Statistics
ADU property tax statistics
41 of 70 figures are HyreADU calculations. What the statute provides, what the county assessors actually say, and what the general levy comes to on a stated increment — with the assumptions printed beside every dollar.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Statutory citation and provenance-class audit
How to use this page
Building an ADU does not reassess your house. It adds a second assessment on top of it. Revenue and Taxation Code § 71 requires the assessor to determine a new base year value for the newly constructed portion, and provides 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 ceiling of 2.0% a year.
The unit is valued at what it adds to market value, not at what it cost to build — San Mateo County’s assessor states it as directly as anyone: “The Assessor will determine the market value (not necessarily the cost) of the construction.” San Diego County’s FAQ says the value added is “usually the full construction cost,” with an over-improvement exception.
The first bill arrives out of cycle. Completion triggers a one-time supplemental assessment under § 75.11, prorated from the completion date to 30 June. San Francisco’s Assessor-Recorder works an ADU example in full, set out below; annualised, its $150,000 increment is $1,774.02 a year.
For scale, at the constitutional 1.0% general levy alone: $1,500 a year on $150,000 of added value, before any voter-approved debt rate, special assessment or Mello-Roos your tax rate area adds.
Those are HyreADU calculations on an added value we chose, not a forecast of your increment. And the finding underneath all of it: no county assessor the study could reach publishes anything naming accessory dwelling units in a property-tax context. Five were attempted.
Every one either had no equivalent page or fell back on generic new-construction guidance. 41 of 70 figures on this page are HyreADU calculations (41 hyreadu calculation, 26 agency published, 3 trade / survey estimate).
Nothing here is tax advice, and the only authoritative answer about your parcel comes from your county assessor.
Read the provenance, not just the number
Property tax is the ADU question where the fear and the law point in opposite directions, and where almost nobody publishes a number that can be checked. Every figure below therefore carries a class, and every dollar figure that is ours carries the assumption it rests on, printed in the same row.
Agency published — the constitutional and statutory text as retrieved from the Legislative Counsel’s code service, and the words a named county assessor put on its own page.
Where an assessor is quoted, the quotation is exact.
HyreADU calculation — the levy ladders, the annualisation of San Francisco’s worked supplemental, the effect of the 2.0% ceiling over ten and twenty years, the levy on an increment equal to an indexed construction median, and the tax expressed as a share of a Fair Market Rent.
All arithmetic, all on stated assumptions.
Trade / survey estimate — the occupancy and rent findings from the 2021 owner survey, used only to say why a tax-against-rent ratio does not describe every owner.
What is absent, deliberately: an estimate of what an ADU adds to assessed value. That number is genuinely local, it is the assessor’s conclusion on the facts of your parcel and your improvement, and inventing it is the failure mode this desk exists to avoid.
Every dollar of “added value” on this page is an input we chose to demonstrate a mechanism, and it is labeled as one every time.
The value study takes the question of what an ADU is worth on its own evidence, and finds that evidence thin.
The distinction that makes the whole subject tractable is that reassessment of the whole property happens on a change in ownership. New construction is a different trigger with a different consequence.
A homeowner has no reason to know that, which is why the publication gap documented below matters more than any single figure on this page.
The figures most worth knowing
Nothing here is tax advice. The only authoritative figure for your parcel is your county assessor’s, and an appeal runs to the county assessment appeals board on a statutory deadline.
The four provisions that do all the work
The law here is unusually settled — three of the four provisions below have been substantively stable for years — and it answers the question homeowners actually ask. Each row is quoted from the text as retrieved on 2026-09-05, with the amendment line so the currency is visible.
| Provision | What it establishes | Operative language as retrieved | Currency | Class |
|---|---|---|---|---|
| Cal. Const. art. XIII A, § 2 | Defines full cash value and names new construction as an event that resets it — for the thing constructed. Subdivision (b) caps the annual inflation factor. | “full cash value” is the 1975–76 valuation “or, thereafter, the appraised value of real property when purchased, newly constructed, or a change in ownership has occurred”; and “the inflationary rate not to exceed 2 percent for any given year.” | Proposition 13 (1978), as amended | Agency published |
| Rev. & Tax. Code § 70(a) | Defines new construction. An ADU is squarely inside it, and so is a garage conversion. | “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 | Agency published |
| 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 | Agency published |
| Rev. & Tax. Code § 75.11 | The mechanism by which the new value reaches you outside the annual roll: 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 | Agency published |
California Constitution and Revenue and Taxation Code as retrieved from leginfo.legislature.ca.gov on 2026-09-05. Full quotations and the surrounding analysis on the property-tax study.
What is not in the code is as important as what is. There is no ADU-specific exclusion from new construction.
SB 1164 (Newman, 2023–24) would have added Revenue and Taxation Code § 74.9 to exclude ADU construction completed between 1 January 2025 and 1 January 2030 for up to ten years; the Board of Equalization’s bill analysis estimated an annual property-tax revenue loss of about $19 million; the bill passed the Senate 29–6 on 22 May 2024 and died in the Assembly Revenue and Taxation Committee on 24 June 2024, hearing canceled at the author’s request.
Pages describing an ADU property-tax holiday under SB 1164 are describing a bill that failed. Section 74.3, which the Education Code cross-references in the school-fee context, is an exclusion for accessibility modifications for a severely and permanently disabled person — not for accessory dwelling units.
We checked, because that cross-reference is exactly the sort of thing that generates a myth.
The mechanism, in the order it happens
Not a timeline of the build. A timeline of the assessment, which lags the build and arrives on its own schedule.
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. A partly built ADU can therefore 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 occupy or let. 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 enrols it as the base year value of the new portion.
Riverside County states the consequence in a sentence: “The value of the existing property increases only by the amount of the addition/new construction.” Santa Clara County is more explicit still: “The short answer is yes, but only on the marginal value of the ADU… The existing home will not be reassessed.”
Market value, not cost — and the two are allowed to differ in both directions. San Diego County’s FAQ says the value added is “usually the full construction cost,” and then names two exceptions: an “over improvement for the area,” where construction cost exceeds the market value added, and an owner-builder who spent substantially less than the resulting increase in value.
Spend heavily on a unit in a tract that does not pay for it, and you can be assessed below the invoice. 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 supplies the practical timing: bills and refunds are generated approximately 60 days from the date on the supplemental notification.
San Francisco’s Assessor-Recorder publishes the arithmetic as a worked example, using an ADU: an increase in value of $150,000 on a unit completed in March 2026, at a combined rate of 1.18268325 per cent, prorated at 0.25 for April to June, giving approximately $443.51.
The annualised equivalent is $1,774.02 — that step is ours, and it is the number a household actually needs for a budget.
The supplemental bill is separate from the 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 an annual factor capped at 2.0%.
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.”
On a sale, everything resets. The whole property is reassessed to the sale price as a change in ownership, and the ADU’s separate base year value disappears into a single new base year for the parcel. That is worth knowing before treating the ADU increment as a permanent, separable line.
The levy, at three rates
Arithmetic on an assumption, not a forecast. The left column is an added assessed value we chose to show the mechanism at scale. The general levy is the constitutional 1.0%.
The other two rates are published figures from two named jurisdictions — San Francisco’s Assessor-Recorder used 1.18268325 per cent in its own ADU example, and the Los Angeles County Assessor’s office has described a combined toll of roughly 1.25 per cent — and they are shown to demonstrate how much voter-approved debt and direct charges can add.
They are not rates for anywhere else, and none of these is your bill.
| Added assessed value | General levy at 1.0% | Per month | At San Francisco’s 1.18268325% | At a ~1.25% combined rate | Class |
|---|---|---|---|---|---|
| $100,000 | $1,000 | $83 | $1,182.68 | $1,250.00 | HyreADU calculation |
| $150,000 | $1,500 | $125 | $1,774.02 | $1,875.00 | HyreADU calculation |
| $200,000 | $2,000 | $167 | $2,365.37 | $2,500.00 | HyreADU calculation |
| $250,000 | $2,500 | $208 | $2,956.71 | $3,125.00 | HyreADU calculation |
| $300,000 | $3,000 | $250 | $3,548.05 | $3,750.00 | HyreADU calculation |
| $400,000 | $4,000 | $333 | $4,730.73 | $5,000.00 | HyreADU calculation |
HyreADU calculation: added market value × the stated rate. The 1.0% column excludes voter-approved indebtedness, special assessments, direct charges and Mello-Roos, all of which vary by tax rate area and can add materially. The two combined rates are named jurisdictions’ published figures applied to values we chose, and are illustrations of magnitude only.
Read the difference between the columns, not just the columns. On $250,000 of added value the general levy alone is $2,500, and at the roughly 1.25 per cent combined rate described in Los Angeles County it is $3,125.00 — about $625.00 a year more.
Anyone budgeting from the constitutional 1 per cent alone is budgeting from a floor. Your tax rate area, not this page, decides the rest.
What the two per cent ceiling does over time
The ADU’s base year value is not frozen; it is capped. Article XIII A, § 2(b) limits the annual inflation factor to 2.0%, and the increment compounds at that ceiling like any other base year value.
Over the holding periods people actually plan around, that compounding is the difference between a one-line budget assumption and a real one. The illustrative increment below is $175,000; substitute your own.
| Years after completion | Factored value of the ADU increment | Annual general levy at 1.0% | Cumulative general levy to date | Class |
|---|---|---|---|---|
| Completion year | $175,000 | $1,750 | — | HyreADU calculation |
| 5 years | $193,214 | $1,932 | $9,107 | HyreADU calculation |
| 10 years | $213,324 | $2,133 | $19,162 | HyreADU calculation |
| 15 years | $235,527 | $2,355 | $30,263 | HyreADU calculation |
| 20 years | $260,041 | $2,600 | $42,520 | HyreADU calculation |
HyreADU calculation. Factored value = $175,000 × 1.02^years, the maximum the constitution permits; the actual factor in a given year can be lower. Cumulative levy sums the general levy over the period on that compounding increment. The existing base year value on the rest of the property is doing the same thing, separately and untouched.
A useful comparison, and a limited one. Ten years of general levy on a $175,000 increment comes to $19,162, and twenty years to $42,520 — against the $230,351 indexed statewide construction median on the cost study, that is about 8.3% and 18.5% of the build.
It is a real operating cost and it belongs in an ROI model. It is not the reason a project does or does not work, and the increment above is one we chose.
If the increment tracked construction cost
Several assessors say plainly that in practice the value added is often derived from cost — San Diego County’s FAQ calls it “usually the full construction cost,” and the Los Angeles County Assessor’s office has described the process as “our staff will determine the cost to build it.” That is a practice statement, not a statutory rule, and § 71 requires market value.
But it makes one arithmetic exercise worth publishing: what the general levy would be if the increment landed at the indexed construction medians on the cost study.
| If the increment equalled… | Increment | Annual general levy at 1.0% | Per month | Ten-year cumulative | Class |
|---|---|---|---|---|---|
| the indexed garage-conversion median | $138,211 | $1,382 | $115 | $15,134 | HyreADU calculation |
| the indexed Los Angeles County median | $153,567 | $1,536 | $128 | $16,815 | HyreADU calculation |
| the indexed statewide median | $230,351 | $2,304 | $192 | $25,223 | HyreADU calculation |
| the indexed Bay Area median | $272,582 | $2,726 | $227 | $29,847 | HyreADU calculation |
| the indexed detached-unit median | $276,421 | $2,764 | $230 | $30,267 | HyreADU calculation |
HyreADU calculation. The increments are the 2018–19 California ADU owner survey medians indexed to August 2026 on the DGS California Construction Cost Index (factor 1.536), which is itself a HyreADU indexation of somebody else’s survey and not a bid. The levy is the constitutional 1.0%, before any local addition.
Three reasons not to treat this as a prediction. The statute requires market value, not cost, and San Diego’s own FAQ names the two cases where they part.
Assessors starting from a cost approach typically use standardized regional costs for a quality class — the Board of Equalization’s Assessors’ Handbook AH 531 is the manual — not your invoice.
And a substantial part of what a household spends on an ADU, including design, fees and site work outside the structure, is not improvement value at all. This table shows the order of magnitude that the mechanism produces. It is not a forecast for a named parcel.
What the county assessors actually publish
What county assessors publish is the finding underneath the arithmetic. 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 officials who alone can answer it have, as far as two separate retrievals could reach, written almost nothing addressed to it.
| County assessor | ADU page in a tax context? | What is published instead | Page date shown | Class |
|---|---|---|---|---|
| San Mateo | No | A general “how construction affects taxable value” page — the most detailed of the five, and silent on ADUs specifically. | Sitemap lastmod 20 May 2025 | Agency published |
| Orange | No | Two general pages: “Building something new” and “Notice of supplemental assessment”. The path /adu returns 404. | None shown | Agency published |
| Riverside | No | A general new-construction page, findable only through sitemap.xml, plus a master FAQ that does not mention ADUs. | None shown | Agency published |
| Contra Costa | No — the sharpest case | The county does publish ADU pages. They belong to Conservation and Development and carry no tax content at all; the supplemental explanation belongs to the Auditor-Controller. | None shown | Agency published |
| Sacramento | Could not be verified | Every subpage path attempted returned 404 against a homepage that loaded. We have no quotes and make no claim about what the county publishes. | None shown | Agency published |
| Santa Clara (a separate retrieval) | Yes | A “Granny Units / Accessory Dwelling Units” page that answers the question in a paragraph and repeats it in the annual new-construction brochure: standing guidance, not a one-off FAQ. | Restated in the 2025–26 brochure | Agency published |
| Los Angeles (a separate retrieval) | Yes | An ADU page for homeowners. The quotable public explanation of method came through the press rather than the page: “Our staff will determine the cost to build it.” | Press statement 12 May 2023 | Agency published |
| San Francisco (a separate retrieval) | Not a dedicated page, but the worked example is an ADU | The Assessor-Recorder’s supplemental-assessment explainer uses an ADU completed in March 2026 with a $150,000 increase, producing a first bill of approximately $443.51. The office states it “does not provide a preliminary estimate of assessed value because we need to review what actually changed.” | 2025 factsheet | Agency published |
| San Diego (a separate retrieval) | No standalone ADU page | The new-construction rule stated generically enough to cover an ADU, with a worked room-addition example and two named exceptions where cost and market value part. | None shown | Agency published |
County assessor guidance retrieved from each county’s own site on 2026-09-05, across two retrievals — five counties for the property-tax study and four for the value study.
Three honest limits on this table. Three of the first five counties were reached by fetching sitemap.xml and constructing paths, because web search was unavailable — that method finds pages that are linked and indexed and misses pages that are not, so the finding is “not found”, not proven absence.
Sacramento County could not be verified at all: every subpage path attempted returned 404 against a homepage that loaded, so we have no quotes and make no claim about what it publishes. And nine counties is not California, which has 58 assessors.
Treat the pattern as a strong signal that county-level ADU tax guidance is rare, not as a measurement of how rare.
The tax against the rent, for the owners who let
An ROI model that omits the property-tax increment overstates the return, and one that treats the increment as catastrophic understates it.
Here is the general levy on a $200,000 illustrative increment expressed as a share of a full year of HUD Fair Market Rent for a one-bedroom unit, by metro.
Fair Market Rent is a 40th-percentile policy construct and not observed ADU rent — it is used here because it is dated, agency-published and defined the same way in every market.
| HUD Metro FMR Area | FY 2026 one-bedroom FMR | Annual at that rent | Annual general levy on $200,000 | Levy as a share of that rent | Class of the share |
|---|---|---|---|---|---|
| Los Angeles-Long Beach-Glendale | $2,328 | $27,936 | $2,000 | 7.2% | HyreADU calculation |
| San Francisco | $2,977 | $35,724 | $2,000 | 5.6% | HyreADU calculation |
| San Jose-Sunnyvale-Santa Clara | $2,982 | $35,784 | $2,000 | 5.6% | HyreADU calculation |
| Sacramento | $1,832 | $21,984 | $2,000 | 9.1% | HyreADU calculation |
| Terner/CCI median rent among surveyed owners who let | $2,000 | $24,000 | $2,000 | 8.3% | HyreADU calculation |
FMR dollars are agency-published (HUD FY 2026, retrieved 2026-09-05; Los Angeles uses the 21 May 2026 revision). The survey rent is Terner/CCI’s, from April 2021 and from owners who let. The increment, the levy and the shares are HyreADU calculations on an added value we chose.
This ratio does not describe every owner, and the survey says why. Chapple, Ganetsos and Lopez found 51 per cent of new California ADUs generating rental income and 16 per cent providing no-cost housing to a relative.
For that 16 per cent the denominator is zero and the tax is an unrecovered operating cost — which is a legitimate choice about housing a family member, not a failed investment.
The ROI calculator accepts a $0 rent for exactly that reason, and has an operating-cost field the tax increment belongs in.
Figures and claims we will not repeat
- “Building an ADU triggers a reassessment of your house”
It does not, and this is the single most consequential misunderstanding in the subject because a homeowner who believes it will not build. Reassessment of the whole property happens on a change in ownership.
New construction is a different trigger with a different consequence, and Revenue and Taxation Code § 71 says in terms that the base year value of the remainder “shall not be changed.” Santa Clara, Riverside, San Mateo and San Diego all state it in their own words.
- An ADU property-tax exemption or holiday under SB 1164
SB 1164 (Newman) would have excluded ADU construction from new construction for up to ten years.
It passed the Senate 29–6 on 22 May 2024 and died in the Assembly Revenue and Taxation Committee on 24 June 2024, hearing canceled at the author’s request.
It is not law, the Board of Equalization’s published list of new-construction exclusions does not include ADUs, and pages describing the relief in the present tense are describing a bill that failed.
- A dollar estimate of what your ADU will add to assessed value
Every added-value figure on this page is an input we chose to demonstrate a mechanism.
The real number is the assessor’s conclusion on the facts of your parcel and your improvement, and San Francisco’s Assessor-Recorder says outright that the office “does not provide a preliminary estimate of assessed value because we need to review what actually changed.” A website that offers you one before the permit is final is guessing.
- A single California property-tax rate
The constitutional general levy is 1.0% and it is the same everywhere. What sits on top of it is not: voter-approved indebtedness, special assessments, direct charges and Mello-Roos all vary by tax rate area.
San Francisco’s own worked example used 1.18268325 per cent, and the Los Angeles County Assessor’s office has described roughly 1.25 per cent. Quoting either as “the California rate” is wrong by a margin that matters over twenty years.
- Construction cost restated as assessed value
Section 71 requires market value, not cost, and San Diego County names both directions in which they part — an over-improvement for the area, where cost exceeds the value added, and an owner-builder who spent substantially less than the resulting increase.
Assessors working from a cost approach use standardized regional costs for a quality class, not your invoice. The “against cost” table on this page is an order-of-magnitude exercise and is labeled as one.
The publication gap is the finding
The law here is unusually clear, and the guidance is unusually absent. That combination is rare and it is worth naming precisely, because it is the reason a myth has survived a settled statute for nearly fifty years.
Of the five county assessors the property-tax study attempted, not one publishes a page naming accessory dwelling units in a property-tax context.
Contra Costa is the sharpest case: the county does publish ADU pages, they belong to Conservation and Development, and they contain no tax content whatsoever — while the supplemental-assessment explanation belongs to a third department, the Auditor-Controller.
A homeowner following the county’s own ADU material never encounters the tax answer at all.
A separate retrieval for the value study found that some counties do better.
Santa Clara publishes a dedicated “Granny Units / Accessory Dwelling Units” page that answers the reassessment question in a paragraph and repeats it in the annual new-construction brochure — standing guidance, not a one-off.
Los Angeles County maintains an ADU page for homeowners. San Francisco uses an ADU as the worked example in its supplemental-assessment explainer, which is the most practically useful thing any of the nine does. So the picture is not uniform. It is thin.
HyreADU analysis. The homeowner is left to generalise from a page about “building something new” and hope the generalisation holds — and it does hold, which is the frustrating part. The statute is on their side.
The two-layer structure protects the base year value they have been sitting on for twenty years. Nothing about the answer is complicated or unwelcome.
It is simply not being said by the only people with the authority to say it, in the words a homeowner would search for.
Publishing a single paragraph naming ADUs, on 58 assessor sites, would retire the most expensive misunderstanding in California ADU work at approximately zero cost.
Citing these figures
Link the property-tax study rather than this digest where you can: the full statutory quotations, the assessor quotes in context, and the retrieval limits live there.
Where a figure is labeled HyreADU calculation, attribute it to HyreADU and carry the assumption with it — “HyreADU calculation: $200,000 of added assessed value at the 1.0% general levy, before voter-approved debt,” not “an ADU costs $2,000 a year in tax.” The assumption is not decoration; without it the figure is false.
Where it is Agency published, cite the code section or the named county assessor page with its retrieval date.
Corrections are welcome and are published on the page with a dated note: hello@hyreadu.com. If your county assessor publishes ADU-specific tax guidance we have not found, that is the single input that would most improve both this digest and the study behind it.
Questions
Will building an ADU reassess my whole house?
How much will an ADU add to my property tax bill?
Is the ADU assessed at what it cost me to build?
What is the supplemental tax bill, and when does it arrive?
Is there a property-tax exemption for ADUs in California?
Does the ADU increment keep growing?
Does converting a garage into an ADU count as new construction?
Which figures on this page are HyreADU’s own?
Is any of this tax advice?
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; HUD FY 2026 Fair Market Rents retrieved 2026-09-05; construction medians indexed to August 2026 CCCI. 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 (Proposition 13) , Section 1 sets the general levy at one per cent of full cash value. Section 2(a) defines full cash value and names new construction as a resetting event for the thing constructed. Section 2(b) caps the annual inflation factor: “the inflationary rate not to exceed 2 percent for any given year.” Retrieved 2026-09-05.
- California Revenue and Taxation Code § 70 — what counts as new construction , “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.” An ADU, and a garage conversion, are inside the definition. Amended by Stats. 2008, Ch. 336. Retrieved 2026-09-05.
- California Revenue and Taxation Code § 71 — a new base year value for the new portion only , 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.” Construction in progress is appraised at full value on each lien date until completion. Amended by Stats. 2017, Ch. 80 (AB 652), effective 21 July 2017. Retrieved 2026-09-05.
- California Revenue and Taxation Code § 75.11 — 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. Retrieved 2026-09-05.
- SB 1164 (Newman, 2023–24) — the ADU new-construction exclusion that did not pass , Would have added Revenue and Taxation Code § 74.9 to exclude ADU construction completed between 1 January 2025 and 1 January 2030 from “new construction” for up to ten years, or until a change in ownership or conversion to non-residential use. Board of Equalization bill analysis estimated an annual property-tax revenue loss of about $19 million. Passed the Senate 29–6 on 22 May 2024; died in the Assembly Revenue and Taxation Committee on 24 June 2024, hearing canceled at the author’s request. Retrieved 2026-09-05.
- San Mateo County Assessor — how construction affects taxable value , “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.” No page naming ADUs in a tax context was found. Retrieved 2026-09-05.
- Orange County Assessor — 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.” “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.” The path /adu returns 404. Retrieved 2026-09-05.
- Riverside County Assessor and Contra Costa County Auditor-Controller , Riverside: “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.” Contra Costa: “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.” Contra Costa’s own ADU pages belong to Conservation and Development and carry no tax content. Retrieved 2026-09-05.
- Santa Clara County Office of the Assessor — Granny Units / Accessory Dwelling Units , “When homeowners contemplate building an ADU… they want to know will my property taxes go up? The short answer is yes, but only on the marginal value of the ADU. Assessors throughout California treat ADU’s like a home addition. The existing home will not be reassessed.” New detached construction is assessed at market value as of completion, or at the 1 January lien date for partially complete construction; conversions are assessed on the alterations. Occupancy — family member or tenant — does not change the assessment. Retrieved 2026-09-05.
- San Francisco Office of the Assessor-Recorder — supplemental assessments, worked with an ADU , “A homeowner completed a new Accessory Dwelling Unit on March 2026. We determined the increased value to be $150,000.” The first supplemental bill is $150,000 × 1.18268325% × 0.25 proration for April to June, “approximately $443.51.” The new-construction factsheet adds that “only the changed or new part of your home may be reassessed at market value, not the entire property,” and that the office “does not provide a preliminary estimate of assessed value because we need to review what actually changed.” Retrieved 2026-09-05.
- San Diego County Assessor/Recorder/County Clerk and the Los Angeles County Assessor , San Diego: “When I build a room addition, is my entire property reassessed? No. Only the value added of the new construction will increase the current assessment,” and the value added “is usually the full construction cost,” with exceptions for an over-improvement for the area and for an owner-builder who spent substantially less than the resulting increase. Los Angeles: the Assessor’s public information officer, quoted in the Los Angeles Times on 12 May 2023, described the method as “straightforward: ‘Our staff will determine the cost to build it,’” added to the current assessment and taxed at 1 per cent plus roughly 0.25 per cent in local levies. Retrieved 2026-09-05.
- Karen Chapple, Dori Ganetsos and Emmanuel Lopez, Implementing the Backyard Revolution (UC Berkeley Center for Community Innovation, 22 April 2021) , 51 per cent of new California ADUs generating rental income; 16 per cent providing no-cost housing to a relative; 8 per cent short-term rentals; median rent among owners who let $2,000. Used on this page only to show that a tax-against-rent ratio does not describe every owner. Retrieved 2026-09-05.
Budget the increment. Do not budget a reassessment that will not happen.
Your existing base year value is protected by statute. The ADU arrives as its own layer, with a supplemental bill that is separate from your regular one and usually outside your impound account. Both of those are facts worth planning around, and neither is a reason not to build.
HyreADU does not assess, appraise, lend or prepare taxes. This page is informational and is not tax, legal or financial advice. It describes California constitutional and statutory text and named county assessor guidance as retrieved on 5 September 2026.
Assessed value is determined by your county assessor on the facts of your parcel and your improvement; every added-value figure on this page is an input HyreADU chose to demonstrate a mechanism and is stated as such in the same row, not a measurement or a forecast of any real property.
The 1 per cent general levy is constitutional and uniform; voter-approved indebtedness, special assessments, direct charges and Mello-Roos vary by tax rate area and are excluded from the general-levy columns.
The two combined rates shown are published figures from two named jurisdictions applied illustratively and are not rates for anywhere else.
County findings rest on retrievals that used sitemap enumeration where search was unavailable, so “no ADU tax page found” means not found, not proven absent; Sacramento County could not be verified at all. California assessment rules are not national.
A property-tax appeal runs to the county assessment appeals board on a statutory deadline.