Cost Segregation · Brief · Working level
Section 179 vs. bonus depreciation for building-adjacent property
Both expensing regimes reach the short-life property a cost segregation study identifies, but they differ on eligibility — 179 uniquely covers roofs and HVAC on nonresidential buildings, while bonus has no dollar cap and no income limit. The differences, and the ordering rule.
After a cost segregation study sorts a building into classes, two regimes compete to expense the short-life property immediately: Section 179 and Section 168(k) bonus depreciation. With 100 percent bonus permanently restored for property acquired after January 19, 2025, bonus wins most head-to-heads on sheer capacity — no dollar cap, no income limit, no election needed. But 179 does one thing bonus cannot: under Section 179(e), it reaches roofs, HVAC, fire protection and alarm, and security systems on nonresidential buildings — 39-year real property that bonus can never touch, because those items (roofs and exterior HVAC especially) fail the interior-only definition of qualified improvement property.
So the practical question is not which regime is better, but which assets to route through each.
How do the two regimes differ?
Head to head. Statutory rules per Sections 179 and 168(k); mechanics in Pub 946, reporting on Form 4562.
| Feature | Section 179 | Bonus (168(k)) |
|---|---|---|
| Rate | 100% of elected amount | 100% for property acquired after Jan 19, 2025; 80/60/40 phase-down for earlier acquisitions |
| Dollar cap | Yes — inflation-adjusted annual maximum, phased out above an investment threshold | None |
| Income limit | Cannot exceed taxable income from active trades or businesses; excess carries forward | None — can create an NOL |
| Building components | Roofs, HVAC, fire protection/alarm, security systems on nonresidential property (§179(e)); QIP | QIP (15-year) only; never structural or exterior items |
| Granularity | Elect asset by asset, any amount per asset | All-or-nothing by asset class (elect out by class) |
| Rental / lodging | Trade-or-business requirement; §179(d) lodging restriction | No equivalent restriction |
| Used property | Eligible | Eligible (post-TCJA), if not previously used by the taxpayer |
| State conformity | More states conform | Widely decoupled — watch state addbacks |
Three differences drive planning. Reach: a new roof on a shopping center is 39-year property with no bonus path; 179 expenses it in full, cap permitting. Capacity: 179's dollar cap and taxable-income limit make it a retail-size tool — a large study's reclassification will blow through it, which is what bonus is for. Precision: because 179 is elective per asset and per dollar, it can fine-tune income (say, to manage a bracket or the QBI deduction) in a way class-wide bonus cannot.
The ordering rule and the routing logic
Where an asset qualifies for both, Section 179 is applied first, reducing basis; bonus then applies to remaining basis; regular MACRS covers the rest. That ordering matters mostly at less-than-100-percent bonus rates; at 100 percent bonus the sequencing is academic for dual-eligible assets, and the routing logic simplifies to:
- 179 for what only 179 can reach — roofs, HVAC, fire protection, security on nonresidential buildings, up to the cap and income limit.
- Bonus for everything else — the study's 5-, 7-, and 15-year property and QIP, in unlimited amounts. How that plays through a study is the subject of cost segregation and bonus depreciation.
When does the choice actually matter?
At permanent 100 percent bonus, 179's remaining significance for real estate is concentrated: the §179(e) building systems, income fine-tuning, and states that allow 179 but decouple from bonus — where routing dollars through 179 avoids a state addback. Conversely, bonus's ability to create losses is only as valuable as the losses are usable, which for rental owners runs straight into the passive-activity screen covered in when cost segregation doesn't make sense. Statutory text for both provisions is at the official U.S. Code; the annually adjusted 179 figures are in the Form 4562 instructions for the year in question.
Frequently asked questions
- What can Section 179 expense that bonus depreciation cannot?
- For nonresidential real property, Section 179(e) reaches certain improvements to the building itself: roofs, HVAC, fire protection and alarm systems, and security systems placed in service after the building. Bonus depreciation cannot touch these unless they qualify as 15-year QIP — and a roof or exterior HVAC unit is not QIP because QIP is limited to interior improvements. Section 179 is also elective asset by asset, allowing precise targeting.
- What are the limits on Section 179 versus bonus?
- Section 179 carries a dollar cap (an inflation-adjusted maximum expensing amount, with a phase-out once total qualifying purchases exceed the investment threshold) and a taxable-income limit — it cannot create a business loss, though disallowed amounts carry forward. Bonus depreciation under Section 168(k) has no dollar cap and no income limit; at the 100 percent rate for property acquired after January 19, 2025, it can create or deepen a loss without restriction.
- Can rental property owners use Section 179?
- Only if the rental activity rises to a trade or business, and Section 179(d) historically restricts property used in connection with lodging — which excludes most residential rental furnishings, with exceptions. Nonresidential landlords operating as a business can use 179 for qualifying improvements like roofs and HVAC. Bonus depreciation has no trade-or-business lodging restriction, which is why it is the default tool for rental real estate.