What Is Section 179D?
Section 179D of the Internal Revenue Code is a federal tax deduction for owners of energy-efficient commercial buildings and — uniquely — for designers of government-owned buildings. Originally enacted by the Energy Policy Act of 2005 and significantly expanded by the Inflation Reduction Act of 2022, Section 179D rewards qualifying energy upgrades with a dollar-per-square-foot deduction. In 2026, that deduction reaches up to $5.36 per square foot for the highest-performing projects meeting prevailing wage and apprenticeship requirements. (IRS.gov / DOE.gov)
Unlike a tax credit — which directly reduces your tax bill dollar-for-dollar — a deduction reduces your taxable income. At a 21% corporate tax rate, a $5.36/ft² deduction on a 200,000 square foot building yields $225,120 in actual tax savings. The deduction applies to qualifying improvements across three major building systems: HVAC and hot water systems, interior lighting, and the building envelope (insulation, windows, roofing, walls, and floors).
The One Big Beautiful Bill Act (OBBBA), Public Law 119-21, established a hard sunset date. Construction must begin by June 30, 2026. Projects starting after that date are ineligible. No extension has been announced or enacted as of this writing.
Who Qualifies for Section 179D?
Eligibility depends on building type and your role in the project. The deduction covers two groups:
Commercial Building Owners
Private owners of commercial buildings can claim 179D for qualifying energy-efficient improvements installed in buildings they own and use in a trade or business. Qualifying building types include:
| Building Type | Qualifies? | Notes |
|---|---|---|
| Office buildings | ✓ Yes | All sizes; must meet ASHRAE 90.1 threshold |
| Retail centers / shopping malls | ✓ Yes | Common area HVAC and lighting upgrades qualify |
| Industrial warehouses | ✓ Yes | Lighting retrofits are the most common qualifying improvement |
| Hotels & hospitality | ✓ Yes | HVAC upgrades and envelope improvements qualify |
| Multifamily (4+ stories) | ✓ Yes | Must be 4 or more stories above grade |
| Healthcare / medical offices | ✓ Yes | HVAC often the primary qualifying system |
| Government-owned buildings | ✓ Via designer | Deduction allocated to the designer, not the public owner |
| Single-family residential | ✗ No | See Section 25C residential credit instead |
| Multifamily (1–3 stories) | ✗ No | Must be 4+ stories for commercial classification |
Designers of Government-Owned Buildings
Architects, engineers, mechanical contractors, and energy consultants who design qualifying energy-efficient improvements for federal, state, or local government-owned buildings can receive an allocated deduction from the government entity. The building owner (the government) allocates the deduction to the designer via a signed certification letter. This is the most common use case for design firms.
To qualify at any deduction level, the energy improvements must achieve at least 25% energy cost savings versus the reference building under ASHRAE Standard 90.1-2007. Improvements that fall below this threshold produce no deduction. A certified energy modeler must document the savings calculation.
2026 Savings Table: Building Type × Deduction Amount
The deduction amount per square foot scales with energy savings percentage. Rates shown reflect IRS inflation-adjusted figures for 2026. Projects meeting prevailing wage and apprenticeship (PWA) requirements earn 5× the base rate.
| Energy Savings vs. Baseline | With PWA (2026 Rate) | Without PWA (2026 Rate) | Example: 100,000 ft² (PWA) |
|---|---|---|---|
| 25% (minimum) | $0.50/ft² | $0.10/ft² | $50,000 |
| 30% | $1.07/ft² | $0.21/ft² | $107,000 |
| 35% | $1.61/ft² | $0.32/ft² | $161,000 |
| 40% | $2.14/ft² | $0.43/ft² | $214,000 |
| 45% | $2.68/ft² | $0.54/ft² | $268,000 |
| 50%+ (maximum) | $5.36/ft² | $1.07/ft² | $536,000 |
Source: IRS.gov — Energy Efficient Commercial Buildings Deduction. Rates are inflation-adjusted annually; 2026 maximum of $5.36/ft² reflects current IRS guidance. Confirm with a qualified tax advisor before filing.
Real-World Example: Office Building HVAC Retrofit
What "Construction Start" Means for the June 30 Deadline
The June 30, 2026 deadline is a construction start deadline — not a completion deadline. A project that breaks ground in late June 2026 and completes six months later is still fully eligible. Understanding exactly what qualifies as a "start" under IRS rules is critical.
The IRS applies two tests to determine whether construction has begun by the deadline. Either test, if satisfied by June 30, 2026, preserves the project's eligibility:
- Physical Work Test: Significant physical work of a permanent nature has commenced at the building site, or at an off-site factory or manufacturing facility for pre-fabricated or modular components. The work must be integral to the qualifying energy system — not preliminary site prep or demolition that would occur regardless of the 179D claim.
- 5% Safe Harbor (Cost Test): The taxpayer has paid or incurred costs equal to at least 5% of the total expected cost of the qualifying energy property. This can include equipment purchases, materials procurement, and labor directly allocable to the qualifying systems.
Signing contracts, requesting proposals, ordering equipment, applying for permits, or conducting energy audits do not constitute a construction start under either IRS test. Physical work or 5% of cost must be documented before June 30, 2026.
Once either test is satisfied, the project maintains its 179D eligibility even if construction extends well past June 30. The deadline is about when you start — not when you finish. Projects with long lead times (large HVAC replacements, full envelope retrofits) should begin procurement and installation well in advance of June 30 to avoid last-minute documentation risks.
Maintain clear records of the construction start date: signed work orders, contractor invoices showing dates, photos, and progress reports. The IRS may request documentation of the start date to verify eligibility. A qualified tax advisor should review your documentation approach before June 30.
Step-by-Step: How to Claim 179D Before June 30
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1Run an energy model to assess baseline savings
A certified energy modeler must compare your building's energy performance against the ASHRAE 90.1 reference building. The model determines your savings percentage — which determines your deduction rate. This step also identifies which systems (HVAC, lighting, envelope) qualify and which don't reach the 25% threshold.
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2Verify prevailing wage and apprenticeship compliance
To earn the 5× enhanced deduction rate (up to $5.36/ft²), workers performing qualifying installation must be paid prevailing wages as determined by the Department of Labor, and a minimum percentage of labor hours must be performed by registered apprentices. This requirement applies to the contractor's workforce — start confirming compliance with your GC now.
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3Get a 179D certification from a qualified inspector
Section 179D requires a certification from a qualified individual (typically a licensed engineer or contractor) who is not related to the taxpayer. The certifier inspects the project and prepares a written report certifying that the installed property meets the energy performance requirements. The IRS provides a model certification format.
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4For government buildings: obtain a signed allocation letter
If you're a designer claiming the deduction allocated from a government owner, you need the government entity to sign an allocation letter. This is typically done at the completion of the project. Begin this conversation now — government procurement timelines can be slow. The letter must reference the specific systems, square footage, and deduction amount being allocated.
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5Begin qualifying construction before June 30, 2026
This is the only step with a hard deadline. Ensure physical work has begun or the 5% cost threshold is met by June 30, 2026. Coordinate with your contractor to schedule installation start dates, procure long-lead equipment, and create a paper trail of costs incurred. With 86 days left, this should begin immediately for any project not already in procurement.
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6Claim the deduction on your federal tax return
The 179D deduction is claimed on Form 3115 (if you missed prior years) or directly on your current-year return. It is taken in the year the qualifying property is placed in service. Your CPA should attach the certification documentation to the return. The deduction reduces your taxable income and cannot exceed the cost of the qualifying property.
Don't Know Your 179D Number?
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Run 179D Calculator → Find a Certified ConsultantFrequently Asked Questions
Section 179D sunsets on June 30, 2026 under the One Big Beautiful Bill Act (OBBBA), Public Law 119-21. This is a construction start deadline, not a completion deadline. Projects that begin qualifying construction before June 30, 2026 remain eligible even if they finish after that date.
The 2026 maximum is $5.36 per square foot for projects that: (1) achieve 50% or greater energy savings versus the ASHRAE 90.1 reference building, and (2) meet IRS prevailing wage and apprenticeship requirements. Without PWA compliance, the maximum is $1.07/ft². Rates below the maximum scale linearly with energy savings percentage from 25% (minimum) to 50% (maximum). Source: IRS.gov.
Two groups qualify: (1) Commercial building owners who install energy-efficient improvements to buildings used in a trade or business, including offices, retail, warehouses, hotels, and 4+-story multifamily. (2) Designers of government-owned buildings — architects, engineers, and contractors — who receive an allocated deduction from the government owner. Residential buildings under 4 stories and personal-use property do not qualify.
The IRS requires either the Physical Work Test (significant physical installation has begun on-site or at an off-site manufacturing facility) or the 5% Cost Safe Harbor (at least 5% of total qualifying property costs have been paid or incurred). Signing contracts, ordering equipment, or obtaining permits alone do not meet either test. Consult a qualified tax advisor to document your start date properly before June 30.
Yes — for government-owned buildings (federal, state, and local). The building owner (the government entity) allocates the 179D deduction to the designer via a signed certification letter. The designer claims the deduction on their own return. This is among the most valuable recurring tax benefits available to architecture and engineering firms with significant public sector project portfolios. The allocation letter must be obtained before filing.