Is Building an ADU Tax Deductible? Rentals, Basis, and Resale

Building an accessory dwelling unit is not tax deductible as a current expense. The IRS treats ADU construction as a capital improvement, so the money you spend gets added to your property’s tax basis instead of being written off the year you spend it.¹ What you can recover, and when, depends on what you do with the unit after it’s finished. Rent it out, and you deduct the cost gradually through depreciation along with your operating expenses. Keep it for personal use, and the payoff waits until you sell.

Why Construction Costs Go Into Basis Instead of a Deduction

The tax code splits spending on your property into two buckets. A repair fixes something that’s broken and keeps the property in its current condition; those costs can sometimes be deducted when incurred. An improvement adds value, extends useful life, or adapts the property to a new use, and those costs have to be capitalized. A new dwelling unit on your lot is a physical enlargement and a major addition, which puts it firmly in the improvement bucket.¹

Capitalizing the cost means adding it to your property’s basis. Basis is the number you’ll use later for depreciation if the unit becomes a rental, and for calculating gain if you sell. Everything required to get the ADU built and ready for use belongs in basis: materials, labor, architectural and engineering fees, permit fees, impact fees, utility hookup charges, and temporary construction utilities. Save receipts and contracts. You may not need them for years, but when you do, the documentation controls what you can claim.

If You Rent the ADU: What You Can Deduct

Renting the ADU at fair market value turns it into residential rental property, which opens up the real tax benefits. You report rental income and expenses on Schedule E, and the net result flows into your 1040.²

Depreciation on the Construction Cost

Depreciation is how you actually recover the money you spent building the unit. The IRS requires residential rental property to be depreciated straight-line over 27.5 years.³ Your depreciable basis is the construction cost minus any portion allocable to land, because land doesn’t wear out. Many owners use their property tax assessment ratio to make that split: if the county assesses land at 25% of total value, 75% of your construction cost is depreciable. Divide the depreciable amount by 27.5 and that’s your annual deduction.

Depreciation isn’t optional. If you skip it, the IRS still treats you as having claimed it when you sell, so you pay recapture tax on depreciation you never actually deducted. There’s no benefit to leaving it on the table.

Operating Expenses

You can also deduct the ordinary costs of running the rental: property taxes allocated to the ADU, insurance, utilities you cover for the tenant, advertising, property management fees, and routine maintenance and repairs.

Loan Interest

If you borrowed to build the ADU and the unit is a rental, the interest is a rental expense on Schedule E rather than an itemized deduction. That’s usually better: it reduces rental income directly without requiring you to itemize. If the same loan covers both your primary residence and the rental ADU, you allocate the interest between personal and rental use. Keep records that show exactly where the loan proceeds went. HELOC funds that get mixed into a general account are hard to trace, and the IRS can disallow interest you can’t tie to the project.

The 20% QBI Deduction

Rental income may qualify for the Section 199A qualified business income deduction, which lets you deduct up to 20% of net rental income if the activity rises to the level of a trade or business. The IRS has a safe harbor for rental real estate: perform at least 250 hours of rental services per year, keep separate books and records, and maintain contemporaneous logs of the work performed.⁴ Handling tenant screening, lease management, maintenance coordination, and bookkeeping yourself can get you to 250 hours. Rentals that miss the safe harbor can still qualify if they otherwise meet the trade-or-business standard.

If You Keep the ADU for Personal Use

An ADU occupied by family, guests, or you with no rent collected produces no annual deductions. No depreciation, no expense write-offs. The construction cost sits in your property’s basis and reduces your taxable gain whenever you sell. Many ADUs built for aging parents or adult children fall into this category, and the owners should expect no tax benefit until the eventual sale.

Interest on a loan used to build a personal-use ADU may still be deductible as home mortgage interest, since the funds substantially improved your home, subject to the overall cap on mortgage debt of $750,000 for mortgages taken out after December 15, 2017.⁵

Short-Term and Mixed-Use Situations

Two in-between scenarios have their own rules.

Rent the ADU for fewer than 15 days during the year and use it as a residence otherwise, and the rental income isn’t reportable at all. You also can’t deduct any rental expenses.⁶ For an owner who hosts a few weekends a year on a short-term platform, this can be a quiet windfall.

When you both use the ADU yourself and rent it during the same year, the threshold is 14 days or 10% of the days it was rented at a fair price, whichever is greater. Personal use above that line makes the ADU a residence in the IRS’s eyes, which caps your deductions at the amount of rental income; you can’t generate a loss to offset other income.⁷⁶ Stay under the threshold and the ADU is treated as a full rental, with the broader deduction rules in play.

The Loss Limits Many Owners Don’t Expect

After depreciation and expenses, a rental ADU often shows a net loss on paper. The passive activity rules decide how much of that loss you can actually use. Rental real estate is passive by default, which means losses generally can’t offset wages or other active income.

There’s a carve-out. If you actively participate in managing the rental, meaning you make the real decisions about tenants, rent, and repairs, you can deduct up to $25,000 of rental losses against other income. That $25,000 allowance starts phasing out when modified adjusted gross income exceeds $100,000, dropping by $1 for every $2 of income above the threshold, and disappears entirely at $150,000.⁸ In the housing markets where ADUs are most common, plenty of owners earn too much to benefit from the carve-out at all.

Unused losses aren’t lost. They carry forward and can offset passive income in future years. When you eventually sell the ADU or the whole property, any accumulated suspended passive losses release and become fully deductible against the gain. A narrower exception exists for taxpayers who qualify as real estate professionals by spending more than 750 hours a year in real property trades or businesses, with those hours making up more than half of all personal services performed, but that’s a tall order for someone with a day job.⁹

What Happens When You Sell

The long-term tax picture comes together at sale. The Section 121 exclusion lets a homeowner exclude up to $250,000 of gain, or $500,000 for married couples filing jointly, on the sale of a primary residence, provided they owned and lived in the home for at least two of the five years before the sale.¹⁰

An ADU that functioned as a separate rental dwelling gets treated separately from the residence at sale. You allocate the sale price and basis between the two using the same method you used for depreciation.¹¹ The residence gain still qualifies for the exclusion. The ADU gain does not, and you’ll owe capital gains tax on that portion.

Depreciation Recapture

This is the part owners most often miss. Section 121 explicitly does not exclude gain attributable to depreciation claimed (or claimable) after May 6, 1997.¹⁰ Every dollar of depreciation you deducted along the way reduced your basis and increased your gain at sale. That accumulated depreciation is recaptured at a maximum rate of 25%, which runs higher than the long-term capital gains rates most taxpayers pay on investment profits.¹² Treat depreciation as a tax loan rather than a tax gift: you get the benefit each year, and the IRS collects part of it back when you sell.

Expect a Property Tax Increase

Building an ADU will almost certainly raise your property taxes. Local assessors track permits and inspections, and the finished unit adds assessed value. Timing and method vary by jurisdiction; some areas reassess during construction, others at completion. For a rental ADU, the additional property tax goes on Schedule E as a rental expense. For a personal-use ADU, it falls under the itemized deduction for state and local taxes, currently capped at $10,000 for most filers. Either way, build the higher bill into your numbers before you break ground.

1
Internal Revenue Service. Tangible Property Final Regulations
2
Internal Revenue Service. About Schedule E (Form 1040), Supplemental Income and Loss
3
Office of the Law Revision Counsel. 26 US Code 168 – Accelerated Cost Recovery System
4
Internal Revenue Service. IRS Finalizes Safe Harbor to Allow Rental Real Estate to Qualify as a Business for Qualified Business Income Deduction
5
Internal Revenue Service. Publication 936, Home Mortgage Interest Deduction
6
Office of the Law Revision Counsel. 26 USC 280A – Disallowance of Certain Expenses in Connection With Business Use of Home
7
Internal Revenue Service. Topic No. 415, Renting Residential and Vacation Property
8
Office of the Law Revision Counsel. 26 USC 469 – Passive Activity Losses and Credits Limited
9
Internal Revenue Service. Publication 925, Passive Activity and At-Risk Rules
10
Office of the Law Revision Counsel. 26 US Code 121 – Exclusion of Gain From Sale of Principal Residence
11
eCFR. 26 CFR 1.121-1 – Exclusion of Gain From Sale or Exchange of a Principal Residence
12
Internal Revenue Service. Topic No. 409, Capital Gains and Losses