In this article
- What Section 179D Does
- Who Qualifies, and Why “Not Especially Green” Is the Wrong Instinct
- New Projects Are Over. Buildings You Already Own Are Not.
- What It’s Worth Depends Heavily on the Year
- How the Catch-Up Works, and What It Actually Nets
- If You’re a Designer Rather Than an Owner
- What We’d Need to Look at Your Portfolio
- Frequently Asked Questions
- What to Do Next
What Section 179D Does
When you build or improve a commercial building, the cost of the lighting, the HVAC and hot water systems and the building envelope goes onto a 39-year depreciation schedule. You recover it a sliver at a time, over four decades.
Section 179D changes that for energy-efficient work. It lets you deduct a portion of that cost immediately, in the year the building is placed in service, instead of waiting thirty-nine years for it.
The deduction is calculated per square foot, and it scales with how much energy the building saves against the applicable standard. That makes it unusual among tax provisions: it rewards square footage, so it gets large fast on the kind of buildings most owners think of as unremarkable.
What That Means in Money
Take a 200,000 square foot facility. Under the rules that applied through 2022, the ceiling was $1.80 per square foot — $360,000 of deduction available immediately rather than spread across thirty-nine years.
It is a deduction, not a credit, so it reduces taxable income rather than tax dollar for dollar. At a 21% corporate rate that $360,000 is worth roughly $75,000 in cash; at individual rates, considerably more. It also reduces the building’s basis, which matters at sale.
The point is simpler than the mechanics: you already paid for the lighting and the HVAC. 179D decides whether you recover that cost this year or over the next four decades.
Who Qualifies, and Why “Not Especially Green” Is the Wrong Instinct
The deduction covers three systems — interior lighting, HVAC and hot water, and the building envelope — and you can qualify on one, two or all three.
Most owners assume their building isn’t efficient enough. Frequently it is. Buildings constructed to modern energy code, or that have had a lighting retrofit or an HVAC replacement at some point, often clear the thresholds without anyone having set out to build a green building. Lighting-only partial deductions under the pre-2023 rules are the single most commonly missed claim we see.
The only way to know is to model it, and modelling is cheap relative to what it finds.
New Projects Are Over. Buildings You Already Own Are Not.
Section 179D terminated for property where construction began after 30 June 2026. That date has passed, so the deduction is closed to new work.
What most owners don’t know is that the deduction on buildings they already own did not go anywhere. If you never had a qualifying building certified, you can still claim it — reaching back as far as 2006 — and in most cases without amending a single return.
The remaining pool is now fixed. It will never grow again. But it is still sitting there.
What It’s Worth Depends Heavily on the Year
The rules changed significantly partway through the eligible period, and most look-back inventory sits in the older column:
| Placed in service | Ceiling |
|---|---|
| 2006–2022 | $1.80 per square foot lifetime per building, inflation-indexed after 2020, available in partials of roughly $0.60 per qualifying system |
| 2023 onward | Roughly $0.58 to $5.81 per square foot, scaled by energy savings and by whether prevailing wage and apprenticeship requirements were met |
The headline figures people have heard are from the post-2022 column. Applying them to a 2015 building will overstate it by roughly threefold.
How the Catch-Up Works, and What It Actually Nets
You don’t amend. Because 179D reduces the building’s basis, and the remaining basis is depreciated, never claiming it is treated as an impermissible method of accounting for depreciation — correctable through a change in accounting method on Form 3115, filed with your current-year return. It is an automatic change, so you file and proceed.
Now the part that gets oversold in this market. Claiming late doesn’t hand you the whole deduction. It hands you the deduction less the depreciation you’ve already taken on that same amount. Counterintuitively, the older the building, the smaller the catch-up.
| 200,000 sq ft building, placed in service 2015 | Amount |
|---|---|
| Gross 179D deduction had it been claimed in 2015 | $360,000 |
| Less: depreciation already taken on that basis (11 of 39 years) | ($101,538) |
| Net catch-up on your current return | $258,462 |
| Approximate cash value at 21% | ~$54,000 |
| Approximate cash value at 37% | ~$96,000 |
Real money on one building, and an owner with six of them is having a different conversation. But anyone quoting you the gross per-square-foot figure as though it were the benefit is not being straight with you.
Two Things That Disqualify Buildings Quickly
- You must have paid for the improvements. If you bought an existing building, you generally cannot claim 179D on energy work a prior owner paid for. This is the most common reason a promising property doesn’t qualify.
- A very recent building may need an amended return instead. A method of accounting isn’t established until it’s been used on two consecutively filed returns, so a building placed in service last year gets amended rather than corrected by method change.
If You’re a Designer Rather Than an Owner
Architects, engineers and design-build firms can receive 179D through an allocation from a government or tax-exempt building owner who can’t use it themselves. That is a genuinely different path with a much shorter window — an amended return within roughly three years, not a twenty-year look-back — and the accounting method change above is expressly unavailable to designers.
If that’s you, read the designer article instead. Don’t let anyone sell you a reach-back you aren’t entitled to.
What We’d Need to Look at Your Portfolio
- Your commercial properties, with square footage and the year each was placed in service
- Whether you paid for the energy-related work — lighting retrofits, HVAC replacement, roof or envelope work — or inherited it
- Whether 179D has ever been claimed on any of them
- For 2023-and-later projects, whether prevailing wage and apprenticeship requirements were met
Three Numbers
What we’ll need: How many commercial properties do you own? · Approximate total square footage · Oldest year placed in service
Enough for us to tell you whether a look-back is worth pursuing before you send anything else.
Frequently Asked Questions
Does the 30 June 2026 date affect buildings I already own?
No. That date governed when construction could begin on new projects. Buildings already placed in service are unaffected.
Do I need an energy study, or can this be estimated?
179D requires certification by a qualified individual, supported by approved energy modeling software and, in most cases, a site visit. It is an engineering deliverable rather than a spreadsheet — which is exactly what makes it defensible.
Does an automatic accounting method change mean the IRS agreed my building qualifies?
No, and it’s worth being clear. The governing procedure says explicitly that consent to the change is not a determination that the taxpayer qualifies, and that examiners may review the underlying certifications. The method change gets you the mechanism; the certification has to stand on its own.
What exactly did “beginning of construction” mean for 179D?
Treasury never defined it for this section. The statute doesn’t define it and doesn’t cross-reference anything that does. Practitioners analogize to standards developed for other energy provisions, but that is an analogy, not authority — and one notice commonly cited for this applies to different Code sections entirely. If you have a project close to the line, document the start thoroughly and take advice.
What happens at sale?
179D reduces the building’s basis, so a larger gain is recognized on disposition. For most hold periods the value of the earlier deduction still comes out ahead, but it should be modelled against your actual plans.
What to Do Next
Tell us how many properties you own, roughly how much square footage, and the oldest year any of them went into service. That’s enough to tell you whether this is worth pursuing.
The deduction on new construction is over. The deduction on the buildings you already own is not.
This article is general information about federal tax rules and is not advice for any specific situation. Eligibility depends on the building, its systems, the year placed in service, who paid for the improvements and your own tax position.
Current as of September 2026.
