Building an accessory dwelling unit is not tax deductible in the year you pay for it. The IRS treats ADU construction as a capital improvement, so the money you spend goes into your property’s tax basis rather than onto your current return. How you eventually recover that money depends on what happens to the unit after it’s built. Rent it out, and you can depreciate the cost over 27.5 years and write off the operating expenses. Keep it for personal use, and the payoff comes only when you sell, in the form of a higher basis and a smaller taxable gain.
Why the Construction Cost Gets Capitalized
The IRS distinguishes repairs from improvements. A repair keeps property in its current condition. An improvement adds value, extends useful life, or adapts the property to a new use. Adding a whole new dwelling unit to your lot is a physical enlargement and a material addition of a major component, which puts it squarely on the improvement side.1Internal Revenue Service. Tangible Property Final Regulations
Because it’s a capital improvement, the full construction cost is added to your property’s basis. That basis is the figure you’ll later use to calculate gain when you sell and to set up depreciation if you rent the unit. Include every expense required to get the ADU ready for use: materials, labor, architectural and engineering fees, permits, impact fees, utility hookup charges, and temporary construction utilities. Keep the receipts. You may not need them for years, but you will need them.
What You Can Deduct If You Rent the ADU
Renting the unit at fair market value throughout the year classifies it as residential rental property, and that classification unlocks the deductions people usually have in mind when they ask this question. All rental income and expenses go on Schedule E, attached to your Form 1040.2Internal Revenue Service. About Schedule E (Form 1040), Supplemental Income and Loss
Depreciation on the Building
Residential rental property is depreciated over 27.5 years using the straight-line method, which means you deduct an equal share of the cost each year.3Office of the Law Revision Counsel. 26 US Code 168 – Accelerated Cost Recovery System Your depreciable basis is the construction cost minus any allocable land value, since land can’t be depreciated. Many owners use the local property tax assessment ratio to split building from land. If your county assessment puts land at 25% of total value, 75% of your ADU cost is depreciable. Divide that by 27.5 to get the annual deduction.
Depreciation isn’t optional. If you skip it, the IRS still treats you as if you claimed it when you sell, so there is no benefit to leaving it on the table.
Operating Expenses
Beyond depreciation, ordinary and necessary rental expenses are deductible: property taxes allocated to the ADU, insurance, utilities you pay for the tenant, advertising, property management fees, and routine maintenance and repairs.
Interest on Money You Borrowed to Build
If you financed the ADU with a loan or HELOC and the unit is a rental, the interest is a rental expense on Schedule E rather than an itemized deduction. That’s often the better outcome, because it reduces rental income directly and doesn’t require you to itemize.
If instead the ADU is for personal use and the loan is secured by your primary residence, the interest may still qualify as home mortgage interest because the funds substantially improved your home. That deduction sits under the overall mortgage debt limit of $750,000 for mortgages taken out after December 15, 2017.4Internal Revenue Service. Publication 936, Home Mortgage Interest Deduction When a single loan covers both the main house and a rental ADU, you have to allocate interest between the personal and rental portions.
Keep clean records showing how you spent the loan proceeds. HELOC funds deposited into a general account and mixed with other spending are hard to trace, and the IRS can disallow the deduction when the paper trail breaks down.
When the ADU Is for Personal Use or Mixed Use
Deductions only come with rental activity, and the rules change quickly once personal use enters the picture.
Personal Use Only
If you house family members, guests, or yourself in the ADU without collecting rent, there are no annual deductions. No depreciation, no expense write-offs. The construction cost stays in your basis and reduces your gain whenever you sell.
Rented Fewer Than 15 Days
A rule worth knowing: if you use the ADU as a residence and rent it for fewer than 15 days during the year, you don’t report the rental income at all. The tradeoff is that you can’t deduct rental expenses either.5Office of the Law Revision Counsel. 26 USC 280A – Disallowance of Certain Expenses in Connection With Business Use of Home
Mixed Personal and Rental Use
When personal use and rental use share the same year, one threshold does most of the work. The unit is treated as your residence if your personal use exceeds the greater of 14 days or 10% of the days it was rented at a fair price.6Internal Revenue Service. Topic No. 415, Renting Residential and Vacation Property Cross that line and the ADU is treated like a vacation home: you report the rental income, but your expense deductions can’t exceed that income, so no rental loss is available to offset your other earnings.5Office of the Law Revision Counsel. 26 USC 280A – Disallowance of Certain Expenses in Connection With Business Use of Home Stay below the threshold and the ADU is generally treated as a rental activity, with fuller access to deductions.
Even when it qualifies as a rental, mixed use requires you to allocate expenses. If the unit was rented for 200 days and used personally for 10, roughly 95% of total expenses, including depreciation, are deductible against rental income. The rest is personal and nondeductible.
The Passive Loss Trap
Here is where many ADU owners find their deductions aren’t as useful as they expected. Rental real estate is a passive activity, and passive losses generally can’t offset wages, salary, or other active income.
There’s a carve-out for active participation. If you make the tenant, rent, and repair decisions yourself, you can deduct up to $25,000 of rental losses against other income. But the allowance phases out once your modified adjusted gross income passes $100,000, shrinking by $1 for every $2 of income above that line, and disappears entirely at $150,000.7Office of the Law Revision Counsel. 26 USC 469 – Passive Activity Losses and Credits Limited
In the housing markets where ADUs are most common, plenty of homeowners are already above that ceiling. Losses aren’t lost, though. Suspended passive losses carry forward and can offset passive income from this or other rental activities in later years. When you eventually sell the ADU or the whole property, any accumulated suspended losses are released and become fully deductible against the gain.
What Happens at Sale
The long-term tax picture is where the ADU decision really settles. Homeowners selling a primary residence can exclude up to $250,000 of gain, or $500,000 for married couples filing jointly, under the Section 121 exclusion, provided they owned and lived in the home for at least two of the five years before the sale.8Office of the Law Revision Counsel. 26 US Code 121 – Exclusion of Gain From Sale of Principal Residence
An ADU that functions as a separate dwelling unit and was used as a rental has to be treated separately from your main home at sale. You allocate the sale price and your basis between the residence portion and the rental portion using the same method you used for depreciation.9eCFR. 26 CFR 1.121-1 – Exclusion of Gain From Sale or Exchange of a Principal Residence Gain on your residence still qualifies for the Section 121 exclusion. Gain on the rented ADU portion does not, and it’s taxable as capital gain.
Depreciation Recapture
This is the part that surprises people. Section 121 explicitly does not apply to gain attributable to depreciation claimed (or that should have been claimed) after May 6, 1997.8Office of the Law Revision Counsel. 26 US Code 121 – Exclusion of Gain From Sale of Principal Residence Every dollar of depreciation you took reduced your basis, which increases the gain when you sell. That amount is recaptured and taxed at a maximum rate of 25%, higher than the long-term capital gains rate most taxpayers pay on other investments.10Internal Revenue Service. Topic No. 409, Capital Gains and Losses
Depreciation is best thought of as a tax loan. You get the benefit while you’re renting, but the IRS takes part of it back at sale, regardless of any exclusion or preferential rate on the rest of the gain.
Property Taxes Will Go Up
Building an ADU will almost certainly raise your property tax bill. Assessors track building permits and inspection records, and a new dwelling unit on the lot pushes up assessed value. Timing and methodology vary: some jurisdictions reassess when construction starts, others when it finishes.
The tax treatment of that increase follows the ADU’s use. On a rental ADU, the added property tax is a Schedule E rental expense. On 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, price the higher property tax into your plans before construction begins.