This question compares a map with a vehicle. Cost segregation is the engineering-based allocation that tells you which parts of a building are 5-year, 7-year, or 15-year property. Section 179 and bonus depreciation are the 2 expensing regimes you can then drive those assets through. The real choice is 179 versus bonus, and for most real estate investors bonus wins, with 2 important exceptions.
How the 2 regimes differ
Section 179 under OBBBA lets a business elect to expense up to $2,500,000 of qualifying property per year, with a dollar-for-dollar phase-out once total Section 179 property placed in service exceeds $4,000,000. It has 2 structural limits bonus does not. First, the deduction cannot exceed your aggregate taxable income from active trades or businesses under IRC section 179(b)(3); anything above that carries forward instead of creating a loss. Second, it is elected asset by asset, which gives fine-grained control.
Bonus depreciation under IRC section 168(k) has no dollar cap and no income limit. It applies automatically to all qualified property in a class unless you elect out under IRC section 168(k)(7), and it can push you into a net operating loss. For property acquired after January 19, 2025, the rate is a permanent 100% under OBBBA, so there is usually no reason to burn the capped 179 election on assets bonus already covers in full.
Where Section 179 is the only option
IRC section 179 reaches 4 categories of nonresidential building work that bonus cannot touch because they are 39-year property: roofs, HVAC, fire protection and alarm systems, and security systems. A self-storage operator replacing a $180,000 roof gets no bonus depreciation on it, but can elect to expense the entire $180,000 under Section 179, subject to the income limit. For rental activities, 179 also requires the activity to rise to a trade or business, a bar a triple-net portfolio may not clear.
Worked example
A fund that operates its rentals as a business buys a $2,000,000 flex-industrial building in 2026 and a cost segregation study identifies $380,000 of 5-year and 15-year property. Later that year it replaces the roof for $150,000. The clean structure: take 100% bonus on the $380,000 of short-life assets, no cap, no income test, and elect Section 179 on the $150,000 roof. Total first-year writeoff: $530,000 plus regular depreciation on the rest.
When the answer flips
Some states allow Section 179 but add back bonus depreciation, so in a nonconforming state 179 can be worth more after-tax. And when income is thin, the 179 income limit means the deduction waits in carryforward rather than becoming a loss you cannot use anyway under IRC section 469. Amadae models both regimes in its savings playbook, screens the property for reclassification potential first, and prepares the return with whichever election the numbers support.