Construction Period Interest Capitalization: Rules and Section 266

Interest you incur on debt tied to a construction project generally cannot be deducted in the year you pay it. Under Internal Revenue Code Section 263A(f), construction period interest capitalization requires you to add that interest to the asset’s cost basis and recover it through depreciation once the property is placed in service. The rule applies to most real property construction and to long-lived or high-cost tangible personal property, with a small business exemption available for taxpayers under an inflation-adjusted gross receipts threshold.

Which Projects Trigger the Rule

The statute applies to “designated property,” and the qualifying rules split by property type.

Real property you produce is always in. Every building, structure, or improvement you construct counts, with no minimum cost and no minimum timeline. If you’re putting up a commercial building, warehouse, or apartment complex, interest capitalization applies from day one of physical work.¹

Tangible personal property is in only if it meets one of three tests:²

The property has a MACRS class life of 20 years or more (utility plants, pipelines, certain specialized manufacturing equipment).
The estimated production period exceeds two years, regardless of cost.
The estimated production period exceeds one year and estimated production cost exceeds $1 million.

The $1 million figure sits in the statute and is not indexed for inflation. A common mistake is applying the timing-and-cost tests to real property. Those tests only matter for tangible personal property. If you’re building something attached to land, the capitalization requirement applies no matter how small or short the job.

Debt unrelated to production stays deductible. A working capital line funding day-to-day operations doesn’t get swept into the calculation just because you happen to have a construction project running at the same time. What matters is whether borrowed funds can be traced or allocated to production expenditures.

One boundary worth naming: qualified residence interest on a personal home is carved out of the allocation, so building your own house doesn’t pull your home mortgage into these rules.²

The Small Business Exemption

The Tax Cuts and Jobs Act added Section 263A(i), which pulls smaller taxpayers out of the UNICAP framework entirely, interest capitalization included. If your average annual gross receipts over the prior three tax years fall below the inflation-adjusted threshold, you’re exempt.³

For tax years beginning in 2025, the threshold is $31 million.⁴ The gross receipts test aggregates related entities, not just the entity doing the construction.

Tax shelters are excluded from the exemption regardless of size. If the IRS classifies your entity as a tax shelter under Section 448(d)(3), the full UNICAP interest rules apply no matter how small the receipts.

When the Construction Period Starts and Ends

Interest is capitalized only during the “production period,” so the start and end dates drive the entire calculation.

Start Date

For real property, the period begins on the date physical construction activity begins. Excavation, grading, and pouring foundations all count as the trigger.²

For tangible personal property, the period begins when accumulated production expenditures, including planning and design costs, reach 5 percent of the total estimated production costs. Physical work doesn’t have to be underway. Cross the 5 percent line on engineering and design alone, and the clock starts.⁵

End Date

The period ends when the property is ready to be placed in service or ready to be held for sale. For a commercial building, that typically means the certificate of occupancy or substantial completion at a state where the building can perform its intended function. For property built to sell, the period ends when production is complete and the property is first held out for sale.²

The 120-Day Suspension and Its Trap

If production activities stop for at least 120 consecutive days, you can suspend capitalization starting with the first measurement period after the stoppage begins, then resume once work restarts.⁵

Here’s what catches people. Delays inherent in the construction process do not count as a cessation. Normal bad weather, scheduled shutdowns, permit delays, design issues, and ground settling are treated as part of the ongoing production period. If your project stalls four months waiting on a building permit, the clock keeps running and you keep capitalizing.

Calculating the Capitalized Amount: The Avoided Cost Method

The regulations require the Avoided Cost Method. The premise: if you hadn’t spent money on construction, you could have used those funds to pay down debt and avoid interest. The method measures how much interest you theoretically could have avoided.⁶

Your actual intentions don’t matter. The method assumes you would have paid down debt, regardless of contractual prepayment restrictions or your own preferences.

The calculation runs off Average Accumulated Production Expenditures (AAPE), a rolling average of what you’ve spent on the project during the capitalization period. AAPE includes direct construction costs, indirect costs required to be capitalized, interest capitalized in prior periods, and the adjusted basis of equipment used in production.⁷ AAPE sets the ceiling on how much interest can be capitalized in any measurement period.

Tier 1: Traced Debt

First, allocate interest from any debt whose proceeds went directly to construction costs. A project-specific construction loan is the classic case. You capitalize the actual interest charged on that traced debt, up to the AAPE amount. Multiple traced loans work through sequentially until traced interest runs out or AAPE is fully absorbed. Any traced interest above AAPE stays deductible.

Documentation carries this tier. Disbursement records and loan agreements need to show where the borrowed money actually went.

Tier 2: Non-Traced Debt

If AAPE exceeds your traced debt, the remainder is treated as funded by your general debt pool. Compute a weighted-average interest rate across your non-traced borrowings (total interest divided by total average principal) and apply that rate to the leftover AAPE. The Tier 2 figure plus the Tier 1 figure equals your capitalized interest for the period.

The calculation runs at least annually until the property is placed in service. Larger projects usually use more frequent measurement periods.

Recovering the Capitalized Interest

Capitalized interest folds into the property’s cost basis and comes back to you through depreciation once the asset is placed in service. The recovery period follows the property, not the interest:⁸

Nonresidential real property: 39 years, straight-line.
Residential rental property: 27.5 years, straight-line.⁹
Long-lived tangible personal property: the asset’s MACRS class life.

Depreciation is reported annually on Form 4562.¹⁰ Keep the capitalized interest component identifiable in your records even though it’s combined with construction costs in the depreciable basis. That separation matters on audit and if depreciation rules shift during the recovery period.

Sell the property before depreciation ends, and any remaining capitalized interest stays in your adjusted basis, reducing taxable gain at sale. The tax benefit isn’t lost; it just moves from annual deductions to a lower gain at disposition.

The Section 266 Election for Voluntary Capitalization

Section 263A(f) tells you when capitalization is required. Section 266 lets you elect it even when it isn’t.¹¹

The election covers taxes, mortgage interest, and other carrying charges on real property during development or construction. It’s most useful in two situations: holding undeveloped land while paying interest on an acquisition loan, or running a small personal-property project that falls below the $1 million designated property threshold.

The trade-off: you give up the current-year deduction and add the interest to basis, which increases future depreciation or reduces gain on sale. Taxpayers with low current income, or landowners with no rental income to shelter, often use it to smooth out the tax impact.

You make the election on a year-by-year, property-by-property basis. If some of the interest on the same property is already subject to mandatory capitalization under 263A(f), apply the mandatory rules first, then elect Section 266 for any remaining qualifying carrying charges.

Recordkeeping Pitfalls

The math is mechanical. The recordkeeping is where taxpayers get hurt. Throughout the construction period, you need to track the start and end dates of physical production activity, all direct and indirect project costs, the terms and disbursements of every loan traced or general, and the weighted-average interest rate on your non-traced debt pool.

Under-capitalizing overstates current deductions and can draw accuracy-related penalties on audit. Over-capitalizing understates current deductions, which effectively hands the government an interest-free loan. Both are wrong; only one tends to draw IRS attention.

Businesses running several projects at once need a separate AAPE calculation and separate traced-debt analysis for each one. Projects generally can’t be pooled unless the regulations specifically permit it. That per-project burden is why larger construction-heavy companies either staff UNICAP compliance internally or bring in outside specialists.

1
eCFR. 26 CFR 1.263A-8 – Requirement to Capitalize Interest
2
Office of the Law Revision Counsel. 26 USC 263A – Capitalization and Inclusion in Inventory Costs of Certain Expenses
3
Internal Revenue Service. Section 263A Costs for Self-Constructed Assets
4
Internal Revenue Service. Revenue Procedure 2024-40
5
GovInfo. 26 CFR 1.263A-12 – Production Period
6
eCFR. 26 CFR 1.263A-9 – The Avoided Cost Method
7
Internal Revenue Service. IRS Memorandum – Accumulated Production Expenditures
8
Office of the Law Revision Counsel. 26 USC 168 – Accelerated Cost Recovery System
9
Internal Revenue Service. Depreciation and Recapture 4
10
Internal Revenue Service. About Form 4562, Depreciation and Amortization
11
Office of the Law Revision Counsel. 26 USC 266 – Carrying Charges