Energy Incentives · Brief · Working level
Section 179D or Section 45L: which incentive fits which building
179D is a per-square-foot deduction for commercial and high-rise buildings; 45L is a per-unit credit for homes and low-rise multifamily. A comparison of who claims what, how much, and under which certification path.
The two building energy incentives are frequently confused because they overlap on exactly one asset class — mid- and high-rise multifamily — and differ everywhere else. Section 179D is a per-square-foot deduction for commercial buildings, keyed to modeled savings against ASHRAE 90.1. Section 45L is a per-unit credit for homes, keyed to ENERGY STAR and Zero Energy Ready certification. Both terminated on the same OBBBA date, June 30, 2026, with different triggers — construction-start for 179D, acquisition for 45L.
The comparison
The table summarizes the operative differences as of the final claim periods.
| Section 179D | Section 45L | |
|---|---|---|
| Character | Deduction (accelerated depreciation; basis reduction) | General business credit (Form 8908; basis reduction with a LIHTC carve-out) |
| Building types | Commercial; residential 4+ stories above grade | Single-family, manufactured homes, multifamily (per ENERGY STAR program scope) |
| Claimant | Owner or funding tenant; designer by allocation for government/tax-exempt buildings | Eligible contractor (builder owning the home during construction) |
| Amount | ~$0.58–$1.16/sf base; ~$2.90–$5.81/sf with PWA (2025, indexed) | $2,500 ENERGY STAR / $5,000 ZERH per home; multifamily $500/$1,000 base, $2,500/$5,000 with prevailing wage; not indexed |
| Standard | 25–50% modeled savings vs. applicable ASHRAE 90.1 (2007 or 2019) | Programmatic: ENERGY STAR or DOE Zero Energy Ready certification |
| Certifier | Licensed PE or contractor in the jurisdiction; DOE-listed software; site visit | Accredited ENERGY STAR/ZERH rater with staged inspections |
| Wage rules | Prevailing wage and apprenticeship for 5× multiplier | Prevailing wage only, multifamily only; no apprenticeship |
| Timing | Placed-in-service year (retrofit path: final certification year) | Year home is acquired for use as a residence |
| Sunset trigger | Construction beginning after June 30, 2026 | Homes acquired after June 30, 2026 |
| Missed-year fix | Owner: Form 3115 method change; designer: amended return | Amended return, open years only |
Deduction versus credit, in dollars
The character difference is the one non-specialists underrate. A $5,000 45L credit is worth $5,000 of tax, less the tax effect of a $5,000 basis (or cost-of-goods) reduction — roughly $3,950 net at a 21 percent rate, permanently. A $5,000 179D deduction is worth $1,050 of tax at 21 percent now, offset by the same deductions forgone over the following decades — a time-value benefit, not a permanent one, exactly like cost segregation. The single exception is the allocated designer deduction, which involves no basis reduction and is therefore genuinely permanent to the designer — see the designer allocation guide. The general framework is laid out in credits versus deductions.
Scale cuts the other way. 179D grows with area: a 300,000-square-foot office at the enhanced 2025 maximum supports a deduction approaching $1.7 million. 45L grows with doors: a 200-unit garden complex meeting prevailing wage supports $500,000 to $1 million of credit. Dense high-rises with small units and large common areas often model better under 179D; low-rise projects with many units are structurally 45L.
The overlap zone and the boundaries
Multifamily at four or more stories above grade sits inside ASHRAE 90.1 and inside the ENERGY STAR Multifamily New Construction program, so a single project can support 179D for the owner and 45L for the eligible contractor — often the same economic party. The pairings that do not work: homebuyers claim neither; a fee builder with no ownership during construction has no 45L; a code-minimum building clears neither standard, particularly against the 90.1-2019 baseline; and a tax-exempt residential sponsor gets 45L only if a taxable eligible contractor exists, while its 179D value survives only through designer allocation. In every case the claim is only as strong as its certification file — the subject of our certification requirements brief.
Frequently asked questions
- Can a multifamily project claim both 179D and 45L?
- Sometimes. A residential building four stories or more above grade falls within ASHRAE 90.1 and can qualify for 179D, while 45L follows the ENERGY STAR Multifamily New Construction program, which covers many of the same buildings. Where both are available, the same project can support a 179D deduction for the owner and per-unit 45L credits for the eligible contractor — subject to basis-reduction coordination.
- Which is worth more, a 179D deduction or a 45L credit?
- Per dollar of face amount, the 45L credit — a credit offsets tax directly, while 179D merely accelerates depreciation at the taxpayer's marginal rate with a basis reduction. But they scale differently: 179D scales with square footage (up to roughly $5.81 per square foot in 2025 with prevailing wage), 45L with unit count ($500 to $5,000 per unit), so large low-density projects and dense high-rises favor different answers.
- Who claims each incentive?
- 179D goes to the building owner (or the tenant funding the improvements), or by allocation to the designer of a government or tax-exempt building. 45L goes only to the eligible contractor — the builder who owned the home during construction. Homebuyers and passive investors who did not build claim neither.