Section 179D explained: the energy-efficient commercial buildings deduction

Section 179D is the Internal Revenue Code's principal incentive for energy-efficient commercial construction: a deduction of up to several dollars per square foot for building envelope, HVAC and hot water, and interior lighting systems that beat a reference ASHRAE standard by at least 25 percent. It is a deduction, not a credit — it accelerates cost recovery rather than offsetting tax dollar for dollar — and after two decades of extensions and one brief period of permanence, it is now on a firm clock. The One Big Beautiful Bill Act (OBBBA), enacted July 4, 2025, terminates 179D for property whose construction begins after June 30, 2026.

That termination date has passed as of this writing. The practical questions now are which in-flight projects still qualify, how much they qualify for, and what a defensible claim looks like.

A short history: 2005 to termination

Section 179D entered the Code in the Energy Policy Act of 2005 as a temporary provision, capped at $1.80 per square foot. For fifteen years it lived on the extenders treadmill — repeatedly expiring and being revived retroactively — until the Consolidated Appropriations Act, 2021 made it permanent and added inflation indexing.

The Inflation Reduction Act of 2022 (IRA) then rebuilt it for property placed in service after 2022: a lower 25 percent savings threshold (down from 50 percent), a sliding-scale amount, a fivefold multiplier for meeting prevailing wage and apprenticeship (PWA) requirements, allocation to designers of buildings owned by any tax-exempt entity rather than only governments, a new deduction path for REITs' earnings-and-profits computations, and an alternative retrofit track under Section 179D(f) based on measured energy-use intensity reductions.

Permanence lasted three years. OBBBA terminated the provision for property the construction of which begins after June 30, 2026. There is no phase-down; the cliff is a beginning-of-construction date, discussed with the parallel Section 45L cutoff in our brief on planning around the June 2026 sunsets.

What qualifies: the three systems

The deduction attaches to "energy-efficient commercial building property" — depreciable property installed in a building located in the United States, within the scope of ASHRAE Reference Standard 90.1, as part of one of three systems:

• The building envelope — walls, roof, insulation, windows, doors.
• Heating, cooling, ventilation, and hot water systems — the mechanical plant, typically the largest energy driver.
• Interior lighting systems — fixtures, lamps, and controls.

"Commercial building" is broader than it sounds. It includes any building within ASHRAE 90.1's scope, which sweeps in multifamily residential buildings four stories or more above grade. Low-rise residential belongs to Section 45L instead; the boundary between the two is mapped in our 179D versus 45L comparison.

The sliding scale and the prevailing-wage multiplier

For property placed in service after 2022, the deduction is computed per square foot on a sliding scale. The floor is 25 percent modeled savings in total annual energy and power costs against the reference building; each additional percentage point of savings adds to the rate, capped at 50 percent savings. Meeting the PWA requirements — Davis-Bacon prevailing wages for laborers and mechanics, plus registered-apprenticeship labor-hour ratios, covered in detail in our prevailing wage and apprenticeship brief — multiplies every figure by five.

The table below shows the statutory base amounts and the inflation-adjusted 2025 figures (Rev. Proc. 2024-40); 2026 amounts adjust again.

Two caps discipline the number. First, the deduction cannot exceed the cost of the energy-efficient property actually placed in service. Second, the IRA replaced the old lifetime per-building cap with a rolling one: the current-year deduction is reduced by 179D deductions taken with respect to the same building in the prior three taxable years (four years for allocated deductions). A building can, in principle, generate deductions repeatedly as it is improved — a design feature that matters less now that the provision is sunsetting.

One transition note: PWA requirements are deemed satisfied for projects whose construction began before January 29, 2023 (60 days after Notice 2022-61), which is why some older projects reach the enhanced rate without wage documentation.

Deduction, not credit — and the basis reduction

Section 179D's economics are frequently oversold, so it is worth being precise. Commercial building systems are ordinarily 39-year property. Section 179D lets the owner deduct up to the capped amount immediately instead of over 39 years, and Section 179D(e) requires a corresponding reduction in the building's basis. The owner does not get extra deductions over the building's life; it gets them sooner. The benefit is the time value of acceleration, at the owner's marginal rate.

That is a genuinely valuable but bounded benefit — the same category of benefit as cost segregation, and subject to the same recapture-adjacent consequences on sale (the reduced basis increases gain, and accelerated deductions on real property convert to unrecaptured Section 1250 gain taxed at up to 25 percent for individuals). The distinction between this and a true credit — which Section 45L is — matters enough that we treat it separately in credits versus deductions.

The one context where 179D behaves differently is the designer allocation: an allocated deduction requires no basis reduction by the designer, because the designer has no basis in the building. That regime has its own rules and its own controversies, covered in the designer allocation guide.

Certification: qualified individuals and DOE-approved software

The deduction is not self-certified. Under Section 179D(d) and Notice 2006-52 (as updated), a qualified individual must certify the energy savings. A qualified individual is a contractor or professional engineer licensed in the jurisdiction where the building sits, and not related (within the meaning of Section 45(e)(4)) to the taxpayer claiming the deduction.

The certification must rest on modeling performed with software on the Department of Energy's published qualified software list, and on a field inspection of the building after it is placed in service, following NREL inspection guidelines. The certification itself is retained in the taxpayer's records rather than attached to the return, but the IRS's 179D practice unit makes it the first document requested on exam. What separates a defensible certification from a cheap one is the subject of our certification requirements brief.

The ASHRAE reference standard

Savings are measured against a hypothetical reference building meeting the "applicable" ASHRAE Standard 90.1. The IRA fixed the rule: the applicable standard is the more recent of 90.1-2007 or the edition affirmed by the Secretary at least four years before the property is placed in service. Under Announcement 2023-1, ASHRAE 90.1-2007 applies to property placed in service before 2027 whose construction began before 2023; 90.1-2019 applies to property whose construction begins in 2023 or later.

This is not a technicality. 90.1-2019 is a substantially tighter baseline than 90.1-2007 — its reference building already assumes efficient lighting and equipment — so the same physical building models to lower percentage savings against it. Projects that began construction in 2023 through mid-2026 need genuinely high-performing design to clear 25 percent, and claims that quote 2007-baseline savings for 2019-baseline buildings are a recurring exam problem.

The retrofit alternative

For existing buildings at least five years old, Section 179D(f) offers an alternative: a qualified retrofit plan measured not by modeling against ASHRAE but by actual reduction in energy use intensity — at least 25 percent, certified after a one-year measurement period following the retrofit. The deduction is taken in the year of the qualifying final certification, not the placed-in-service year, and is capped at the aggregate adjusted basis of the retrofit property. The measurement-period lag means retrofits placed in service in 2025 may generate deductions in 2026 or 2027; the OBBBA cutoff is tested against when construction of the retrofit began, not when the certification lands.

When 179D does not make sense

Neutrality requires the negative cases:

• Loss taxpayers and low rates. A deduction is worth its rate. An owner in losses, or an entity whose income flows to tax-exempt partners, gets little or nothing currently.
• Short expected hold. Basis reduction plus a near-term sale can convert the timing benefit into roughly offsetting gain, net of the rate spread.
• Bonus-depreciation overlap. With 100 percent bonus depreciation permanently restored for property acquired after January 19, 2025, much of what 179D accelerates for shorter-lived components may already be deductible through a cost segregation study; 179D's marginal value is largest for 39-year structural systems (envelope, built-in HVAC) that bonus does not reach.
• Marginal energy performance. Against the 90.1-2019 baseline, code-minimum buildings generally do not qualify. A study fee spent to confirm 24 percent savings buys nothing.
• IRS scrutiny. Exam activity concentrates on modeling integrity, the applicable reference standard, certifier independence, and — above all — designer allocations, where the Service has an active compliance campaign.

What to do now

For construction that began by June 30, 2026, nothing about the mechanics has changed: model, certify, deduct in the placed-in-service year, reduce basis. For prior years, a missed 179D deduction on property the taxpayer owns is generally a change in accounting method claimable via Form 3115 with a Section 481(a) adjustment — no amended return needed — while designers must receive allocations and claim on original or amended returns for open years. Either way, the certification file, not the tax computation, is where these claims are won or lost.