closing.
Choosing a provider on price alone. A study is the foundation for a deduction the IRS can examine. Ask who signs the report, which of the six approaches in the Audit Techniques Guide it follows, and who answers when an examiner calls.
A study needs at least $180,000 of depreciable basis after land to be worth the fee. If you plan to sell within a couple of years, recapture can take back most of what you deducted and the time value is small. If the loss is passive and you have no passive income, no real estate professional status, and no qualifying short-term rental, the deduction sits suspended until you have something to use it against, and paying for a study today to deduct in 2031 is a weak trade. And Section 179 is pointless on a building whose business shows no taxable income this year, because the election carries forward instead of producing a refund.
Neither is better across the board. For the reclassified components a study identifies, bonus depreciation is usually the main tool, because it has no dollar cap or taxable income limit. Section 179 is useful for specific nonresidential improvements, such as roofs, HVAC, fire protection, alarm, and security systems, that bonus depreciation does not reach.
Generally no. Section 179 real property improvements are limited to nonresidential real property, so residential rental owners rely on cost segregation and bonus depreciation.
No. The deduction is limited to the taxable income from your active trades or businesses, and any excess carries forward. Bonus depreciation can create a loss, subject to the loss limitation rules.
The maximum deduction is $2,560,000, with a phase-out beginning when qualifying purchases exceed $4,090,000. Confirm the figures with your tax advisor when you file.
You may still be able to claim the missed depreciation through a lookback study and Form 3115, which uses a change in accounting method and a Section 481(a) adjustment rather than an amended return. This is generally available only while you still own the property. See our guide to catching up missed depreciation with Form 3115.
The IRS has published guidance on how it evaluates studies, and a fully engineered and accounted study that follows that guidance is the best protection. Since 2004 the IRS has described in its Audit Techniques Guide which methods it finds methodical and which it views with caution, and a study built under Approaches 1 or 2 with a traceable component schedule is the one an examiner can follow.
If you own commercial, residential rental, or short-term rental property that you purchased, built, or renovated, send us the address, the purchase price and the closing date. Cost Seg America returns a free proposal within 24 hours showing what a fully engineered and accounted study would likely reclassify on your building. Every study comes with audit defense for as long as you own the building, with no time limit, no hour cap and no additional fee.
Talk to Jim Dougherty and his team at 1-888-365-5023 or info@costsegamerica.com.
This article is for general educational purposes and is not tax, legal, or accounting advice. The examples are illustrative, not actual client results, and your results will depend on your property, your facts, and current law. Dollar limits are inflation-adjusted annually and should be confirmed for the year in which you file. Consult your CPA or tax advisor before making any tax decision.
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