Section 179 vs. Bonus Depreciation: What Applies to a Building?

Two different tools, not two versions of the same tool

Section 179 and bonus depreciation both let an owner take a deduction faster than the standard depreciation schedule, and both get lumped together in conversation as though they are interchangeable. They are not. They apply to different pieces of a building, work under different limits, and answer different questions.

Bonus depreciation, under section 168(k), is a percentage write-off applied automatically to qualifying property once its depreciation class is known. Section 179 is an election an owner makes, component by component, subject to annual limits. A cost segregation study is what identifies which components are eligible for which treatment in the first place; without the study, there is nothing to elect or apply bonus to beyond the building's default 39-year or 27.5-year schedule.

The confusion between the two is understandable, because both mechanics get discussed in the same conversation, often on the same building, often in the same tax year. But an owner or a contractor pitching this to a client is better served treating them as two separate line items on a checklist, not two names for one thing.

What section 179 qualified real property actually covers

On nonresidential property only, section 179 qualified real property lets an owner expense specific building systems placed in service after the building itself was first placed in service: roofs, HVAC equipment, fire protection and alarm systems, and security systems. That is the full list. It does not extend to flooring, cabinetry, decorative lighting, or the site improvements a cost segregation study typically finds in the 5-, 7-, and 15-year buckets.

Two limits cap how much of that expense an owner can actually use. There is an annual dollar cap on the total amount that can be expensed under 179 across all qualifying property, and there is a taxable-income limit: 179 expensing cannot create or increase a business loss, meaning the deduction is capped at the business's taxable income for the year, with any excess carried forward. Section 179 does not apply to residential rental property at all, only to nonresidential commercial buildings.

The timing requirement matters as much as the component list: the system has to be placed in service AFTER the building itself was already in service. A brand-new building's original roof or HVAC system does not qualify under this specific rule; it is the replacement roof or replacement HVAC system on an already-operating building that opens the door.

What bonus depreciation covers, and why 2025 changed the math

Bonus depreciation applies to the 5-year, 7-year, and 15-year property a cost segregation study identifies, carpet and most flooring, decorative lighting, cabinetry, appliances, certain electrical and plumbing serving equipment on the 5-year side, certain fixtures and furniture at 7 years, and land improvements like paving, fencing, landscaping, site utilities, and outdoor lighting at 15 years. It does not touch the 39-year (commercial) or 27.5-year (residential rental) structural shell, the roof deck, walls, and central structure stay on the long schedule regardless of bonus.

The 2025 OBBBA law restored 100% bonus depreciation permanently for qualifying property acquired after January 19, 2025. Property acquired between 2023 and that date sits on the earlier phase-down schedule (80%, 60%, then 40% depending on the acquisition year). See the full bonus depreciation timeline for how that phase-down applies year by year to an older acquisition still being depreciated today.

How the two stack on a real project

On a commercial renovation, say a roof and HVAC replacement paired with new flooring and updated site lighting, both mechanics typically apply to different line items in the same project. The flooring and lighting, once a study reclassifies them into 5-year or 15-year property, get 100% bonus depreciation automatically. The roof and HVAC replacement, if they were placed in service after the building's original in-service date, are section 179 qualified real property candidates, subject to the annual and income limits.

A study is what makes the second half of that sentence possible at all. Without an engineered study identifying the roof and HVAC replacement as separate assets with their own basis, an owner has no eligible property to make a 179 election against, and no way to know what the old, replaced components are worth writing off under partial asset disposition either. The three mechanics, bonus, 179, and disposition, all run off the same underlying classification work.

Why this matters for a contractor's proposal, not just a CPA's return

For the contractors in our audience, roofers, HVAC companies, general contractors, this comparison is the one a building owner's finance team eventually asks about, because it is the difference between "this whole job depreciates over 39 years" and "most of this job gets written off starting this year." Framing the roof or HVAC replacement as a 179 candidate, and the finishes and site work as bonus-eligible, is a more precise conversation than a blanket claim about depreciation.

The test is always the same regardless of who raises it: describe what the mechanics do, and route the specific number to a study. Whether a given job's mix of components clears the annual 179 cap or lands cleanly in bonus-eligible buckets is a question the free estimate at qualify is built to answer before anyone commits.

Frequently asked questions

Can an owner use both section 179 and bonus depreciation on the same building in the same year?

Yes, because they typically apply to different components. A roof replacement might be a 179 candidate while new flooring and site lighting identified by the same study are bonus-eligible property. The two are not competing for the same dollar; they are usually covering different line items.

Does section 179 apply to a short-term rental or residential rental property?

No. Section 179 qualified real property applies only to nonresidential property. Residential rental owners rely on bonus depreciation for the 5-, 7-, and 15-year components a study identifies, not on a 179 election.

What happens if a business doesn't have enough taxable income to use a full 179 deduction?

Section 179 expensing is capped at the business's taxable income for the year; it cannot create or increase a loss. Any amount above that limit carries forward to future years, unlike bonus depreciation, which has no income limit and can contribute to a loss.

Is a structural roof replacement always a 179 candidate?

It can qualify as section 179 qualified real property on nonresidential buildings if placed in service after the building's original in-service date, subject to the annual dollar cap and the taxable-income limit. Whether a specific roof job clears those limits depends on the business's full return, which is a question for the owner's CPA.

Do I need a cost segregation study to use either 179 or bonus depreciation?

A study is what identifies which components exist as separately depreciable assets with their own basis in the first place. Without that classification, an owner has no eligible property to elect 179 against and no components correctly identified as bonus-eligible short-life property.