How to appeal your assessment after the assessor added ADU value

Last updated 2026-07-11

Why did my assessed value jump after I built an ADU?

Assessors track new construction, and an ADU is new construction. When a permit gets pulled and finaled for a garage conversion, a backyard cottage, or a basement apartment, that triggers what most jurisdictions call a "supplemental" or "interim" assessment. The assessor adds the estimated market value of the new unit to your existing assessed value, and your next tax bill reflects both.

The mechanism varies by state. In California, Proposition 13 limits reassessment to the value of new construction only, so the rest of your house stays at its existing base year value. ^[1] Texas runs full-market reassessments annually whether you built anything or not, so the ADU just adds to an already-moving target. ^[2]

Here's the part that matters for your appeal. The number the assessor assigns to your ADU is usually a mass-appraisal estimate, not a careful individual appraisal. They look at your permit, note the square footage, apply a cost-per-square-foot schedule, and produce a figure. That schedule can be stale. It can miss functional problems with the unit. And it almost never accounts for the fact that lenders and buyers discount ADU value heavily on small lots in specific markets. That gap between the assessor's cost-based estimate and real market value is exactly where your appeal lives.

How much can an ADU add to your assessed value?

Anywhere from roughly $80,000 to over $300,000, and the spread depends on your state's assessment method, the size of the unit, and local construction costs.

In California, a 500-square-foot ADU permitted in 2023 in the Bay Area might land on a supplemental roll with a construction-cost value of $175,000 to $300,000. ^[3] The California Board of Equalization publishes annual cost tables that assessors reference, and those tables have climbed sharply since 2020. In Texas, an appraisal district might add $80,000 to $150,000 for a similar unit in a mid-tier suburban market, depending on finish level and whether it has a separate utility hookup.

The chart below shows how ADU construction costs, which form the assessor's baseline, varied by region in recent years, based on construction industry data. Your assessor is using something close to these figures. If you can show actual sales of comparable properties with ADUs that transact below those cost benchmarks, you have a real case.

One thing to keep straight: assessed value and taxable value are not always the same number. Many states apply an assessment ratio (say, 80% of market value) before applying the mill rate. Knowing your state's ratio lets you reverse-engineer what market value the assessor actually implied for your ADU. ^[4]

What is the deadline to appeal an ADU assessment?

The deadline is often 30 to 60 days from the date the county mails your supplemental or corrected notice, and that clock runs separately from your annual assessment appeal window. Missing it is the single most common way homeowners lose the right to appeal. The ADU trap is timing: the notice can arrive at a different time of year than your regular annual notice.

Most states send a separate supplemental or interim notice when new construction is added. That notice starts its own clock. The table below lists typical deadlines, but verify with your specific county because local rules vary.

State	Typical appeal window after notice	Where to verify
California	60 days from supplemental notice mailing	County Assessor website
Texas	30 days from Notice of Appraised Value	Local Appraisal District
Illinois (Cook County)	Varies by township; roughly 30 days	Cook County Assessor
New York (outside NYC)	Grievance Day, 3rd Tuesday in June typically	Town Assessor
Georgia	45 days from notice	County Board of Tax Assessors
Virginia	Varies; often 3 years for supplemental	State Code § 58.1-3983

In California, the supplemental assessment system is governed by Revenue and Taxation Code sections 75 through 75.72. ^[5] The 60-day clock runs from the date the county mails the supplemental notice, not from the date you open it or spot the change in your online record. Permit an ADU in spring, have the county final it in August, and you should expect a supplemental notice by fall with a deadline in November or December.

Texas works differently. The appraisal district may add the improvement mid-year and send a corrected notice. Texas Property Tax Code Section 25.19 requires the district to deliver notices, and Section 41.44 gives owners 30 days to protest. ^[2] Don't wait for your next annual notice. The protest clock runs from the date on the corrected one.

Cook County, Illinois, runs on a township system, so your window depends on which township your property sits in. The Cook County tax assessor tax bill has a separate reassessment cycle by township that changes your timing. Check the assessor's published reassessment schedule before you assume you have the standard window.

What evidence actually wins an ADU appeal?

Comparable sales win ADU appeals. The assessor valued your ADU with a cost approach, and you counter with what real buyers pay, because buyers in your market often won't pay dollar-for-dollar for ADU space the way construction costs suggest.

Here's the method. Find three to six properties in your neighborhood that sold within the past 12 months (24 months in a slow market) with ADUs similar to yours in size and type. Look up what they sold for. Then find comparable sales of similar properties without ADUs. The difference between those two groups is the market's revealed value for the ADU. If that number comes in lower than what the assessor implied, that's your argument.

Where to find comps:

• Your county recorder's deed database (free, public)
• Zillow and Redfin for sale price history, verified against public records
• Multiple Listing Service (MLS) data through a real estate agent, who will often pull comps for free if you ask
• Your county assessor's own comparable sales lookup, which many publish online

Permit and construction records help too. If your ADU cost you $95,000 to build and the assessor placed $160,000 on it, your contractor invoices and permit cost statement are direct evidence the cost approach is inflated. Keep every receipt.

Functional obsolescence arguments work in specific situations. An ADU built over a detached garage with no separate entrance, low ceilings, or a shared HVAC system has real limitations buyers discount. Document them with photos. A blurry phone photo taken the day of your hearing beats no photo at all.

A formal appraisal from a licensed MAI appraiser is the most persuasive single document you can bring, but it costs $500 to $1,500 and takes weeks. Worth it if your assessment increase implies more than $3,000 to $5,000 in annual extra taxes. For smaller stakes, the comp research alone is usually enough.

How do I find the sales comps I need for this appeal?

Start on your county assessor's own website. Look up three to five addresses near you that you know have ADUs. Most homeowners skip this step, and it's the easiest one. A lot of counties now flag ADU units in the property record. They may call them "secondary unit," "guest house," "granny flat," or "in-law unit." Pull those records, note the assessed value, then cross-reference with your county recorder to find sale dates and prices.

For Los Angeles County, the Assessor's portal lets you search by parcel and see improvement details. ^[6] Santa Clara County has a similar system. ^[7] If your county has no searchable portal, your county recorder's grantor-grantee index is the backup, though it's more work.

One honest limitation: in many markets, ADU sales comps are thin. There simply aren't many sold properties with verified ADUs in a tight radius. If you can't find six, use what you have and supplement with square-footage-adjusted comps of the main house only, then argue the assessor overstated the ADU's contribution to total value. A thin comp market is actually a reasonable argument for conservatism on your part, and the board should acknowledge that uncertainty.

In California, the State Board of Equalization's Assessment Appeals Manual (Assessors' Handbook Section 521) describes how boards should weigh evidence when comparable sales are limited. ^[8] Cite the manual in your appeal letter and you signal you've done real homework.

What does the formal appeal process look like step by step?

The agency names vary by state, but the structure is close to identical everywhere. Six steps, start to finish.

Step 1: Get your notice and confirm the deadline. The supplemental or corrected notice states your new assessed value and the last day to appeal. Write that date on your calendar the same day the notice arrives.

Step 2: Request the assessor's work file. Many states give you the right to see the assessor's property record card and the notes or schedules used to value your ADU. In California, you can request it before filing. In Texas, the appraisal district must make your appraisal records available for inspection. ^[2] Knowing how they built their number tells you exactly what to attack.

Step 3: File the appeal. The form goes by different names: "petition for review," "application for changed assessment," "notice of protest," or "assessment appeal application." File before the deadline even if your evidence isn't fully assembled. Missing the deadline is permanent. Gathering more evidence can happen after you file.

Step 4: Attempt an informal review. Most jurisdictions have an informal settlement stage where an assessor's office reviewer looks at your evidence and can correct the value without a full hearing. Bring your comps. Be polite and specific. A lot of ADU appeals settle right here, because the original value came from an automated schedule and the reviewer can see the market data you brought.

Step 5: Formal hearing. If informal review fails, you present to a local board, usually the Assessment Appeals Board, Board of Equalization, or Appraisal Review Board. You typically get 15 to 30 minutes. Bring a one-page summary, your comps in a grid, and your photos. Speak to the gap between the assessor's cost-based number and what properties actually sell for.

For homeowners in Gwinnett County, Georgia, the Gwinnett County tax assessor process includes a 45-day window and an internal review step before the Board of Equalization hearing.

Want a structured way to build your comp grid and write your appeal letter without hiring a contingency firm? The TaxFightBack appeal kit walks through each step with fill-in templates sized to your state's process. That said, everything in this article is enough to do it from scratch.

Can I argue the ADU shouldn't have been assessed separately at all?

Sometimes, yes. The assessor needs a legal basis for adding value, and a few situations make the addition improper.

The ADU was never permitted. If you bought a house that already had an unpermitted unit, the assessor generally can't add new construction value unless they reclassify the unit as an existing taxable improvement based on actual inspection. Some jurisdictions do assess unpermitted structures when they discover them, which is a separate argument, but if the unit was already there when you bought the place, there's no new construction trigger.

The permit was issued but the unit wasn't substantially complete during the tax year. Most states require substantial completion before a supplemental assessment attaches. If the assessor jumped the gun, the timeline of your permit and the final inspection date matter.

The ADU qualifies for a limited exclusion. California AB 1033, signed in 2023, and California's existing ADU law (Government Code 65852.2) affect how some ADUs are treated for transfer purposes, though they don't directly exempt ADUs from property tax. ^[9] Some counties do publish assessor guidance on how to treat attached versus detached ADUs under Proposition 13. Ask your county assessor's office directly whether the unit qualifies for any limited exclusion.

The unit is a replacement of prior square footage, not new construction. If you tore down a storage structure and put up a finished ADU, the assessor should only add the net increase in value, not the total new square footage. Check the property record card to see what stood there before.

What happens to my property taxes after a successful ADU appeal?

If the board reduces your assessed value, the county adjusts your tax bill and, in most cases, issues a refund or credit for any overpayment during the appeal period. The timing depends on your jurisdiction.

In California, a successful appeal produces a corrected tax bill and a refund of overpaid taxes with interest (currently 3% per annum under Revenue and Taxation Code Section 5096). ^[5] The refund check usually arrives within a few months of the board's written decision.

In Texas, if the Appraisal Review Board cuts your value, the district certifies the corrected value and the taxing units (school district, county, city) recalculate your bill. Overpayments carry 5% interest. ^[2]

Here's what many homeowners miss. A successful appeal doesn't necessarily cap future assessments. Win by showing your ADU added $80,000 in market value rather than the $150,000 the assessor claimed, and next year's assessor can look at that market again. In states without a California-style Proposition 13 base-year freeze, the fight can recur annually. In California, the reduced supplemental value becomes your new Proposition 13 base for the ADU portion, which protects it going forward.

In Montgomery County, Maryland, and many other high-value jurisdictions, properties with ADUs get flagged for triennial or annual review. The Montgomery County property tax system runs triennial assessments, so a reduction today locks in for three years.

What if the assessor's ADU value seems right but my overall assessment is still too high?

Then appeal the whole assessment, more than the ADU line. The ADU addition sometimes reveals a broader problem: the base assessment on the main house was already aggressive, and the new unit just made the total undeniable.

You can argue both things at once. The ADU addition was overstated, and the underlying house value was already above market. Present comps for the full property including the ADU that show total market value below the total assessed value. Boards respond to a clean single-number argument: "The assessor says my property is worth $875,000. The six most comparable sales in my neighborhood average $740,000. I'm asking for $740,000."

For homeowners in high-cost California markets, the Santa Clara property tax appeal system stays busy because base-year values in some neighborhoods are genuinely low while new construction assessments can be aggressive. If you're in Santa Clara County with a new ADU, check both the supplemental notice and your base assessment notice.

In Los Angeles, the Los Angeles County property tax appeal system handles tens of thousands of cases a year. The Assessment Appeals Board there publishes detailed procedures including evidence requirements, and they see ADU cases regularly now given the construction boom driven by California's 2016 to 2020 ADU law changes.

Do I need to hire an attorney or contingency firm to appeal an ADU assessment?

No. Most residential appeals, ADU appeals included, are handled by homeowners with no professional representation. The process is built to be accessible, and the evidence you need (comparable sales, your construction costs, photos) is stuff you already have or can gather for free.

Contingency firms take a cut of your first-year tax savings, typically 25% to 50% of the reduction. ^[10] On a $100,000 assessed value reduction in a county with a 1.2% effective tax rate, that's $1,200 in annual savings, with the firm keeping $300 to $600 of year one. You keep nothing extra in years two and three when that base year value keeps saving you money.

When does a professional make sense? If your total assessed value including the ADU sits above $1 million, the stakes justify a licensed property tax consultant or an MAI appraiser. Complex commercial or mixed-use situations, like a triplex with a new fourth unit, also benefit from professional help. But a standard single-family home with a newly built backyard cottage? Do it yourself.

An attorney is basically never necessary for a residential assessment appeal at the informal or board stage. They matter if you land in actual tax court, which is rare for residential cases.

Common mistakes homeowners make when appealing an ADU assessment

Missing the deadline is number one. Already covered, but it bears repeating, because the supplemental notice arrives separately from your annual tax bill and it's easy to drop in a pile and forget.

Arguing cost instead of market value. The assessor used cost-based valuation. Don't respond by arguing your contractor charged less, though that's useful supporting evidence. The legal standard is market value, which is what buyers would pay. Redirect every argument back to that standard.

Bringing only your own appraisal. Boards treat homeowner-selected appraisals as biased, even legitimate ones. Pair any appraisal with your own independent comp research so the board sees the market evidence directly, not only through the lens of your hired appraiser.

Accepting the informal review number too fast. The informal reviewer may offer a small reduction, say $20,000, to move you along. If your evidence supports $60,000, decline and go to the formal hearing. You're not penalized for rejecting an informal offer.

Not getting the decision in writing. After a win, ask for the written decision and keep it permanently. You'll need it if the assessor tries to re-add inflated value in a future year.

For Bexar County homeowners who just permitted an ADU and got a corrected notice, the Bexar County tax assessor protest process runs through the county Appraisal Review Board. The informal settlement step there is active and worth pursuing before you schedule a formal hearing.

Frequently asked questions

How long do I have to appeal after the assessor adds ADU value?

It depends on your state. California gives you 60 days from the mailing date of the supplemental notice. Texas gives you 30 days from the corrected Notice of Appraised Value. Georgia is 45 days. New York uses a fixed Grievance Day (typically the third Tuesday in June) regardless of when the change was made. Always verify with your specific county, because local rules can shorten or extend the state default.

Can I appeal if my ADU was unpermitted and the assessor found it during an inspection?

Yes, you can still appeal the value assigned. You can't easily argue the unit shouldn't be assessed at all once the assessor has documented its existence, but you can challenge whether their valuation is accurate. Pull comparable sales of properties with similar unpermitted or non-conforming units in your area. Unpermitted ADUs typically sell at a discount to permitted ones because buyers face legal exposure, and that discount should reduce your assessed value.

Will appealing my ADU assessment cause the assessor to look more closely at my whole property?

This fear is common but mostly unfounded. Assessors don't generally retaliate by raising other parts of your assessment because you filed. The appeal process is quasi-judicial, and an appeal is your legal right. In California and Texas, boards generally cannot raise your value above what you're appealing from without separate proceedings. Filing is almost always the right move if you have solid evidence.

What if I can't find comparable sales of properties with ADUs in my neighborhood?

Use what you can find, even two or three sales rather than six. Supplement with paired-sales analysis: find homes without ADUs that sold near yours, then argue that the assessor's implied value for your ADU exceeds what the market demonstrably pays for that square footage in your area. Thin comp markets are common with ADUs right now because the construction surge is recent, and boards understand that.

Does California's Proposition 13 protect me from the full ADU value being added?

Proposition 13 limits reassessment to the value of new construction only, so it doesn't stop the assessor from adding ADU value, but it does protect the rest of your house from being bumped up. The ADU value is added to your existing base-year value, then capped at 2% annual increases going forward. California Revenue and Taxation Code sections 75 through 75.72 govern how supplemental assessments work inside the Prop 13 framework.

How do I find out exactly how the assessor calculated the ADU value?

Request your property record card and the assessor's notes or cost schedule from the county assessor's office. In most states this is a public record you can request before or after filing. In California and Texas, the property record (called the appraisal record in Texas) is legally required to be made available to you. Look for the square footage used, the cost-per-square-foot rate applied, and any depreciation or adjustment factors.

Can I appeal the ADU addition even if I haven't received a formal notice yet?

In most jurisdictions, you need a notice to start the clock, and you can't file without a specific assessed value to contest. If you permitted an ADU recently and haven't gotten a supplemental notice, watch your mail and your county assessor's online portal closely. Some counties notify only by mail. If the notice goes to a prior owner's address or a lender, you could miss it without knowing.

What is the typical success rate for property tax appeals?

Reliable national data is sparse. The most commonly cited figure is that roughly 30% to 50% of residential appeals that reach a formal hearing win some reduction, based on various state reports. Nobody has clean national aggregate data on this. Your odds improve sharply when you bring actual comparable sales evidence rather than arguing on principle. ADU appeals often succeed at the informal stage because the assessor's cost schedule is visibly out of step with thin market data.

If I win my appeal, will the reduction apply to future tax years automatically?

For the year or years covered by the appeal, yes. In California, a successful supplemental appeal sets a lower Proposition 13 base year value for the ADU, which carries forward with only 2% annual increases. In other states with annual reassessment, the reduction applies to the contested year only, and the assessor can reassess next year at market value again. Get the board's written decision and track your assessment each year going forward.

Does building an ADU always increase my property taxes?

Almost certainly, to some degree. Even in California under Prop 13, the new construction value is added to your base. The real question is whether the assessor added the right amount. In high-construction-cost markets the assessor's schedule often overstates the market value an ADU contributes, especially for smaller units on constrained lots. The tax increase is real. The argument is about how large it should be.

Should I hire an MAI appraiser or can I do the comparable sales research myself?

For most residential ADU appeals, self-researched comparable sales are enough at the informal and board hearing stages. A licensed MAI appraisal costs $500 to $1,500 and adds credibility, but it's rarely decisive on its own. If your assessment increase implies more than $3,000 to $5,000 in extra annual taxes, the appraisal cost likely pays for itself. For smaller disputes, do the comp research yourself and save the money.

Can I appeal an ADU assessment in the same appeal as my regular annual assessment?

It depends on timing. If the supplemental notice arrives before the annual appeal deadline, you may be able to combine them into one appeal by contesting the total assessed value. If they arrive on separate timelines, you'll likely file separately. Ask your county assessor's office directly whether the supplemental and annual appeals can be consolidated. Getting this wrong can get one of the appeals dismissed as untimely.

What if the assessor added square footage that is wrong, more than the per-square-foot rate?

Incorrect square footage is one of the cleanest appeals you can make. Pull your building permit, the architectural plans, and the final inspection certificate. If those documents show 480 square feet and the assessor's property record card shows 620 square feet, that discrepancy alone can win you a reduction with no comp analysis at all. Always verify the physical facts on your property record card before building a market-value argument.

Sources

1. California State Board of Equalization, Proposition 13 Overview: California's Proposition 13 limits reassessment to the value of new construction only; the rest of the property retains its base-year value.
2. Texas Comptroller of Public Accounts, Property Tax Code: Texas Property Tax Code Section 25.19 requires appraisal districts to deliver notices; Section 41.44 gives property owners 30 days to protest; overpayments carry 5% interest.
3. Lincoln Institute of Land Policy, Significant Features of the Property Tax (assessment ratios by state): Many states apply an assessment ratio below 100% of market value before applying the mill rate; knowing the ratio lets owners reverse-engineer the implied market value.
4. California Revenue and Taxation Code, Sections 75-75.72 (Supplemental Assessments) and Section 5096 (Refund Interest): California's supplemental assessment system is governed by R&TC 75-75.72; the 60-day appeal window runs from mailing date; refunds on successful appeals accrue interest at 3% per annum under R&TC 5096.
5. Los Angeles County Assessor, Property Search Portal: The LA County Assessor's portal allows parcel-level property searches showing improvement details including ADU classifications.
6. Santa Clara County Assessor's Office: Santa Clara County publishes a searchable property assessment database that includes improvement type and square footage detail used by homeowners for comp research.
7. California Department of Housing and Community Development, ADU Law Summary (Government Code 65852.2): California Government Code 65852.2 and related legislation govern ADU permitting statewide; AB 1033 (2023) introduced condo-conversion rules for ADUs but does not exempt them from property tax.
8. National Taxpayers Union Foundation, Property Tax Appeal Guide: Contingency property tax consulting firms typically charge 25% to 50% of first-year tax savings as their fee.
9. Virginia Code § 58.1-3983 (Local Tax Review): Virginia allows property owners up to three years from the tax due date to apply for a correction of an erroneous assessment, including supplemental assessments.
10. Cook County Assessor's Office, Assessment Appeal Information: Cook County assessment appeals follow a township-by-township reassessment schedule; the appeal window is approximately 30 days from the township reassessment notice.