Research study
What an ADU does to your property tax bill — and what no assessor will tell you
The statute is clear and the county guidance is missing. Both of those are findings, and the second one is the reason this page exists.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Statutory citation and retrieval-date audit
The finding
Building an ADU does not reassess your house. It adds a second assessment on top of it. Revenue and Taxation Code section 71 provides that the assessor determines a new base year value for the newly constructed portion, and that “the base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed.” Your existing Proposition 13 base year value survives intact and keeps inflating at the capped rate.
The ADU is valued at what it adds to market value, not at what it cost to build. San Mateo County’s assessor states the rule as directly as anyone: “The Assessor will determine the market value (not necessarily the cost) of the construction, and add that value to the existing property assessment.” The first bill arrives out of cycle. Completion of new construction triggers a one-time supplemental assessment under Revenue and Taxation Code section 75.11, prorated from the completion date to the end of the tax year on 30 June.
And the finding nobody publishes: no county assessor we could reach has a page that names accessory dwelling units in a property tax context. Five were attempted, and every one either has no equivalent or falls back to generic new-construction guidance that treats an ADU exactly like a bedroom addition.
Nothing here is tax advice. Valuation is the assessor’s, appeal rights run to the county assessment appeals board on a deadline, and the only authoritative answer about your parcel comes from your county assessor.
Read this first
- Nothing here is tax advice, and we cannot tell you what your bill will be
Assessed value is determined by your county assessor on the facts of your parcel and your improvement. Nobody can compute it from a webpage.
The worked arithmetic further down this page is arithmetic on an assumption we invented to show the mechanism, and it is labeled as such every time it appears.
If a real number matters, the county assessor is the only source of it, and a property tax appeal runs to the county assessment appeals board on a statutory deadline.
- We could not verify Sacramento County at all
Sacramento County’s assessor site returned its homepage and nothing else. Every subpage path we attempted — new construction, supplemental assessments, the FAQ index, the property owners section — returned 404.
It is a SharePoint-style site whose link structure is not discoverable without a search engine, and web search was unavailable for this retrieval.
We have no quotes from Sacramento County and make no claim about what it publishes. That is a gap in our method, not a finding about the county.
- “No ADU page found” is not “no ADU page exists”
Three of the five counties were reached by fetching
sitemap.xmland guessing paths, because web search was unavailable. That method finds pages that are linked and indexed in the sitemap. It misses pages that are not.The honest form of our finding is: on 5 September 2026, using sitemap enumeration and direct fetches, we found no county assessor page naming accessory dwelling units in a tax context. We think that absence is real and meaningful. We are not going to call it proven.
- Five counties is not California
California has 58 county assessors. We reached five. We chose them for population and for spread rather than at random, and a different five might have produced a different answer. Treat the pattern as a strong signal that county-level ADU tax guidance is rare, not as a measurement of how rare.
Why this question is asked so much and answered so badly
Almost every California homeowner considering an ADU has the same fear in roughly the same words: that adding a unit will "trigger a reassessment" and blow up a Proposition 13 base year value they have been sitting on for twenty years.
It is the single most consequential misunderstanding in the subject, because a homeowner who believes it will not build, and the belief is wrong.
The fear is not irrational. It is a half-memory of a real rule. Reassessment of the whole property is what happens on a change in ownership.
New construction is a different trigger with a different consequence, and the statute is explicit about the difference — but a homeowner has no reason to know that, and, as this page documents, their county assessor is not telling them.
HyreADU analysis: the interesting thing is not that the law is confusing. The law is unusually clear here. The interesting thing is the publication gap.
An ADU is now among the most common forms of residential new construction in urban California, the tax question is one of the first three a homeowner asks, and the assessors who alone can answer it have — as far as we could reach — written nothing addressed to it.
The homeowner is left to generalise from a page about "building something new" and hope the generalisation holds.
What the law actually provides
Four provisions do all the work. Each is quoted from the text as retrieved on 2026-09-05, with the amendment line so you can see how settled it is.
| Provision | What it establishes | Operative language as retrieved | Currency |
|---|---|---|---|
| Cal. Const. art. XIII A, § 2(a) | Defines "full cash value" and names new construction as one of the events that resets it — for the thing constructed. | "full cash value" is the 1975–76 assessor’s valuation "or, thereafter, the appraised value of real property when purchased, newly constructed, or a change in ownership has occurred after the 1975 assessment." | Proposition 13 (1978), as amended. The 2 per cent cap sits in subdivision (b): "the inflationary rate not to exceed 2 percent for any given year." |
| Rev. & Tax. Code § 70(a) | Defines what counts as new construction. An ADU is squarely inside it. | "Any addition to real property, whether land or improvements, including fixtures, since the last lien date"; and any alteration "that constitutes a major rehabilitation thereof or that converts the property to a different use." | Amended by Stats. 2008, Ch. 336. Stable for eighteen years. |
| Rev. & Tax. Code § 71 | The provision that answers the whole question. A new base year value for the new part only; the rest is left alone. | On completion "the entire portion of property which is newly constructed shall be reappraised at its full value, and that value shall be the base year value" — while "the base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed." | Amended by Stats. 2017, Ch. 80 (AB 652), effective 21 July 2017. |
| Rev. & Tax. Code § 75.11 | The mechanism by which the new value reaches you out of the normal annual cycle: the supplemental assessment. | Where completion falls between 1 June and 31 December the supplemental assessment is "the difference between the new base year value and the taxable value on the current roll"; where it falls between 1 January and 31 May a second supplemental assessment carries "the value change due to the new construction." | Amended by Stats. 2019, Ch. 669 (SB 196), effective 1 January 2020. |
The four provisions that determine how an accessory dwelling unit is assessed in California. Sources: California Constitution and Revenue and Taxation Code as retrieved from leginfo.legislature.ca.gov on 2026-09-05.
HyreADU note on what is not here: there is no ADU-specific exclusion from new construction in the Revenue and Taxation Code. Section 74.3, which the Education Code cross-references in the school-fee context, is an exclusion for modifications "for the purpose of making the dwelling more accessible to a severely and permanently disabled person" — not for accessory dwelling units.
We checked, because the cross-reference is the sort of thing that generates a myth.
The mechanism, in the order it happens to you
Not a timeline of the build. A timeline of the assessment, which is a different thing and lags the build.
While you are building: construction in progress
Section 71 provides that "construction in progress" is appraised at full value on each lien date until completion. In practice that means a partly built ADU can appear on the roll before it is finished, at whatever the assessor judges the incomplete work to be worth.
This surprises people, because it arrives before there is anything to rent. It is not a penalty and it is not permanent — on completion the section requires the whole newly constructed portion to be reappraised and that value becomes the base year value.
On completion: a new base year value for the ADU only
The assessor determines the market value the new construction adds and establishes that as the base year value of the new portion. Riverside County states the consequence in one sentence: "The value of the existing property increases only by the amount of the addition/new construction."
Market value, not cost. San Mateo County is the most explicit of the five: "The Assessor will determine the market value (not necessarily the cost) of the construction, and add that value to the existing property assessment." This cuts both ways.
Spend $400,000 on a unit that adds $250,000 of market value and you are assessed on the $250,000. Build cheaply in a strong submarket and the reverse applies.
Then: a one-time supplemental bill, prorated
The new value does not wait for the next annual roll. Orange County: "New construction that adds value to the property will generate a one-time supplemental assessment," and "Supplemental tax bills are prorated from the date of the transfer or completion of new construction to the end of the tax year (June 30)."
Contra Costa’s Auditor-Controller gives the practical timing: "Supplemental bills and refunds are generated approximately 60 days from the date listed on your Supplemental notification." Orange County adds the case people forget: "If the supplemental assessment is a negative amount, a refund may be generated for a portion of the taxes that have already been paid."
The supplemental bill is separate from your regular bill and is not paid through an impound account by default. That is the single most common cause of an unexpected delinquency after a construction project, and it is a cash-flow fact rather than a tax rule.
Afterwards: two layers, both capped
From the following roll year the property carries the original base year value and the ADU’s base year value, each inflating by the annual factor capped at 2 per cent. Neither is re-appraised again until that portion changes ownership or undergoes further new construction.
San Mateo County states the protection plainly: "The existing property, however, is not reappraised; its assessed value will not change except for the annual inflation adjustment of up to two percent."
What five county assessors actually publish
The heart of this page. One row per county assessor, recording what we found, what we could not find, and the exact words where a quote exists. Retrieved 2026-09-05.
| County assessor | ADU page in a tax context? | What is published instead | Operative quote as retrieved | Page date shown |
|---|---|---|---|---|
| San Mateo (smcacre.gov) | No. The most detailed of the five on new construction generally, and silent on ADUs specifically. | “How construction affects taxable value”, a general new-construction page. | “The existing property, however, is not reappraised; its assessed value will not change except for the annual inflation adjustment of up to two percent.” And: “The Assessor will determine the market value (not necessarily the cost) of the construction, and add that value to the existing property assessment.” And: “New construction adds incremental value to an existing property and will generate a supplemental assessment.” | Sitemap lastmod 20 May 2025. The only one of the five carrying any date signal at all. |
| Orange (ocassessor.gov) | No. The path /adu returns 404. | Two general pages: “Building something new” and “Notice of supplemental assessment”. | “New construction that adds value to the property will generate a one-time supplemental assessment”, with the added value representing “the market value of the new improvements”. And: “Supplemental tax bills are prorated from the date of the transfer or completion of new construction to the end of the tax year (June 30).” And: “If the supplemental assessment is a negative amount, a refund may be generated for a portion of the taxes that have already been paid.” | None shown on either page. |
| Riverside (rivcoacr.org) | No. /adu, /assessor-faqs, /Assessor-FAQ and /faqs all return 404. | A general “new construction” page, located only via sitemap.xml, plus a master FAQ that does not mention ADUs. | “The value of the existing property increases only by the amount of the addition/new construction.” And: “The market value of the addition is determined and added to the existing value of property.” From the FAQ: “Any change of ownership or completion of new construction that changes the assessed value of a property will trigger a reassessment.” | None shown. |
| Contra Costa (contracosta.ca.gov) | No — and this is the sharpest case. The county does publish ADU pages. They belong to other departments and contain no tax content at all. | The county’s ADU pages sit with Conservation & Development and are zoning documents. The supplemental-assessment explanation belongs to the Auditor-Controller, not the Assessor. | Of the Conservation & Development ADU page: it “contains no information whatsoever about property tax reassessment, supplemental assessments, valuation methods, or any tax-related consequences.” From the Auditor-Controller’s supplemental bills page: “State law requires the Assessor to reappraise property as of the date of the change-in-ownership or completion of new construction”, and “Supplemental bills and refunds are generated approximately 60 days from the date listed on your Supplemental notification.” | None shown. |
| Sacramento (assessor.saccounty.gov) | Unknown — we could not verify anything. | The homepage rendered and contains no ADU mention. Every subpage path attempted returned 404: new construction, supplemental assessments, the FAQ index and the property owners section. | No quotes. We retrieved nothing beyond the homepage and make no claim about this county. | n/a |
County assessor publication on accessory dwelling units, five California counties, retrieved 2026-09-05. Quotes are reproduced as returned by direct fetch of each county’s own site.
HyreADU method note, carried forward from the retrieval log without softening: all fetches ran URL to markdown to a summarizing model, and quotes are what that pipeline reported.
Web search was unavailable, so three of the five counties were reached by sitemap enumeration and path guessing. “No ADU page found” is therefore a statement about our search, not a proof about the county.
The absence is the finding
Writing this page as though the county pages we found were about ADUs would be easy, and wrong. They are not about ADUs.
They are about "building something new", and an ADU is one instance of that.
Every quote in the table above is a general new-construction quote that we are applying to ADUs — and we are applying it, the county is not.
HyreADU analysis: the generalisation is almost certainly correct. Nothing in section 70 or 71 carves an ADU out, no county page suggests a different treatment, and the mechanism is the same one that applies to a bedroom addition.
But "almost certainly correct by generalisation from a page about something else" is a materially weaker basis than "the assessor says so", and a homeowner deciding whether to spend $250,000 deserves to know which one they are standing on.
HyreADU recommendation: ask the assessor directly and in writing, before you build.
Two questions get you a documentary answer: how will the ADU be valued for the supplemental assessment, and will any part of the existing improvement be reappraised as a result of the work.
The second question is the one worth asking, because the answer under section 71 should be no, and a written no is worth having.
The four terms people mix up
- Reassessment (change in ownership)
- The event people are afraid of. A change in ownership resets the full cash value of the property that changed hands. Building an ADU is not a change in ownership and does not trigger it.
- New construction assessment
- What actually happens. Under Rev. & Tax. Code § 71 the newly constructed portion is appraised at full value on completion and that becomes its base year value, while "the base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed." Two assessments on one parcel, not one bigger assessment.
- Supplemental assessment
- The delivery mechanism, under Rev. & Tax. Code §§ 75.10 and following. Because the value change happens mid-year, a separate prorated bill covers the period from completion to the end of the tax year, and it can be negative, in which case a refund may follow. It arrives separately from the annual bill and is commonly missed by impound accounts.
- Market value contribution
- The measure of what gets added. Not the construction cost, not the contract price, not the loan amount. San Mateo County: "the market value (not necessarily the cost) of the construction". A cost overrun does not raise the assessment, and a bargain build does not lower it.
What to do about it, in order
A reproducible sequence rather than a reassurance. Steps one and two are worth doing before you sign a construction contract.
- 1 Find your current base year value on the annual bill, not the Zillow estimate
The number the ADU is added to is your assessed value, which for a long-held California property is usually far below market.
Knowing the gap tells you how much of your Proposition 13 protection is at stake — and, under section 71, the answer is that none of it is, which is easier to believe once you have seen the two numbers side by side.
- 2 Write to the county assessor before you build, and keep the reply
Ask how the ADU will be valued for supplemental assessment purposes, and ask explicitly whether any portion of the existing improvement will be reappraised as a result of the work. Written answers from the assessor beat any website, this one included, and they are the document you will want if a later valuation surprises you.
- 3 Budget for a supplemental bill that your lender will not pay
The supplemental bill is issued separately and is typically not covered by an escrow impound account set up on the original assessment. Contra Costa puts the arrival at roughly 60 days from the notification date. Treat it as a line item in the project budget, not a surprise.
- 4 Check the valuation when it lands, against market value rather than your build cost
The statutory measure is what the improvement adds to market value. If the assessed increment looks closer to your invoice total than to a defensible market contribution, that is the point at which the question becomes worth raising — and the appeal route is the county assessment appeals board, on a filing deadline that is short and unforgiving.
- 5 If the numbers are material, get advice from someone who can see your parcel
A California property tax attorney or a licensed appraiser working in your county. Not a website. This page describes a statutory mechanism; it cannot see your assessment, your tax rate area, your direct charges or your improvement.
What we could not verify
Sacramento County entirely. The assessor’s homepage rendered and every other path we tried returned 404. We have no quotes and make no claim. This is the largest single hole in the page and it is a hole in our method, not a statement about the county.
The State Board of Equalization’s Assessors' Handbook. The BOE publishes the handbook series that county assessors work from, and section AH 410 on assessment of new construction would be the authoritative statement of practice above the county level.
We could not retrieve it on this date and have cited the Constitution and the Revenue and Taxation Code directly instead. Nothing on this page is attributed to the BOE.
Any BOE Letter to Assessors addressing accessory dwelling units. If one exists it would be the single best source for this page, and it would settle whether the generalisation from generic new-construction guidance is the department’s own view. We could not search for one. This is the first thing to check in the next edition.
The other 53 county assessors. Unattempted. We reached five and we say five.
Whether any county applies the rule differently in practice. The contract for this page anticipated variation in how assessors apply the rule.
We found no evidence of variation, and we also found almost no published county material at all — which means we are reporting an absence of evidence, not evidence of uniformity.
Those are different, and only the first one is what we measured.
Questions
Will building an ADU reassess my whole house in California?
Is the ADU assessed on what it cost me to build?
What is the supplemental tax bill and when does it arrive?
Roughly how much will an ADU add to my annual bill?
Do I get taxed while the ADU is still being built?
Is there any property tax exclusion for ADUs in California?
Does converting an existing garage count as new construction for tax purposes?
Which county assessor publishes the best guidance on this?
Written and audited by
HyreADU Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a builder. We read the permit extract, the statute, the HCD return or the fee schedule ourselves, and publish each figure with its source and retrieval date.
Where a number cannot be traced to a primary source, we leave it out and say what we could not verify. Our store-based claims cover California only.
- CA
- the only state this desk will make store-based claims about
- 5
- jurisdictions with extracted ADU permit evidence
- 735
- CSLB-verified companies in the California store
- 0
- national claims from a one-state store
How this desk works
- Primary sources only. Permit counts come from the city or county that issued the permit. Production counts come from HCD’s Annual Progress Report. Rents come from HUD or the Census. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- This is a California site. The company store is 734 California firms and one New Mexico firm. Permit evidence exists for five named jurisdictions: Los Angeles, San Francisco, Sacramento, San José and unincorporated Marin. A number from that store is titled to those places, never to the United States.
- A permit is not a completion, and a license is not an ADU grade. California licenses no ADU classification. Being named on an ADU permit is evidence of engagement in that jurisdiction, not of quality, completion, or work anywhere else. Owner-builder permits are excluded from contractor counts.
- Calculation is labeled as calculation. Figures we derive are never presented as something HCD, HUD, the Census or a city published. Terner Center research is cited as Terner’s, never restated as ours.
- We do not design, permit or build ADUs, and we take no payment for placement, ranking or a favorable mention. Pages that look like rankings are not: they publish public-record counts and let the reader decide.
- Nothing here is legal, tax or financial advice. Zoning, underwriting and appraisal practice vary by jurisdiction, lender and appraiser. The useful next step on a specific lot is the planning counter and a licensed professional.
Data as of California Constitution article XIII A and Revenue and Taxation Code §§ 70, 71 and 75.11 retrieved from leginfo.legislature.ca.gov 2026-09-05; county assessor guidance retrieved from each county’s own site 2026-09-05. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
- California Constitution, article XIII A, section 2 — full cash value and the inflation cap , Retrieved language: "full cash value" is "the county assessor’s valuation of real property as shown on the 1975–76 tax bill" or thereafter "the appraised value of real property when purchased, newly constructed, or a change in ownership has occurred after the 1975 assessment"; exclusions from "newly constructed" for disaster reconstruction and seismic retrofitting; the 2 per cent limit in subdivision (b), "the inflationary rate not to exceed 2 percent for any given year". Retrieved 2026-09-05.
- California Revenue and Taxation Code § 70 — definition of "newly constructed" , Retrieved language at subdivision (a): "Any addition to real property, whether land or improvements, including fixtures, since the last lien date"; and any alteration since the last lien date "that constitutes a major rehabilitation thereof or that converts the property to a different use". Amended by Stats. 2008, Ch. 336, Sec. 1. Retrieved 2026-09-05.
- California Revenue and Taxation Code § 71 — new base year value for the new portion only , The provision this page turns on. Retrieved language: "the base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed"; construction in progress "appraised at its full value on the lien date"; on completion "the entire portion of property which is newly constructed shall be reappraised at its full value, and that value shall be the base year value." Amended by Stats. 2017, Ch. 80, Sec. 2 (AB 652), effective 21 July 2017. Retrieved 2026-09-05.
- California Revenue and Taxation Code § 75.11 — supplemental assessment computation , Retrieved language on the two completion windows, on net supplemental assessments where multiple events fall in one assessment year, and on the roll deadlines. Amended by Stats. 2019, Ch. 669, Sec. 2 (SB 196), effective 1 January 2020. Retrieved 2026-09-05.
- California Revenue and Taxation Code § 74.3 — accessibility exclusion (checked and ruled out) , Retrieved to confirm it is NOT an ADU provision. Subdivision (a) excludes from "newly constructed" the construction, installation or modification of portions of an existing dwelling "for the purpose of making the dwelling more accessible to a severely and permanently disabled person." Amended by Stats. 1994, Ch. 146, Sec. 191. Cited here only to close off a plausible misreading created by the Education Code cross-reference at § 17620(a)(1)(C)(ii). Retrieved 2026-09-05.
- San Mateo County Assessor–County Clerk–Recorder — How construction affects taxable value , The clearest of the five and the source of the market-value-not-cost quote. "The existing property, however, is not reappraised; its assessed value will not change except for the annual inflation adjustment of up to two percent." "The Assessor will determine the market value (not necessarily the cost) of the construction, and add that value to the existing property assessment." "New construction adds incremental value to an existing property and will generate a supplemental assessment." Sitemap lastmod 20 May 2025. The page does not mention accessory dwelling units. Note smcacre.org redirects to smcacre.gov. Retrieved 2026-09-05.
- Orange County Assessor — Building something new , "New construction that adds value to the property will generate a one-time supplemental assessment", with the added value representing "the market value of the new improvements". No date shown on the page. The path /adu returns 404. Note ocgov.com/gov/assessor redirects to ocassessor.gov. Retrieved 2026-09-05.
- Orange County Assessor — Notice of supplemental assessment , "Supplemental assessments are value adjustments required by law when a property: 1. Changes ownership or 2. Undergoes new construction." "Supplemental tax bills are prorated from the date of the transfer or completion of new construction to the end of the tax year (June 30)." "If the supplemental assessment is a negative amount, a refund may be generated for a portion of the taxes that have already been paid." No date shown. Retrieved 2026-09-05.
- Riverside County Assessor–County Clerk–Recorder — New construction , Located via the site sitemap after /adu, /assessor-faqs, /Assessor-FAQ and /faqs all returned 404. "The value of the existing property increases only by the amount of the addition/new construction." "The market value of the addition is determined and added to the existing value of property." No date shown. The county’s master FAQ page contains no ADU mention and states: "Any change of ownership or completion of new construction that changes the assessed value of a property will trigger a reassessment." Retrieved 2026-09-05.
- Contra Costa County Auditor–Controller — Supplemental bills , The county’s supplemental explanation belongs to the Auditor-Controller rather than the Assessor. "State law requires the Assessor to reappraise property as of the date of the change-in-ownership or completion of new construction." "Supplemental bills and refunds are generated approximately 60 days from the date listed on your Supplemental notification." No date shown. Retrieved 2026-09-05.
- Contra Costa County Department of Conservation and Development — Accessory Dwelling Units , Cited as a negative result and the sharpest illustration of the publication gap: the county has an ADU page, it belongs to planning, and on retrieval it contained no information about property tax reassessment, supplemental assessments, valuation methods or any tax consequence. Retrieved 2026-09-05.
- Sacramento County Assessor — homepage (the unverified county) , Recorded as a retrieval failure rather than a source. The homepage rendered and contains no ADU mention; every subpage path attempted returned 404, including FAQ/Pages/NewConstruction.aspx, FAQ/Pages/default.aspx, PropertyOwners and Supplemental-Assessments.aspx. No quotes were obtained and no claim about Sacramento County is made on this page. Retrieved 2026-09-05.
- California Education Code § 17620 — school fees and the 500 square foot assessable space threshold , Retrieved in the course of ruling out an ADU property tax exclusion. Subdivision (a)(1)(C)(i) applies the levy to residential construction "only if the resulting increase in assessable space exceeds 500 square feet", and (a)(1)(C)(ii) cross-references the Rev. & Tax. Code § 74.3 accessibility exclusion. Amended by Stats. 2010, Ch. 541, Sec. 1 (AB 2048). Retrieved 2026-09-05.
The tax question should not decide this on its own
Section 71 protects the base year value on the rest of your property, and the added levy is 1 per cent of what the unit adds.
Run the whole picture — build cost, fees, rent and the supplemental bill — rather than one line of it. The calculator refuses to give you a single confident number, on purpose.
HyreADU does not design, permit or build accessory dwelling units, does not practice law and does not provide tax advice. This page is informational and is not tax advice, legal advice or financial advice.
It describes California constitutional and statutory text and county assessor publications as retrieved on 5 September 2026, and it cannot tell you what your assessment, your tax rate area or your bill will be.
Assessed value is determined by your county assessor on the facts of your parcel; appeal rights run to the county assessment appeals board on a statutory deadline.
Any dollar figure on this page labeled a HyreADU calculation is arithmetic on an assumption we chose to illustrate a mechanism, not an estimate of any real property.
The useful next step is your county assessor, in writing, and — where the sums are material — a California property tax professional.