Does an ADU Increase Property Taxes in Utah?

The short answer, at a glance

There is no special statewide "ADU property tax exemption" in Utah. The number that matters is Utah's ordinary residential exemption, applied to the value your ADU adds.

Yes — but not by one fixed amount

In Utah, three things decide your increase: how much market value the ADU adds, whether that value qualifies for the residential exemption, and your local tax rate. Change any one and the number changes. That's why a flat "expect $2,000 a year" figure from a national article is a guess, not an answer.

Here's what actually drives it:

1. 1.Added market value — what the ADU adds to what your property would sell for (not what you paid to build it).
2. 2.Taxable percentage — 55% for value that qualifies as a primary residence, up to 100% for value that doesn't.
3. 3.Local tax rate — set by the overlapping taxing districts (county, city, school district, and special districts) that serve your parcel.

How the property-tax increase is calculated

A useful planning estimate is: added market value × taxable percentage × your local tax rate = estimated annual increase. For a unit that qualifies for the residential exemption, the taxable percentage is 55%. For value that doesn't qualify, it can be 100%.

The examples below use a 1.0% rate for illustration only — your actual rate is set by your local taxing districts, so use the rate shown on your own valuation notice or county tax record for a real estimate.

These are estimates. Your county's market value, exemption status, and local tax rate control the actual amount. Utah's property tax rates are low by national standards, but they vary by tax area.

You're taxed on added value, not on what you spend

The county values the ADU's contribution to your property's market value — which isn't simply your builder's invoice. Construction cost can inform an appraisal, but the taxable figure is the market value the assessor assigns, not the amount you spent. A $150,000 build might add more or less than $150,000 in value depending on your neighborhood and the finished result. Utah assessors are required to value property at fair market value annually.

Estimate your own increase

Plug in your numbers to see your estimated annual and monthly property-tax increase, both with and without the residential exemption.

Once you have a ballpark, the more useful question is whether the whole project pencils out for your lot.

Will your whole property be reassessed if you build an ADU?

Not the way out-of-state guides suggest. Utah already re-values your entire property to market value every year — ADU or not — so building one doesn't "trigger" a punishing whole-home reassessment. The ADU simply adds its appraised value on top of a valuation that was already moving with the market. In a normal single-parcel project, the ADU affects your existing parcel's value; it doesn't create a separate standalone tax bill.

Why the "only the ADU gets reassessed" reassurance is a California rule

Many popular ADU-tax pages tell you your existing home's assessed value "never changes," and only the new unit is added. That's a feature of California's Proposition 13, which freezes a home's assessed value until it sells. Utah has no such freeze. Utah assessors value property at fair market value, update those values annually, and physically review each property at least once every five years. So the ADU doesn't shock the system — but it does raise your total assessed value in a market-driven way.

How the assessor learns about your ADU

Permit and new-construction records typically reach your county assessor, who may appraise the new work before adding its value to your assessment. The value of new construction is added to your existing assessed value, and the exact process varies by county. You'll usually see the change reflected on your next valuation notice. If you ever build without a permit, that's a separate and serious problem — don't count on it flying under the radar.

The 45% exemption and your ADU: long-term vs. short-term rental

This is the single biggest factor most guides skip. A unit used as a primary residence — by you, a family member, or a long-term tenant — keeps Utah's 45% residential exemption, so only 55% of its value is taxed. A nightly or short-term rental is "transient use" and does not qualify for the exemption, so that value can be taxed at 100% — which can nearly double the tax on that portion.

Same $150,000 ADU value added, two different uses:

That's roughly $675 a year in difference on the same unit, driven entirely by how you use it.

How this maps to your situation

One more nuance: Utah's "one exemption per household" limit applies to your own domicile. But the exemption is also available on a unit that is a tenant's primary residence — so an owner living in the main home who rents the ADU to a long-term tenant can generally keep the exemption on both.

Which ADU type raises property taxes the most?

Detached ADUs and large additions usually create the clearest new value, because they add a whole new structure or new finished square footage. Basement/internal conversions and garage conversions can still raise your assessment, but often by less, because they convert space you already have.

The honest rule of thumb: the more market value the project adds, the more likely it raises your tax bill. Don't count on "a basement ADU won't raise my taxes" or "a garage conversion avoids reassessment" — both can add assessable value. See our legal basement apartment guide, garage conversion guide, and detached ADU guide for what each type involves.

When will the increase show up on your bill?

Utah property is valued as of January 1, and valuation notices arrive in July or August. If your ADU is under construction or newly finished around that date, when and how much shows up depends on your project's status and your county's practice — so the tax impact rarely appears the day construction ends.

• January 1 is the valuation date. Your property is assessed based on its condition and market value as of that day, including how far along any construction is.
• Notices land mid-year. Your annual valuation notice (July/August) is where you'll first see the ADU's added value and the projected tax.

Don't assume the tax impact appears the moment construction ends. Watch your next valuation notice and county tax record after the project is permitted, inspected, or added to county records.

Property tax isn't the same as permits, impact fees, utilities, insurance, or income tax

Property tax is only one line in your ADU budget. Keep it separate from the one-time and situational costs below so you don't confuse one bill for another.

If you rent the ADU, the rent is taxable income, and expenses and depreciation may be deductible — but that's a conversation for a qualified tax professional, not something to plan around from a web page. For the full budget picture, see our Utah ADU cost guide and ADU rental income guide.

What to do if the assessor's value seems too high

If the valuation notice after your ADU looks wrong, start with the property record, not the tax total. Utah's appeal process is about the market value or your exemption status — not simply that the bill feels high — and it's deadline-driven.

First, check the record for factual errors:

• Total square footage, and finished vs. unfinished area
• Number of structures and units
• Land acreage
• Property classification (primary residential vs. other)
• Whether the residential exemption is applied
• Whether the ADU was mischaracterized (e.g., counted larger or more finished than it is)

Evidence that helps: a recent appraisal, closing or refinance documents, comparable sales, your construction scope and permit records, and photos of any unfinished or non-livable space. This is general planning information, not tax or legal advice — for a formal dispute, use the official appeal process, and for income-tax questions, use a qualified professional.

Should property taxes stop you from building an ADU?

Usually, property tax alone isn't the deciding factor — but it can matter if the ADU has thin rental margins, expensive utility work, short-term-rental plans, or a high detached-build cost. The real question isn't "will taxes go up?" It's "does the whole project still make sense after taxes, financing, utilities, permits, and realistic use?"

Here's the honest version: an ADU is not automatically a smart project just because Utah has become more ADU-friendly. The right project depends on your lot, city, utilities, budget, and how you'll use it. The property-tax increase is real, but it's rarely the thing that makes or breaks a plan — feasibility usually does. That's the piece worth pinning down before you spend on plans, permits, or quotes.

Your Utah ADU property-tax checklist (before you build)

Frequently asked questions

Sources we checked

Property tax rules, rates, and local ordinances change. We re-verify state exemption details after each legislative session, county rates and valuation timing each summer, and appeal deadlines before mid-year.

• 1.Utah State Tax Commission — Primary Residential Exemption: the 45% exemption / 55% taxable value, who qualifies, the transient-use exclusion, and valuation-notice timing.
• 2.Utah Constitution, Article XIII, §3; Utah Code §§ 59-2-102, 59-2-103, 59-2-103.5 — authority and definitions for the residential exemption and primary residence.
• 3.Utah County Assessor — FAQ and Real Property: assessment at fair market value, annual updates, five-year review, new-construction appraisal, and appeal deadlines.
• 4.Summit County Assessor — Primary Residence Exemption: short-term/nightly rentals and vacation use do not qualify for the exemption.
• 5.Utah Office of Property Rights Ombudsman — Accessory Dwelling Units (and Utah Code §11-36a-202): the internal-ADU definition and the internal-ADU impact-fee rule.
• 6.Other Utah county assessor pages reviewed for consistency (Weber, Washington, Davis, Cache, and Salt Lake County) confirm the same 55% taxable value and transient-use exclusion.