An ADU affects your property taxes by adding its assessed value to your existing parcel assessment, so your bill goes up by roughly the ADU’s value multiplied by your local tax rate rather than through a full reassessment of the whole property. Effective property tax rates run from about 0.3% to over 2% depending on where you live, which means a $150,000 ADU can add anywhere from $450 to $3,300 a year. If you rent the unit out, let family live there without charging rent, or eventually sell the property, the federal tax picture gets more involved.
What Triggers the Higher Assessment
When your ADU is finished, the local assessor gets notice through the building department, usually at final inspection or certificate of occupancy. The assessor values the new construction at its current market rate and adds that amount to what your parcel was already assessed at. Your original home keeps its prior assessed value. You’re not paying taxes as though the whole property just sold at today’s prices.
In many jurisdictions this produces a supplemental tax bill covering the portion of the tax year remaining after completion. That bill arrives separately from your regular annual statement and reflects only the added value of the ADU. The following year, the unit’s value rolls into your standard annual assessment.
How the Assessor Values Your ADU
For new construction, assessors usually start with the cost approach: what you spent on materials and labor, adjusted for depreciation. Comparable sales of ADU-equipped homes are hard to find in most markets, so the cost method fills the gap.
Two other methods can push the number in either direction. The income approach estimates market rent for a similar unit, applies a capitalization rate, and derives a present value, and the assessor can use it whether or not the unit is actually rented. The sales comparison approach looks at nearby single-family sales and adjusts upward for the extra square footage. Which method the assessor leans on matters, because the ADU’s assessed value does not have to match your construction budget. Hot rental markets can push it above cost; slower markets can leave it below.
Estimating Your Tax Increase
Multiply the ADU’s assessed value by your local effective property tax rate. That is the approximate annual increase. A $200,000 detached ADU in an area with a 1.2% effective rate adds roughly $2,400 per year.
Conversions come in lower. A junior ADU carved out of an existing bedroom or garage with no new square footage might add only $30,000 to $60,000 in assessed value, because the assessor is valuing the conversion rather than ground-up construction. Detached new builds consistently produce larger assessment increases than conversions within the existing footprint.
Exemptions and Freezes to Check Before You Build
Homestead and homeowner exemptions on your primary residence generally continue after you add an ADU, as long as you still live in the primary home. The exemption reduces the taxable value of the parcel by a fixed amount that varies by jurisdiction.
Some states and localities offer targeted ADU exemptions to encourage affordable housing, allowing a delay in reassessment or a partial exclusion if the unit is rented to low-income tenants or reserved for family. These programs usually require filing a claim with the assessor before construction begins, and missing that deadline typically means the full assessment applies automatically.
Senior and veteran assessment freeze programs deserve a closer look. In some jurisdictions, only the new improvement gets assessed at current value while the freeze on the original home stays intact. In others, major improvements can reset the freeze entirely. Confirm with your assessor before breaking ground if you rely on one of these programs.
Getting the Assessment Right the First Time
The best way to keep the valuation fair is to hand the assessor organized records. Have ready an itemized breakdown of construction costs with receipts for materials and contractor payments, your building permits, approved site plans, and inspection records that verify square footage and room count. Many counties also send a property statement or new construction form that captures completion date, total improvement cost, and intended use. Be precise: discrepancies between your numbers and the building department’s records can trigger a physical inspection.
If the income approach would produce a lower valuation than the cost approach in your case, include rental agreements or local market rent data that supports it. Photos of the finished unit help the assessor make a fair determination without a site visit. Correcting an inflated assessment after the fact takes considerably more effort than getting the initial number right.
How to Appeal the Bill
If the supplemental bill comes in higher than your records justify, you can appeal. File an appeal application with the local assessor’s office or the clerk of the assessment review board. Deadlines are strict and vary, but 30 to 90 days from the notice date is common. Miss it and you almost always forfeit your right to contest that cycle.
At the hearing you present your evidence: actual construction costs, comparable property data, or declining market conditions. In most jurisdictions the assessor’s valuation carries a presumption of correctness, meaning the burden is on you to show it is wrong. Coming in with organized cost records and a specific dollar figure you believe the ADU should be assessed at makes a stronger case than a general complaint that the bill seems high. If the board rules in your favor, the bill gets adjusted and any overpayment is typically refunded.
If You Rent the ADU Out
Rental income from the ADU gets reported on Schedule E of Form 1040.1Internal Revenue Service. About Schedule E Form 1040 Supplemental Income and Loss You can deduct a range of expenses against that income: mortgage interest allocated to the ADU, property taxes attributable to the unit, insurance, repairs, utilities, and depreciation. Those deductions often absorb a large share of the rental income for tax purposes, especially in the early years when depreciation is largest.
Under the Modified Accelerated Cost Recovery System, a residential rental structure like a detached ADU is depreciated over 27.5 years using the mid-month convention.2Office of the Law Revision Counsel. 26 USC 168 Accelerated Cost Recovery System A $200,000 ADU (excluding land value) yields roughly $7,270 a year in depreciation deductions. That deduction reduces taxable rental income now, but it creates an obligation later when you sell.3Internal Revenue Service. Publication 527 Residential Rental Property
If you use the ADU part-time as a rental and part-time for personal purposes, expenses can only be deducted for the days the unit was actually rented or actively available for rent. Keep a calendar log of rental versus personal use days.
Capital Gains When You Sell
Section 121 lets you exclude up to $250,000 in capital gains from the sale of your principal residence, or $500,000 for married couples filing jointly, if you’ve owned and lived in the home for at least two of the five years before the sale.4Office of the Law Revision Counsel. 26 USC 121 Exclusion of Gain From Sale of Principal Residence A detached ADU that you’ve rented out is treated as a separate dwelling unit, so gain must be allocated between the primary residence portion and the rental portion. The gain on the primary residence qualifies for the Section 121 exclusion; the gain allocated to the ADU does not.5eCFR. 26 CFR 1.121-1 Exclusion of Gain From Sale or Exchange of a Principal Residence
On top of losing the exclusion for the ADU’s share, you’ll face depreciation recapture on the total depreciation you claimed or were entitled to claim. That amount is taxed at a maximum rate of 25% as unrecaptured Section 1250 gain, plus a potential 3.8% net investment income tax.6Internal Revenue Service. Property Basis Sale of Home Etc 5 Ten years of depreciation on a $200,000 ADU at roughly $7,270 a year comes to about $72,700 in recapture; at 25%, that is roughly $18,175 in additional tax, separate from any capital gains tax on appreciation.
There is one exception worth flagging. If your ADU is a junior unit that shares the same dwelling unit as your primary residence and you never claimed depreciation, IRS regulations do not require allocation between residential and non-residential use, and the entire gain may qualify for the Section 121 exclusion.5eCFR. 26 CFR 1.121-1 Exclusion of Gain From Sale or Exchange of a Principal Residence
If Family Lives There Rent-Free
Letting a family member live in your ADU without charging rent can trigger a federal gift tax reporting obligation. The IRS treats providing rent-free housing as a gift of the property’s use, valued at the fair market rent you would otherwise charge.7Internal Revenue Service. Gifts and Inheritances
For 2026, the annual gift tax exclusion is $19,000 per recipient.8Internal Revenue Service. Whats New Estate and Gift Tax If fair market rent for your ADU would exceed $19,000 over the year (roughly $1,583 a month), you’re required to file Form 709 with your tax return. Filing doesn’t necessarily mean you owe gift tax, since any amount above the annual exclusion simply reduces your lifetime gift and estate tax exemption, but failing to file when required can create problems later, especially for estate planning. Married couples can each apply the $19,000 exclusion through gift-splitting, doubling the threshold to $38,000 per year.
What Happens If Heirs Inherit the Property
When heirs inherit a property that includes an ADU, the entire property receives a stepped-up basis to fair market value at the date of the owner’s death. Appreciation during the original owner’s lifetime, including the value the ADU added, effectively disappears for capital gains purposes. Heirs who sell shortly after inheriting typically owe little or no capital gains tax because their basis matches current market value.
The stepped-up basis also wipes out the depreciation recapture problem. If the original owner claimed $100,000 in depreciation on the ADU over the years, that history does not carry over. The heirs’ basis starts fresh at the date-of-death value, which is one of the strongest tax advantages of holding rental property until death rather than selling during your lifetime.