Unpermitted work discovered during assessment: what happens next

Last updated 2026-07-09

What actually happens when an assessor discovers unpermitted work?

Your assessed value goes up, usually right away. That is the short answer.

Assessors are required by law in every state to assess property at its actual use and condition, not its permitted condition. If your house has a finished basement, a converted garage, or an addition that never got a permit, the assessor is still supposed to count it. California's State Board of Equalization states that "all real property is taxable unless the Legislature has provided a specific exemption" ^[1], and square footage is square footage regardless of whether a permit was pulled.

Discovery happens a few different ways. A routine re-inspection cycle, a neighbor complaint to the building department, a sale that required a permit search, or an aerial imagery update can all put unpermitted work on the assessor's radar. Some counties now run AI-assisted aerial photo analysis that compares permit records against visible structures. Los Angeles County has used aerial imagery cross-referenced with building permit records to flag structures that appear on satellite images but not in permit databases ^[2].

Once the assessor identifies the improvement, they typically send you a Notice of Supplemental Assessment or an amended Notice of Assessed Value showing the new higher figure. The clock for your appeal starts from that notice date, not from the discovery date. Missing that window is the single most expensive mistake homeowners make in this situation.

The building department is a separate agency from the assessor's office. Assessors often have no formal duty to report unpermitted work to code enforcement, but they may share data. Whether you get a code enforcement knock on the door depends heavily on local inter-agency data-sharing agreements. Some counties have tight integration. Others operate in silos. Don't assume an assessment notice means code enforcement is coming, and don't assume it isn't.

Does unpermitted work always increase your property taxes?

Almost always, yes. But the increase may be smaller than you expect, and sometimes the unpermitted status works in your favor on value.

Here is why. Permitted improvements are assessed at full replacement or market value contribution. Unpermitted work is assessed at a discount in many jurisdictions because a buyer cannot easily finance a home with unpermitted square footage using a conventional mortgage, the seller usually has to disclose the unpermitted status, and the buyer inherits the liability to retrofit or demolish. Those buyer risks suppress market value.

A 2019 study of residential sales in California found that unpermitted additions sold at discounts ranging from roughly 5% to 25% below comparable permitted properties, depending on the size of the addition and local code complexity ^[3]. That discount is something you can argue to your assessor.

So the real question is not "will my taxes go up" but "by how much relative to what a permitted version of this improvement would cost." If the assessor adds a finished basement at full permitted-addition rates but that basement has no egress windows and could not legally be occupied, you have a genuine argument for a lower value. Document the deficiencies. Photograph them. Get a contractor quote for what it would cost to bring the space into compliance. That quote is direct evidence of reduced market value.

If you are in Cook County or another jurisdiction with a formal comparable-sales appeal process, you can also search recent sales of homes with disclosed unpermitted square footage to use as comps.

How much can your tax bill increase when unpermitted square footage is added?

It comes down to three numbers: the assessed value added, your local assessment ratio, and your effective tax rate.

Here is a simple example. Say your assessor adds 400 square feet of unpermitted finished space at $120 per square foot (a rough Midwest median for finished basement space as of 2024, per RS Means construction cost data) ^[4]. That is $48,000 in added assessed value. If your jurisdiction assesses at 100% of market value and your effective rate is 1.2%, your annual tax increase is about $576. In a high-rate jurisdiction like parts of Illinois, where effective rates can top 2.5%, that same addition could add $1,200 or more per year.

The table below shows how the math plays out across different scenarios.

Added sq ft	$/sq ft used	Added assessed value	Rate 1.0%	Rate 1.5%	Rate 2.5%
200	$100	$20,000	$200/yr	$300/yr	$500/yr
400	$120	$48,000	$480/yr	$720/yr	$1,200/yr
600	$130	$78,000	$780/yr	$1,170/yr	$1,950/yr
800	$140	$112,000	$1,120/yr	$1,680/yr	$2,800/yr

These are rough estimates. Your county's cost-per-square-foot schedule, depreciation tables, and assessment ratio all shift the real number. You can usually find the assessor's cost schedule on their website or request it under your state's public records law. Knowing their number before you appeal is the most useful thing you can do.

For context, the average effective property tax rate in the United States was 0.87% of home value in 2023, per ATTOM Data Solutions, but state-level medians run from 0.28% in Hawaii to 2.33% in New Jersey ^[5].

Can the assessor retroactively tax unpermitted work for prior years?

Yes, in many states, and this is the part that surprises people most.

Escape assessments (also called supplemental assessments or back assessments) let the assessor go back and tax improvements that should have been on the rolls but weren't. The look-back period varies a lot by state.

California, under Revenue and Taxation Code Section 531.3, allows escape assessments up to eight years back when the assessor can show the improvement was omitted due to the owner's failure to report it ^[6]. A converted garage you finished ten years ago could generate back taxes for the last eight years, plus interest.

Texas does not use the same escape mechanism, but Texas Tax Code Section 25.21 lets the appraisal district add omitted property for the current and two prior years ^[7]. New York State allows up to six years of back assessment for omitted improvements under RPTL Section 556 ^[8].

Interest accrues on back assessments in most states. California charges 1.5% per month on delinquent escape assessment amounts, which compounds fast. If you are facing a retroactive assessment, get the math in writing from the assessor's office before you decide whether to fight it or negotiate a payment plan.

The practical takeaway: if you have unpermitted work and you are thinking about a sale, a refinance, or any permit activity on your property, get ahead of this. The escape liability does not vanish when you sell. In some states the lien follows the property.

Should you voluntarily disclose unpermitted work to the assessor?

This is the question nobody gives a straight answer to. Here is mine: probably not, with one big exception.

Voluntary disclosure does not reduce your tax liability. You still owe taxes on the improvement once it is added to the rolls, and you still owe any applicable back assessments. What you get in return is basically nothing, except possibly goodwill that has no legal weight. Some assessor offices claim they will reduce or waive penalties on escape assessments if you come forward voluntarily. Get that in writing before you rely on it.

The exception is if you are selling or refinancing. Your state may require disclosure of known unpermitted work on the seller's disclosure statement. California Civil Code Section 1102 requires sellers to disclose material facts affecting value, and unpermitted work qualifies ^[9]. Non-disclosure in a sale creates legal exposure that dwarfs any tax savings from staying quiet. In that situation, you are better off retroactively permitting what you can before listing.

If you are staying put and the work is genuinely invisible from the street or from the air, the risk calculus is different for everyone. I am not in a position to give you legal advice, but I will say this: the homeowners who get hurt worst are the ones who assumed unpermitted work would never surface and then faced an eight-year escape assessment with interest right before they needed to refinance.

What's the difference between the assessor finding unpermitted work and code enforcement finding it?

These are two separate legal processes with two separate sets of consequences, and they run on separate tracks.

The assessor's job is to value your property for tax purposes. They have broad authority to inspect property, compare records, and add improvements to the tax roll. Their finding results in a higher assessed value, a tax bill, and possibly a back assessment. That is the end of the assessor's authority. They cannot order you to demolish anything or pay a fine.

Code enforcement operates under your local municipality's building code and ordinances. If they find unpermitted work, they can issue a Notice of Violation, require you to obtain a retroactive permit (sometimes called a permit for existing construction), or, in cases where the work cannot meet code, require you to remove it. Fines for code violations vary enormously, from a few hundred dollars to tens of thousands for egregious or repeated violations.

The two agencies share data in some places and not in others. Gwinnett County in Georgia coordinates property data across departments. Montgomery County in Maryland maintains separate databases with limited cross-sharing. You cannot know your specific county's setup without asking or reading their inter-agency data policy.

One uncomfortable truth: if the assessor logs your unpermitted addition on a field inspection and it lands in their records, there is a non-zero chance that record becomes accessible to code enforcement at some point, especially during a future permit application when you must disclose existing conditions.

How do you appeal an assessment that includes unpermitted work?

The appeal process works the same way it does for any assessment dispute. What changes is your evidence strategy.

Step one: get the assessor's work product. Request the property record card and the cost breakdown they used to value the improvement. In most states this is available without a formal public records request. Just call and ask. The record card tells you the square footage they counted, the quality grade they assigned, and the cost-per-square-foot schedule they used. All three are worth checking.

Step two: identify the errors. Common ones include wrong square footage (they measured from aerial imagery, which misses overhangs and walls), wrong quality grade (they tagged "average" quality on a space that is actually low grade, no insulation, no HVAC), and no depreciation applied for unpermitted status.

Step three: build your evidence. You want a contractor estimate for bringing the space into code compliance, showing the cost gap between its current state and permitted value. You want photos documenting deficiencies: inadequate ceiling height, no egress, unfinished walls. And if you can get them, comparable sales of properties with disclosed unpermitted square footage in your area. MLS databases sometimes flag these, and a real estate agent can pull them.

Step four: file within the deadline. Deadlines range from 30 days in some Texas counties to 90 days in California from the notice date. Miss the deadline in most states and you waive your right to appeal for that assessment year. Look up your county's deadline before anything else.

If you want a structured, step-by-step process for the appeal itself, the TaxFightBack DIY Appeal Kit walks through evidence gathering, comparable selection, and the hearing format without requiring you to hire a contingency firm.

For homeowners in specific high-volume markets, see our guides on LA County property tax and Santa Clara property tax appeals, both of which have their own quirks around unpermitted ADUs.

Does retroactively permitting the work help with your property taxes?

Retroactive permitting, sometimes called a permit for existing construction or a building permit after the fact, means you go to the building department, pay a fee (often double the standard permit fee as a penalty), pass all required inspections, and get the work officially recorded.

For property taxes, retroactive permitting almost never helps you. Once the assessor has added the improvement to the roll, the improvement stays on the roll whether it is permitted or not. Permitting it does not remove value. It may actually raise assessed value slightly if the permit fee and required upgrades count as additional improvements.

Where retroactive permitting does help: mortgage financing, insurance, and resale. Most lenders will not approve a purchase loan or cash-out refinance on a home with disclosed unpermitted square footage above certain thresholds. Fannie Mae guidelines let lenders include unpermitted ADU square footage in appraisals only under specific conditions, and many lenders take a more restrictive position than the guidelines require ^[10].

If code requires you to tear out the work because it cannot meet code, that is different. Demolition of the unpermitted improvement removes it from existence, which is a factual basis to appeal for removal from the tax roll. Document the demolition with photos and a contractor's statement, then file a correction request with the assessor.

What if the unpermitted work was done by a previous owner?

For tax purposes, it does not matter.

Property taxes follow the property, not the person who made the improvements. If you bought a house with an unpermitted addition already in place, you are responsible for the taxes on it the moment the assessor discovers and assesses it. That is true even if you paid market value for the house not knowing the work was unpermitted.

Your remedy, if any, is against the seller. Most states require seller disclosure of known material defects and unpermitted work. If the seller failed to disclose and you can prove they knew (or should have known), you may have a claim for misrepresentation. But that is a civil legal matter, separate from your tax liability.

The practical protection when buying: always order a permit history search as part of due diligence. Most county building departments provide permit searches online or over the counter. For a few hundred dollars, a real estate attorney or title company can pull a full permit history. Compare it against the improvements that show up in the home inspection. If finished square footage in the house does not match the permitted square footage on record, you have a gap worth explaining before closing.

In high-value markets like Santa Clara or NYC, where ADU conversions and basement finishes are everywhere, this mismatch between actual and permitted square footage is practically the norm. Buyers should budget for potential escape assessments.

What are your deadlines and how do you find them?

Your appeal deadline starts from the date on your assessment notice, not the date the assessor visited or the date the unpermitted work was first discovered. The notice date is what matters. Write it down the day the notice arrives.

Here is a real-data snapshot of appeal filing windows for supplemental and amended assessments in major states. These apply to new or amended notices, including those triggered by unpermitted work discovery.

State	Appeal deadline from notice	Governing authority
California	60 days from notice date	Cal. R&T Code §1603 [6]
Texas	30 days from notice date (or May 15, whichever is later)	Texas Tax Code §41.44 [7]
New York	Varies by locality; typically 30-45 days from tentative roll	RPTL §524 [8]
Illinois	30 days from assessment notice	35 ILCS 200/16-55 [11]
Florida	25 days from TRIM notice (mailed in August)	Fla. Stat. §194.011 [12]
Georgia	45 days from notice of assessment	O.C.G.A. §48-5-311 [13]

These deadlines are hard. Most states have essentially no exceptions for missing them. Not illness, not travel, not ignorance of the law. If you get a notice, treat the deadline like a court date.

For county-specific deadlines and filing locations, the assessor's website is always the authoritative source. See the Bexar County tax assessor and Bibb County tax assessor guides for examples of how these play out locally.

Can you get the unpermitted work removed from your assessment if you never intended to keep it?

Yes, if you actually remove it. No, if you just say you intend to.

If the unpermitted improvement still exists on the property, it is assessable. Period. The assessor is not required to take your word that you plan to demolish it. Some homeowners try to argue that because the improvement could not get a permit, it has zero value. That argument almost never works on its own, because the improvement still occupies space and can still be used.

If you genuinely want the improvement off the tax roll, demolish it, document the demolition thoroughly (dated photos, contractor's statement, dumpster haul-away receipts), and file a correction or amended return with the assessor. In California you would file an Assessment Appeal Application or a Request for Value Review depending on the timing. In Texas you would file a Notice of Protest citing incorrect value based on changed property conditions.

One gray area: if the building department orders demolition as a code violation remedy and you comply, your case for removal is even stronger, because you have a government order documenting that the improvement is gone. Keep that order.

Removing the improvement does not erase any back assessment for the years it existed. You owe taxes for the years the improvement was on the property, even if it no longer is.

Frequently asked questions

Will the assessor report my unpermitted work to code enforcement?

Not automatically, and not in every jurisdiction. Assessors and code enforcement are separate agencies. In some counties they share data systems; in others they operate independently. The assessor's job is to value property, not enforce building codes. That said, if you later apply for a permit for other work on the property, the building department will see existing conditions and may flag anything unpermitted at that point.

How far back can the assessor tax me for unpermitted improvements?

It depends on your state. California allows escape assessments up to eight years back under Revenue and Taxation Code Section 531.3. Texas allows two prior years plus the current year under Tax Code Section 25.21. New York allows up to six years under RPTL Section 556. Interest accrues on back assessments in most states, so the total liability can grow significantly. Confirm your state's specific look-back period with your county assessor's office.

Does getting a retroactive building permit reduce my property taxes?

Rarely. Once an improvement is on the tax roll, it stays there whether it is permitted or not. Retroactive permitting helps with financing, insurance, and resale but does not remove value from the assessment. If anything, the upgrades required to pass inspection might increase value slightly. The only way to remove an improvement from the tax roll is to physically demolish it and document the demolition.

What if the previous owner built the unpermitted addition, not me?

Property taxes follow the property, not the owner who made the improvement. You are responsible for taxes on any improvement the assessor discovers, regardless of when it was built or who built it. Your legal recourse is against the seller for failure to disclose, not against the assessor. Always pull a permit history before buying any home, especially in markets with high rates of informal construction like California or New York.

Can the assessor enter my property to look for unpermitted work?

Assessors generally have statutory authority to inspect property for assessment purposes, but most state laws require reasonable notice and limit entry to exterior inspections without owner consent for interior access. In practice, most discovery happens through aerial imagery, permit record cross-checks, or voluntary access during sales. Refusing entry is your right in most states but may result in the assessor estimating value from exterior observation, which can cut either way.

How do I find the assessor's cost-per-square-foot schedule they used?

Request the property record card directly from your assessor's office. Most counties provide it free of charge, either online through their parcel search tool or by phone or mail request. The record card shows the quality grade, square footage, and cost schedule applied. In many states the assessment manual or cost manual itself is a public record you can request separately. Knowing their number is essential before you can argue it is wrong.

What evidence should I bring to appeal an assessment that added unpermitted square footage?

Bring: (1) photos documenting deficiencies such as lack of egress, no insulation, low ceiling height, and unfinished walls; (2) a contractor estimate for what it would cost to bring the space to code; (3) comparable sales of homes with disclosed unpermitted square footage showing a market discount; and (4) the assessor's own cost schedule so you can challenge their square footage count or quality grade directly. All four together make a strong factual record.

Is unpermitted work worth less for assessment purposes than equivalent permitted work?

It should be, and you can argue that it is. Unpermitted square footage carries buyer liability, financing restrictions, and potential code enforcement costs. Studies of California sales data found unpermitted additions sold at discounts of roughly 5% to 25% below comparable permitted properties. That market discount is a legitimate argument for a lower assessed value. Document the specific deficiencies and the cost to remedy them to quantify the discount for the assessor or appeals board.

What happens to unpermitted work when I sell the house?

In most states you must disclose known unpermitted work on the seller's disclosure statement. California Civil Code Section 1102 requires disclosure of material facts affecting value. The buyer will typically negotiate a price reduction, require you to retroactively permit the work, or require removal as a condition of sale. Lenders may also refuse to finance the purchase unless the situation is resolved. Non-disclosure creates legal liability that can survive closing.

What is an escape assessment and how is it different from a regular assessment?

An escape assessment is a retroactive tax assessment covering years when an improvement should have been on the tax roll but wasn't. It is not a penalty in the code enforcement sense; it is back taxes owed on value the assessor missed. Most states allow escape assessments going back two to eight years with interest. A regular assessment reflects current year value. An escape assessment adds past-year liability on top of whatever your current assessment says.

How do I know if my county shares assessor data with code enforcement?

Call both agencies and ask directly. Ask the assessor's office whether property record updates are shared with the building department, and ask code enforcement whether they receive referrals or data flags from the assessor. You can also request your county's inter-agency data sharing policy under your state's public records law. There is no universal rule; it varies county by county and sometimes changes when counties upgrade their software systems.

Can I appeal a supplemental assessment for unpermitted work after the deadline has passed?

In most states, no. Missing the appeal deadline is almost always fatal to your appeal rights for that year. A handful of states allow late appeals with a showing of good cause, but the bar is high, typically requiring documented incapacitation or a demonstrated error by the assessor, not simply forgetting. The moment you receive any assessment notice, check the deadline on the notice or on your assessor's website and put it on your calendar immediately.

Does my homeowner's insurance cover unpermitted work?

Generally no, or only partially. Most homeowner's policies exclude coverage for structures built without required permits, or will pay only what a compliant, permitted structure would have cost, not the actual replacement cost of the unpermitted work. After a loss, an insurer may also use the unpermitted status to deny or reduce a claim for the damaged portion. Review your policy's language on permitted construction and talk to your agent before assuming coverage exists.

Sources

1. California State Board of Equalization, Property Tax Overview: All real property in California is taxable unless the Legislature has provided a specific exemption
2. Los Angeles County Assessor, Assessment Process Overview: Los Angeles County uses aerial imagery cross-referenced with building permit records to identify unpermitted structures
3. Journal of Real Estate Research, 'The Effect of Unpermitted Additions on Residential Sale Prices' (2019): Unpermitted additions in California sold at discounts of roughly 5% to 25% below comparable permitted properties depending on addition size and local code complexity
4. Gordian RSMeans Construction Cost Data, Residential Cost Data 2024: Rough Midwest median cost for finished basement space approximately $120 per square foot as of 2024
5. ATTOM Data Solutions, 2023 Property Tax Report: Average effective U.S. property tax rate was 0.87% in 2023; state medians range from 0.28% in Hawaii to 2.33% in New Jersey
6. California Revenue and Taxation Code, Sections 531.3 and 1603: California allows escape assessments up to eight years back; property owners have 60 days from notice to appeal supplemental assessments
7. Texas Tax Code, Sections 25.21 and 41.44: Texas allows omitted property to be added for current year and two prior years; appeal deadline is 30 days from notice or May 15, whichever is later
8. New York Real Property Tax Law, Sections 524 and 556: New York allows back assessment of omitted improvements up to six years; appeal periods vary by locality, typically 30 to 45 days
9. California Civil Code Section 1102 (Transfer Disclosure Statement): California sellers must disclose material facts affecting value, including unpermitted work, on the statutory Transfer Disclosure Statement
10. Illinois Compiled Statutes 35 ILCS 200/16-55, Assessment Appeals: Illinois property owners have 30 days from assessment notice to file an appeal with the county board of review
11. Florida Statutes Section 194.011, Assessment Notices and Petition Filing: Florida property owners have 25 days from the TRIM notice (mailed in August) to petition the Value Adjustment Board
12. Georgia Code Section 48-5-311, Property Tax Appeals: Georgia property owners have 45 days from the county notice of assessment to file an appeal