The Section 179D energy-efficient commercial building deduction ends on June 30, 2026. Not gradually, not with a phase-down, not with a transition provision. On that date, any project that hasn’t yet met one of the IRS “begin construction” tests loses eligibility, permanently. The One Big Beautiful Bill Act signed into law in July 2025 made the sunset statutory, no extension mechanism included, no extension legislation pending.
At the time of publication, there are roughly ten weeks left. That window is tight — but it’s not too tight for projects where the construction and design work is genuinely close to ready to start. This article is the last-call guide for commercial real estate developers, owners, and the design professionals (architects, engineers, contractors) who can claim the deduction on government and non-profit buildings. The audience is the person with a project currently in late-stage design or early procurement, who needs to know whether “beginning construction” before the deadline is achievable and what the accelerated schedule looks like.
For older projects already completed — anything placed in service from 2006 forward — the deduction can still be claimed retroactively through a Form 3115 change-in-accounting-method filing. That path is covered briefly at the end of this article and remains valuable post-sunset. The main event, though, is the urgent scheduling question: if you want to qualify a new project, what does it take to beat the deadline?
Section 179D in 2026: what’s ending and when
Quick context for readers who haven’t tracked this incentive recently.
Section 179D rewards energy-efficient commercial building construction and retrofits. For 2026 projects, the deduction is worth $0.59–$1.19 per square foot at the base rate, or $2.97–$5.94 per square foot if the project meets Prevailing Wage and Apprenticeship (PWA) requirements. For a 100,000-square-foot facility hitting the bonus rate, that’s roughly $594,000 in federal tax deduction.
The deduction applies to:
- Commercial building owners who install or retrofit qualifying energy-efficient systems.
- Architects, engineers, and contractors responsible for the design of energy-efficient systems in government-owned or tax-exempt-entity-owned buildings, through an allocation letter from the building owner.
Qualifying improvements include interior lighting, HVAC and hot water systems, and building envelope (insulation, roofing, windows). The project must reduce total energy and power costs by at least 25% compared to the ASHRAE 90.1 reference baseline.
The rate improvements from the Inflation Reduction Act (IRA) remain in force through the sunset. What OBBBA changed was not the deduction amount but the timeline: projects must begin construction on or before June 30, 2026. Projects that start in time can complete later and still claim the full deduction.
The Q2 2026 acceleration decision framework
The honest question for any reader considering whether to push a project into construction before June 30: is the deduction worth the risk of rushing a project that wasn’t ready?
For a 100,000-square-foot commercial project at the bonus rate, the 179D deduction is approximately $594,000 in federal deduction, which at a typical commercial tax rate translates to somewhere around $125,000–$208,000 in actual tax savings. For larger projects, this scales proportionally. For a 500,000-square-foot distribution facility the number approaches $1 million in tax savings.
That math justifies meaningful acceleration effort — but not unlimited risk. Three scenarios:
Accelerate: The project was already targeting late 2026 or early 2027 construction start. Design is 80%+ complete. Permits are expected within weeks. Long-lead equipment is already specified and can be ordered on signed LOIs. Moving the construction start from August 2026 to June 2026 is a 2-month acceleration with manageable risk.
Don’t accelerate: The project is still in schematic design. Major permits haven’t been submitted. Long-lead equipment hasn’t been specified. The owner is considering “trying to qualify” for 179D. At this stage, forcing a June 30 start introduces more risk than the deduction value justifies — rushed designs produce rework, cost overruns, and frequently non-qualifying energy performance.
Depends: The project is in the messy middle — design at 50–70% complete, permits being processed, some long-lead items specified. Here the decision depends on how much schedule slip the owner can absorb if the acceleration compresses other phases. Usually the right answer is to chase the 5% Safe Harbor (discussed below) rather than the Physical Work Test, because it requires less coordinated project readiness.
A project that’s genuinely not ready to start by June 30 should not be forced to start. The cost of building the wrong project is substantially more than the deduction is worth.
What “begin construction” actually means
The IRS provides two separate tests, and a project only needs to satisfy one of them by June 30, 2026.
Test 1: Physical Work Test
Significant physical work of a permanent nature must begin at the building site or at an off-site factory for prefabricated or modular components. The work must be integral to the qualifying energy property.
What counts:
- Foundation work (footings, slabs)
- Vertical construction (framing, structural steel erection)
- Installation of qualifying MEP systems
- Off-site factory work on integral components (e.g., custom switchgear, prefabricated panels)
What doesn’t count:
- Site clearing and grubbing
- Planning, design work, permitting
- Signing contracts
- Ordering equipment without delivery or work begun
- Preliminary activities that would occur regardless of the 179D claim
The continuity requirement applies: once physical work begins, construction must continue more or less continuously until the project is placed in service. Four calendar years is the Continuity Safe Harbor — projects placed in service within four years of construction start are automatically deemed to meet the continuity requirement.
Test 2: 5% Safe Harbor (Cost Test)
The taxpayer must pay or incur costs equal to at least 5% of the total expected cost of the qualifying energy property. These costs must be genuine — actually paid or legally incurred under binding contracts.
What counts toward the 5%:
- Equipment purchases where costs are paid or incurred (not just ordered)
- Long-lead items under binding contract where the manufacturer is incurring costs
- Labour directly allocable to the qualifying energy property
- Material purchases for the qualifying systems
What doesn’t count:
- Planning and design fees (generally excluded)
- Non-qualifying scope costs
- Deposits on orders where the vendor hasn’t yet incurred costs
Like the Physical Work Test, the 5% Safe Harbor carries a continuity requirement: continuous efforts to advance the project must follow. The four-year Continuity Safe Harbor applies here too.
The 5% Safe Harbor is usually the easier test to satisfy in the last weeks before the deadline because it doesn’t require crews on site. Paying for and taking delivery of major long-lead equipment can cross the 5% threshold without requiring the coordinated construction mobilisation that the Physical Work Test implies.
Accelerated-schedule case example: 6-month commercial build
A representative commercial project that a developer might push to qualify:
- Project: 80,000 sq ft commercial office / light industrial mixed-use building
- Total cost: ~$18M
- Target 179D benefit: ~$475,000 deduction at bonus rate
- Original schedule: Break ground September 2026, complete March 2027
- Accelerated schedule: Break ground late June 2026, complete April 2027
The accelerated version compresses pre-construction by 10 weeks rather than compressing construction itself. Key moves:
| Weeks | Activity |
|---|---|
| Weeks 1–2 (now) | Permit package finalisation, AHJ expediting |
| Weeks 2–4 | Long-lead equipment procurement (HVAC, switchgear, lighting) — targets 5% Safe Harbor |
| Weeks 4–6 | Civil contractor mobilisation, building permit approval |
| Weeks 6–8 | Site grading begins (but this alone doesn’t satisfy Physical Work Test) |
| Weeks 8–10 | Foundation work begins — clearly satisfies Physical Work Test |
| Week 10 | June 30 deadline. Both tests satisfied. |
| Months 3–11 | Normal construction sequence proceeds |
Note that the long-lead equipment procurement in weeks 2–4 is doing double duty: preserving schedule on items that would otherwise be critical path later, and building costs toward the 5% Safe Harbor as insurance in case foundation work is delayed past June 30.
Accelerated-schedule case example: 9-month warehouse build
For a larger distribution-style project, the accelerated schedule compresses differently:
- Project: 250,000 sq ft tilt-up warehouse
- Total cost: ~$32M
- Target 179D benefit: ~$1.49M deduction at bonus rate
Key moves:
- Procurement of long-lead items (racking, HVAC, lighting) can reach 5% ($1.6M) relatively easily with binding contracts for switchgear, HVAC systems, and structural steel.
- Foundation work on warehouses is typically earlier and simpler than on commercial office — a slab-on-grade pour satisfies Physical Work Test.
- The Continuity Safe Harbor gives a four-year placed-in-service window, so construction can proceed at standard pace after the deadline is met.
For warehouse owners, the 5% Safe Harbor is often easier than Physical Work Test given how much of warehouse cost concentrates in long-lead mechanical, electrical, and specialty equipment. A structural steel order with a binding contract and in-flight factory work at the mill routinely crosses the 5% threshold.
See Warehouse Build Schedule: A 9-Month Industrial Construction Gantt for the full non-accelerated schedule.
What happens after June 30, 2026
Three concurrent realities.
For new projects starting after June 30: 179D is not available. No transition provision, no phase-down, no extended eligibility for “projects already in design.” The sunset is hard. Project financial models for post-deadline starts should not include 179D benefit.
For projects that started before June 30: Full eligibility preserved under the Continuity Safe Harbor if placed in service within four years (by end of 2030 for June 2026 starts). Normal construction proceeds. Form 7205 claimed at the placed-in-service date.
For older projects placed in service 2006 through mid-2026: Retroactive claims remain available through Form 3115 Change in Accounting Method. This is the “Look-Back Study” path — a specialised engineering and tax analysis that identifies qualifying energy-efficient improvements on previously-constructed buildings. No amended returns required; the deduction is claimed in the current tax year through the 3115 filing.
Look-Back Studies are more valuable than most commercial building owners realise. Any commercial building constructed or substantially renovated between 2006 and 2025 that incorporated efficient lighting, HVAC, or envelope improvements is potentially eligible. The analysis typically costs $5,000–$20,000 and produces deductions in the $50,000–$500,000+ range depending on building size and efficiency achieved.
When acceleration isn’t worth it
Several scenarios where pushing a project to start by June 30 is likely to produce more harm than benefit:
Design is incomplete. Rushing a partial-design project into construction produces change orders, cost overruns, and commonly non-qualifying energy performance. The deduction depends on actually achieving the 25% energy savings threshold — a project designed in a rush often doesn’t hit it.
PWA compliance isn’t clearly achievable. The bonus deduction rate (5x the base) requires prevailing wage and apprenticeship compliance across the project. Rushing start dates without time to confirm labour practices can cost the 5x multiplier, reducing the deduction to the base rate — at which point the economic justification for acceleration weakens substantially.
Permits aren’t ready. A project without permits cannot meaningfully meet either the Physical Work Test or the 5% Safe Harbor — both require real activity that permits enable. “Beginning construction without permits” risks AHJ enforcement and can actually disqualify the 179D claim if the work has to be redone.
Long-lead items aren’t reliable. Ordering $1M of switchgear to hit the 5% Safe Harbor only works if the switchgear is actually going into the project. Ordering equipment that may need to be re-specified later is wasteful and doesn’t reliably satisfy the safe harbour.
Owner isn’t committed. The deduction is a tax benefit for the commercial building owner (or, in government and non-profit cases, the designer). If the owner isn’t committed to the 179D strategy — including the documentation discipline it requires — accelerating the schedule produces cost without the offsetting benefit.
FAQ
Q: How much is the 179D deduction actually worth?
For 2026 projects at the bonus rate (with PWA compliance): $2.97–$5.94 per square foot, inflation-adjusted. Base rate (without PWA): $0.59–$1.19 per square foot. For a 100,000-square-foot project at the top bonus rate, that’s approximately $594,000 in federal deduction, translating to roughly $125,000–$208,000 in actual tax savings at typical commercial tax rates.
Q: Can my project qualify if construction hasn’t started yet?
Only if you can satisfy either the Physical Work Test or the 5% Safe Harbor by June 30, 2026. With about ten weeks remaining at time of publication, projects in late-stage design with permits near-ready and long-lead equipment specified can still qualify. Projects in early design likely cannot.
Q: Does signing a contract count as “beginning construction”?
No. Contract signature alone does not satisfy either test. Under the Physical Work Test, actual physical work must begin. Under the 5% Safe Harbor, costs must be paid or incurred — meaning the vendor has actually performed work, delivered goods, or is contractually bound in a way that legally obligates the cost.
Q: Can I claim 179D on an older building I already own?
Potentially yes, through a Look-Back Study and Form 3115 filing. This covers energy-efficient improvements on buildings placed in service from 2006 forward. The Look-Back Study is a specialised engineering-tax analysis typically costing $5K–$20K and frequently producing deductions of $50K–$500K+.
Q: What about the prevailing wage and apprenticeship requirements?
PWA compliance is what unlocks the 5x bonus rate. It requires (1) paying prevailing wages per the Davis-Bacon Act for all laborers and mechanics, and (2) ensuring apprentices perform a specified percentage of total labor hours. Good-faith efforts can serve as a safe harbour for apprenticeship if genuine apprenticeship availability is limited. Documentation throughout the project is essential — PWA audits can disqualify claims that appeared compliant in theory but lacked support.
Q: What’s the difference between 179D and solar tax credits?
Different sections of the tax code. 179D covers energy-efficient commercial building property — lighting, HVAC, envelope. The renewable energy production and investment tax credits cover solar, wind, battery storage, etc. Both were modified by OBBBA but with different deadlines. Solar tax credits have their own July 4, 2026 begin-construction deadline covered in Solar + Battery Storage Project Timeline for US Utility Developers.
Q: Can I use a combination of 179D and other incentives?
Yes, with coordination. 179D can be combined with cost segregation studies, 45L residential energy credits (where applicable), and various state-level incentives. The coordination rules prevent claiming the same cost under multiple federal provisions, but the overall tax optimisation strategy typically layers 179D with other tools.
Q: Will Congress extend the deadline?
No extension legislation is currently pending. Industry groups (AIA, design firms) have been lobbying for extension since OBBBA passed, without success. Plan the schedule around the existing June 30, 2026 deadline; do not bank on extension.
Q: What documentation do I need to support the claim?
Construction contracts, schedules, invoices, engineering plans, daily logs, equipment orders, physical work records, energy modelling by DOE-approved software, prevailing wage certified payroll, apprenticeship hour tracking, and (for designers of tax-exempt buildings) allocation letters from the owner. Document everything from day one; gaps surface during IRS review and can disqualify the claim.