Section 179D—the energy-efficient commercial buildings deduction worth up to $5.81 per square foot—terminates for projects that break ground after June 30, 2026. Here's what specifiers, contractors, and building envelope pros need to lock in before the window closes.
A Permanent Incentive, Suddenly Sunsetting
For nearly two decades, Section 179D of the Internal Revenue Code has been one of the most reliable federal levers behind high-performance envelope design. It provides federal tax deductions for placing in service eligible energy-efficient property in commercial buildings, and was amended by the Inflation Reduction Act for taxable years beginning after Dec. 31, 2022. Then the rules changed.
The One Big Beautiful Bill Act (Public Law 119-21) added a termination provision: 179D shall not apply to property the construction of which begins after June 30, 2026. That is no longer a distant policy note. With roughly two weeks left until the cutoff, every glazing contractor, envelope consultant, and design team with a qualifying project in the pipeline needs to know exactly where they stand.
What 179D Actually Covers
The deduction is broader than many specifiers realize. Energy-efficient property installed as part of the following building systems may be eligible for this deduction: interior lighting; heating, ventilating, and air conditioning (HVAC); hot water (service water heating); and/or building envelope.
For BuildOpenings readers, the envelope category is the relevant one—and it is wide. Section 179D focuses on energy-efficient commercial building property. The Department of Energy explains that eligible property may include systems tied to interior lighting, HVAC, hot water, and the building envelope, for both new construction and building upgrade projects. That covers high-performance curtain wall, storefront, window wall, thermally broken framing, advanced IGUs, and even retrofit measures like commercial window film.
The Numbers Specifiers Need
The deduction is meaningful enough to change the math on whether a project goes with a code-minimum window or a triple-glazed, low-U system:
- For 2025, the 179D deduction offers up to $5.81 per square foot for qualifying energy-efficient commercial building improvements when projects meet prevailing wage and apprenticeship requirements.
- Incentive amounts can reach up to $5.94 per square foot, providing a significant boost to tax strategy on new or remodeled facilities.
- On a 100,000-square-foot office tower, that's the difference between a $581,000 deduction and zero.
The Deadline Isn't What You Think
The most important nuance—and the one most likely to trip up GCs—is that June 30 is a construction start deadline, not a completion deadline.
The Section 179D clean energy tax deduction sunsets after June 30, 2026, but eligible building owners, contractors or designers can still claim the deduction for a qualifying building project even after that date passes. However, to satisfy eligibility requirements, projects must meet either the physical work test or the 5% cost test by no later than June 30.
In practice, that means projects that break dirt or hit 5% of total cost before July 1 can still capture 179D—even if the curtain wall doesn't go up until 2028. Provided eligibility requirements are met, taxpayers may still claim Section 179D after June 30—even up until the end of a standard 39-year depreciation schedule. But there are less than six months left to meet eligibility requirements.
Practical Implications for Envelope Pros
For architects and specifiers: This is a near-term reason to push owners to commit to high-performance envelope specs now. Companies with energy-efficient projects in their pipeline should evaluate projects to determine if planning and design phases can be accelerated to meet the deadline. This expedited timeline may strain design teams and require earlier commitment to project specifications than initially planned. Expect requests to lock down glazing performance values, framing systems, and envelope assemblies earlier in DD than usual.
For general contractors and CMs: The 5% cost test is your friend. Mobilization, early material orders, and foundation work all count. Document everything.
For manufacturers and fabricators: Expect a Q2 push on orders for thermally improved framing, high-performance IGUs, low-E coatings, and any envelope assembly that helps a project clear the 25%-energy-reduction threshold. After June 30, the demand signal shifts.
For tax-exempt owners: The allocation pathway remains valuable. The program allows certain tax-exempt entities—including government agencies and nonprofit organizations—to allocate deductions to designers responsible for energy-efficient systems, expanding participation across the construction value chain. Schools, municipalities, and nonprofits with shovel-ready envelope retrofits have a narrow window to lock in designer allocations.
What Comes After
Developers and property owners will need to explore alternative funding mechanisms, such as state-level incentives or private financing, to support energy-efficient projects post-2026. State stretch codes, utility rebate programs, and city-level performance mandates (NYC's LL97, Boston's BERDO, Washington's Clean Buildings Act) will increasingly carry the weight that 179D used to share. For envelope pros, the federal carrot is going away. The state and municipal sticks are not.