How Does the 179D Deduction Work? Eligibility, Amount, and Deadline

The Section 179D deduction lets you subtract up to $5.94 per square foot from federal taxable income when a commercial building’s energy-efficient improvements cut modeled energy and power costs by at least 25% against an ASHRAE baseline and the project meets federal prevailing wage and apprenticeship rules. Projects that skip the labor standards are capped at $1.19 per square foot. So how does the 179D deduction work in practice? You install qualifying lighting, HVAC, or envelope improvements, have a licensed engineer or architect model and certify the energy savings, and claim the deduction on Form 7205 for the tax year the property is placed in service. One deadline drives current planning: under Public Law 119-21, Section 179D does not apply to property whose construction begins after June 30, 2026.¹

Who Can Claim It

Two groups qualify. If you own a commercial building and install energy-efficient property, you claim the deduction directly. Multifamily residential buildings count when the structure is at least four stories tall, because the ASHRAE 90.1 standard excludes low-rise residential buildings of three stories or fewer.²

The second path exists for buildings owned by entities that pay no federal income tax. Government agencies at every level, tribal governments, Alaska Native Corporations, and tax-exempt organizations can all participate, and because they have no tax liability to offset, they allocate the deduction to the designer of the energy-efficient systems.³ The designer is the architect, engineer, or contractor primarily responsible for creating the building’s energy-efficient features, and that person needs a signed allocation letter from the owner before claiming anything on their own return.⁴

What Improvements Qualify

Three categories of building systems are eligible: interior lighting, heating, cooling, ventilation and hot water systems, and the building envelope. The envelope covers the physical barrier between inside and outside, including insulation, windows, roofing, and exterior walls.

For property placed in service in 2023 or later, there is no partial deduction for upgrading just one system. The installed property has to be part of a plan that cuts the building’s total annual energy and power costs by at least 25% across all qualifying systems compared to a reference building modeled to the applicable ASHRAE standard.³

How Much the Deduction Is Worth

The dollar amount slides with efficiency. For 2026, the base tier starts at $0.59 per square foot at the 25% savings floor and rises by $0.02 for each additional percentage point of savings, topping out at $1.19 per square foot at 55% savings. Those are the rates for projects that don’t meet federal labor standards.⁵

Projects that satisfy prevailing wage and apprenticeship requirements get five times the base amount, so the range runs from $2.97 per square foot at 25% savings to $5.94 per square foot at the maximum.⁶ Both figures are indexed for inflation and adjusted annually by the IRS.

The tier gap matters. On a 50,000-square-foot building hitting maximum efficiency, the base deduction is $59,500 and the prevailing wage deduction is $297,000.

The Lookback Cap

You can’t stack unlimited deductions on the same building. Amounts you deducted in the prior three tax years, or four years if the deduction was allocated to a designer, reduce the maximum available for the current year.³

The Prevailing Wage and Apprenticeship Rules

Getting the higher tier means meeting two labor standards from the Inflation Reduction Act. All laborers and mechanics on the project must be paid at least the prevailing wage rate for their classification and location, as set by the Department of Labor under Davis-Bacon standards. Registered apprentices must also perform a specified percentage of total labor hours.⁷

Slip-ups don’t automatically kill the bonus. If you underpaid workers unintentionally, you can still qualify by paying the wage difference plus interest at the federal short-term rate plus six percentage points, along with a $5,000 penalty per underpaid worker. Intentional underpayment doubles that penalty to $10,000 per worker.⁸ For apprenticeship shortfalls, the penalty is $50 multiplied by the total labor hours where the requirement wasn’t met, and it may not apply at all if you can show a good faith effort to comply or had a qualifying project labor agreement in place.⁶

Which ASHRAE Standard Applies

Savings are measured against ASHRAE Standard 90.1. Your building is modeled against a theoretical reference building that just barely meets the standard, and the certifier calculates the percentage of energy costs your design saves against that baseline.

Version matters. For property placed in service before January 1, 2027, the applicable benchmark is ASHRAE Standard 90.1-2007. The newer 90.1-2019 standard applies only to buildings that both began construction on or after January 1, 2023, and have property placed in service on or after January 1, 2027.³ Most projects claiming the deduction in 2026 are measured against a standard published nearly two decades ago, which makes the 25% threshold more achievable than it sounds.

Retrofits of Existing Buildings

Section 179D includes a separate track for retrofits, in subsection (f). Rather than comparing your building to a hypothetical ASHRAE reference, this path measures actual energy use intensity before and after the retrofit. The building must have been placed in service at least five years before you establish the retrofit plan, and the plan itself must be a written document prepared by a qualified professional projecting at least a 25% reduction in energy use intensity.¹

The retrofit deduction is claimed in the tax year that includes the date of the final certification, which happens more than one year after the property is placed in service and confirms that actual energy use dropped by at least 25%.⁴ The June 30, 2026 construction deadline applies here too.

Certification

You can’t self-certify. A qualified individual has to verify that the building meets the energy savings thresholds. The statute defines qualified professionals as licensed architects or licensed engineers, and qualified individuals as people recognized by an organization certified by the Secretary of the Treasury.¹

The certification must rely on energy modeling done with qualified computer software whose designer has certified it meets the calculation procedures required by the Secretary. The Department of Energy maintains the approved list.⁹ The certifier documents the exact percentage of energy savings, verifies that installed systems match the design specifications, and produces a technical report you keep on file.

Filing and Basis Reduction

Claiming the deduction requires Form 7205. The form asks for the building’s address, the date the property was placed in service, the energy savings percentage, the building’s square footage, and information about the certifier. You also indicate whether you’re claiming as the building owner or as an allocated designer.⁵

Form 7205 calculates the deduction, and the total flows to your regular return. Corporations filing Form 1120 include it on line 25 and attach Form 7205. Other entities use the corresponding line.⁵ You don’t submit the full certification report with your return, but you have to retain supporting documentation for at least three years from the date the return is due or filed, whichever is later.¹⁰

One trade-off catches some taxpayers off guard: the building’s tax basis must be reduced by the amount of the deduction claimed. If you deduct $200,000 under Section 179D, your depreciable basis drops by $200,000, which means smaller depreciation deductions in future years.¹¹ An immediate deduction is generally more valuable than the same amount spread across 39 years of depreciation, but the reduced future depreciation belongs in any long-term modeling.

The June 30, 2026 Construction Deadline

Section 179D does not apply to property whose construction begins after June 30, 2026. The termination date was enacted as part of Public Law 119-21, signed in July 2025.¹ Projects that have already begun construction before that date remain eligible even if the building is placed in service later.

The “beginning of construction” standard has historically been interpreted through IRS guidance to mean either starting physical work of a significant nature or incurring at least 5% of the total project cost. Congress could extend or modify the deadline, as it has done with prior iterations of Section 179D, but the safer approach is to plan against the deadline currently on the books.

1
Office of the Law Revision Counsel. 26 USC 179D – Energy Efficient Commercial Buildings Deduction
2
IRS. IRC 179D Energy Efficient Commercial Buildings Deduction
3
Internal Revenue Service. Energy Efficient Commercial Buildings Deduction
4
Department of Energy. 179D Energy Efficient Commercial Buildings Tax Deduction
5
Internal Revenue Service. Instructions for Form 7205 (12/2025)
6
Internal Revenue Service. Frequently Asked Questions About the Prevailing Wage and Apprenticeship Under the Inflation Reduction Act
7
U.S. Department of Labor. Prevailing Wage and the Inflation Reduction Act
8
Federal Register. Increased Amounts of Credit or Deduction for Satisfying Certain Prevailing Wage and Registered Apprenticeship Requirements
9
Department of Energy. Qualified Software for Calculating Commercial Building Tax Deductions
10
Internal Revenue Service. Instructions for Form 1120 (2025)
11
Office of the Law Revision Counsel. 26 U.S. Code 179D – Energy Efficient Commercial Buildings Deduction