Building an accessory dwelling unit raises your property taxes, but usually less than homeowners fear. So how does an ADU affect property taxes in practice? In most jurisdictions, the assessor appraises the ADU alone at market value and adds that figure to your existing assessed value. Your primary home’s assessment stays put. Multiply the ADU’s added value by your local tax rate and you have a close estimate of the annual increase.
How the Assessment Actually Changes
When you finish an ADU, the local assessor treats it as new construction. The office determines the market value of the ADU by itself and adds it to your property’s existing assessed value. The rest of the parcel is not reappraised just because you built an addition. That matters: a full-property reassessment could raise your bill sharply, and this approach protects you from that.
The math is simple. If your ADU adds $200,000 in assessed value and your local property tax rate is 1%, your annual property taxes go up by roughly $2,000. Rates (sometimes called mill rates) vary widely by county and municipality, so plug in your own. Some jurisdictions also issue a prorated bill for the portion of the tax year remaining after the ADU is completed, meaning your first bill may cover less than a full year.
What Determines the ADU’s Taxable Value
Assessors weigh several factors when assigning a value to the new unit:
- Square footage. Larger units get higher assessments. Assessors compare your ADU’s size against local cost-per-square-foot benchmarks for similar construction.
- Construction costs. Building permits and contractor invoices give the assessor a baseline for what the structure is worth.
- Quality of finishes. Higher-end materials, appliances, and fixtures push the assessed value up compared to basic construction.
- Market comparables. Assessors review recent sales of nearby properties with ADUs and rental rates for similar units to establish fair market value.
If your ADU is designed as a rental, the assessor may also use an income-based approach. This method estimates value based on the rent the unit could generate, adjusted for operating expenses and vacancy rates. In practice, assessors often lean on construction cost data for new builds and use the income approach as a secondary check.
Programs That Limit the Tax Increase
Many states have assessment caps that restrict how much your property’s taxable value can rise each year. Even if your ADU adds significant market value, these caps may limit the annual increase to a fixed percentage, commonly between 2% and 10% depending on the state. The cap applies to ongoing annual reassessment. The initial addition of the ADU’s value to the tax rolls is typically not subject to the cap.
Some jurisdictions offer targeted exemptions for ADUs built to house senior citizens or people with disabilities. These programs may freeze the taxable value of the residence, including the ADU, or exclude the ADU’s value from the assessment rolls entirely for a set period. To qualify, you generally file paperwork confirming the occupant meets the eligibility requirements, and you must maintain that qualifying use. If the occupancy changes and the ADU no longer meets the program’s requirements, your jurisdiction can retroactively apply the taxes you avoided, plus interest.
A handful of jurisdictions also treat conversion ADUs more favorably than new construction. Turning an existing garage or basement into a living unit is presumed to add less market value than building a new structure from scratch. Rules vary widely, so ask your local assessor’s office what programs are available before you begin construction.
The Reassessment Timeline
Reassessment begins once your local building department issues a final inspection or certificate of occupancy for the ADU. In most jurisdictions the building department automatically notifies the tax assessor’s office that new construction has been completed. From there:
- An appraiser from the assessor’s office may visit the property to verify size, layout, and construction quality. Some jurisdictions handle this review remotely using permit records and aerial imagery.
- The assessor sets a market value for the ADU and adds it to your existing assessed value. You receive a notice of assessed value showing the updated total.
- Your next property tax bill reflects the higher assessment. Some jurisdictions issue a separate prorated bill covering the period between the ADU’s completion date and the end of the tax year.
- You have a limited window to challenge the new assessment, typically 30 to 90 days depending on the jurisdiction.
Some counties process new construction assessments within a few weeks. Others take several months, especially when a physical site visit is required.
Documents to Keep for the Reassessment
Organized records make the assessment smoother and give you a stronger position if you appeal. Keep:
- Building permits, which establish the scope and legality of the work and give the assessor a starting point.
- Construction cost records: signed contracts, invoices, and receipts for materials and labor that document what you actually spent.
- The certificate of occupancy, which confirms the ADU passed final inspection. Cross-check the square footage listed here against your other records.
- Any new construction statement your jurisdiction requires you to file reporting completion. Your local assessor’s office or building department can provide the correct form.
Accurate cost records matter most. If the assessor’s valuation significantly exceeds your documented construction costs, you have a concrete basis for an appeal.
How to Appeal the ADU Assessment
If you receive a notice of assessed value and believe the ADU has been overvalued, you can file a formal appeal with your local assessment review board. The deadline varies: some jurisdictions allow as few as 30 days from the date on the notice, others up to 90. Missing the deadline usually means you accept the assessment for that tax year.
To build an effective appeal, compare the assessor’s valuation against your documented construction costs, the square footage on your certificate of occupancy, and recent sales of comparable properties with ADUs in your area. If the assessor used an income approach, gather data on actual rental rates for similar units nearby. Present specific discrepancies. Review boards respond to factual arguments, not general objections that the number feels high. Many jurisdictions also offer an informal meeting with the assessor before a formal hearing, which can resolve straightforward disputes.
Federal Taxes If You Rent the ADU
If you rent your ADU, the income is taxable at the federal level. You report rental income and deductible expenses on Schedule E (Form 1040), Supplemental Income and Loss.
Reporting Income and Expenses
All rent you collect goes on Schedule E as income. You can deduct ordinary and necessary rental expenses, including property taxes allocable to the ADU, insurance, maintenance and repairs, utilities you pay for the tenant, property management fees, and depreciation of the structure. You cannot deduct the value of your own labor.
When you rent only the ADU while living in the primary home, you deduct only the portion of shared expenses that applies to the rented space. If the ADU makes up 25% of the property’s total square footage, you allocate 25% of shared costs like property taxes and insurance to the rental.
The 15-Day Rule
Rent the ADU fewer than 15 days during the year and you do not report any of the rental income, and you cannot deduct rental expenses. The rule applies regardless of how much rent you collect during those days.
Depreciation
The ADU structure (not the land underneath) can be depreciated over 27.5 years using the straight-line method. Depreciation begins when the ADU is ready and available for rent, not necessarily when a tenant moves in. It reduces your taxable rental income each year, but it also reduces your cost basis in the property, which matters when you sell.
Net Investment Income Tax
Rental income may also be subject to the 3.8% net investment income tax if your modified adjusted gross income exceeds $200,000 (single), $250,000 (married filing jointly), or $125,000 (married filing separately).
Capital Gains When You Sell
When you sell a home that includes an ADU, the federal capital gains exclusion under Section 121 lets you exclude up to $250,000 of gain ($500,000 for married couples filing jointly) if you owned and used the property as your principal residence for at least two of the five years before the sale. A rental ADU creates two wrinkles.
Nonqualified Use
Any portion of the property used for something other than your principal residence, like a rental ADU, may count as “nonqualified use.” Gain allocated to periods of nonqualified use does not qualify for the Section 121 exclusion. The allocation is based on the ratio of time the property (or portion of it) was used for nonqualified purposes compared to the total time you owned it. Periods of nonqualified use before January 1, 2009 are not counted against you.
Depreciation Recapture
Even if the rest of the gain qualifies for the exclusion, depreciation you claimed (or could have claimed) on the rental ADU is not eligible. That depreciation is taxed as ordinary income at a maximum rate of 25% when you sell. Depreciate $50,000 over several years and you owe tax on that $50,000, regardless of whether the overall sale qualifies under Section 121.
Timing matters. The longer you rent the ADU before selling, the larger the share of gain allocated to nonqualified use and the more depreciation you must recapture. Some homeowners stop renting and convert the ADU to personal use before selling, though this strategy has limitations and is worth discussing with a tax professional.
Costs Beyond Property Tax
Property tax is the recurring cost, but a few others sit alongside it. Permit-stage development impact fees fund public infrastructure and are charged separately from property taxes; many jurisdictions reduce or waive them for units under 750 square feet. Homeowners insurance premiums almost always rise because the added structure increases replacement cost and, if you rent, liability exposure. Some standard policies do not automatically extend to detached rental structures, so confirm coverage with your insurer before construction. And building an ADU without permits does not avoid the tax: unpermitted structures cannot legally be rented in most jurisdictions, can trigger fines or removal orders, and complicate any future sale when lenders and appraisers require permitted structures on record.