Research study

How many unpermitted ADUs are there? What the record actually supports

Name: Unpermitted accessory dwelling units: prior estimates, denominators and route audit, retrieved September 2026
Creator: HyreADU Research Desk
Published: 2026-09-05
License: https://creativecommons.org/licenses/by/4.0/

A unit built without a permit is, by construction, missing from the dataset that would count it. This page publishes what the prior research really found, on which denominators, and where it has been misquoted.

Updated September 2026 · Data as of Prior estimates as reported in the Freddie Mac Economic & Housing Research Insight of July 2020, retrieved 2026-09-05; California HCD Annual Progress Report Table A2 extract, HCD file last updated 4 September 2026; California Government Code Chapter 13 retrieved from leginfo 2026-09-05

Written by HyreADU Research Desk Primary-source research and data analysis

Audited by HyreADU Research Desk Denominator, attribution and retrieval-date audit

34–80% of single-family housing units in three surveyed LA neighborhoods, 2009 Cuff, Higgins and Dahl (2010), as reported in the Freddie Mac Insight of July 2020: a 2009 field survey of three Los Angeles neighborhoods with high foreclosure counts found this share “likely to have illegal ADUs”. Their finding, not ours.

>90% of secondary units reported by surveyed Bay Area homeowners lacked required permits, 2011 Chapple and others (2012), as reported in the same Freddie Mac Insight. A different denominator from the figure above, and not combinable with it.

No count of unpermitted ADUs published at any geography Searched the federal permit and housing survey documentation and the California state data returns. A permit dataset cannot contain a unit that has no permit. Retrieved 2026-09-05.

The finding

No source publishes a count of unpermitted accessory dwelling units, at any geography, and the obstacle is definitional rather than clerical. Permit datasets are records of permits. A unit built without one leaves no row.

Freddie Mac states the problem plainly in the only national ADU study with a published method: “illegal units—also called ‘shadow housing’—will not be represented in the data.” Two prior estimates are cited constantly, and almost always wrongly. Cuff, Higgins and Dahl surveyed three Los Angeles neighborhoods in 2009 and found that 34 to 80 per cent of single-family housing units in those areas were likely to have illegal ADUs.

Chapple and others surveyed San Francisco Bay Area homeowners in 2011 and found that more than 90 per cent of secondary units lacked required building permits.

Those two percentages are of different things: a share of houses in three purposively selected neighborhoods, and a share of existing secondary units among survey respondents.

Multiply them, or quote either as “the share of ADUs that are unpermitted in California”, and you have a number neither study supports.

Both are field research from 2009 and 2011, before California legalized ADUs at scale. Every year since 2018 has added permitted units to the denominator, 178,495 permit units in California alone through 2025, which mechanically lowers the informal share even if not one informal unit has been built or legalized.

The one route that could bound the problem from below is the legalization record, and it is young. Applications under the statutory amnesty pathway for pre-2020 units at Government Code section 66311.7, and under local tracks such as Oakland’s, are a floor on the informal stock.

No statewide count of them was retrievable on this date, and none is estimated here.

HyreADU publishes no estimate of the unpermitted stock. Not a range, not a “conservative” figure, not a scaled version of somebody else’s. What we publish is what each prior study measured and what it will bear.

Read this first

We publish no estimate of the unpermitted stock, and that is the finding
It would be easy to take one of the prior percentages, apply it to a housing-unit count and publish a headline. That number would be indefensible for reasons this page sets out in detail, and producing it would be worse than publishing nothing. What we publish is the anatomy of the question.
The two prior studies are read here through a secondary source
We retrieved Cuff, Higgins and Dahl (2010) and Chapple and others (2012) as they are reported in Freddie Mac’s July 2020 Insight, which is where their literature review sets them out.
We did not obtain either original paper on this date. That is a real limitation: we are reporting a summary of a finding, and we attribute it to its authors while being clear about the route.
Nothing here is a national figure
Both field studies are Californian, one in Los Angeles and one in the Bay Area, and both are more than fifteen years old. Nothing about them generalises to other states, other housing stocks or other enforcement regimes, and we make no attempt to make them do so.
This is not advice about a specific unit
If you have or are buying a property with an unpermitted unit, the questions that matter are legal and jurisdiction-specific: whether an amnesty track applies, what the local enforcement posture is, and what a lender and an insurer will do.
That is a conversation for the permitting agency and a professional, and our companion page on legalizing an unpermitted ADU covers the process rather than the count.

Why this count is structurally hard

Most housing quantities are hard to measure because the survey is expensive or the sample is small.

This one is hard for a different reason: the defining characteristic of the thing being counted is its absence from the administrative record. An unpermitted unit is not a unit that was recorded badly.

It is a unit that was never recorded, because recording it was precisely what did not happen.

That closes off the usual routes in a specific order. Permit data is out by definition.

The federal housing surveys are out because, as our instrument audit shows, they have no accessory category at all — so even a perfectly enumerated informal unit would appear as an ordinary one-unit or two-unit structure, indistinguishable from a permitted one.

Assessor records are out because an owner who did not seek a permit generally did not invite a reassessment either.

What remains are three imperfect routes, and they are the three that the literature actually uses. Field observation: walk or fly the neighborhood and count what is there against what is recorded.

Owner survey: ask homeowners directly and hope they answer honestly. Revealed preference: count the units that come forward under an amnesty. Each of the two prior estimates used one of the first two. Nobody has yet published the third at scale.

HyreADU analysis: the third route is the one that will eventually produce a defensible floor, and it is only now becoming possible.

California’s statutory amnesty pathway and the local programs running alongside it generate an administrative record of units that previously had none. Every completed legalization is one unit of hard evidence.

It will always be a floor rather than an estimate — the units that stay hidden stay hidden — but a floor from a real record beats a range from a 2009 sample applied to a 2026 stock.

The prior estimates, and exactly what each measured

The two studies that every discussion of unpermitted ADUs eventually cites. Read the denominator column before the percentage column — that is where the misquotation happens.

The two prior field estimates, drawn on separate axes because they are percentages of different populations. The left panel is a share of houses in three selected neighborhoods; the right is a share of the secondary units that surveyed homeowners said they had. Nothing meaningful is obtained by putting them on one scale, which is why we have not. Chart: HyreADU Research Desk, from the table above. Underlying findings are Cuff, Higgins & Dahl (2010) and Chapple and others (2012) as reported in Freddie Mac, Economic & Housing Research Insight, July 2020, retrieved 2026-09-05.

Study	Year of fieldwork	Geography	Method	Finding	Denominator — what the percentage is OF
Cuff, Higgins & Dahl (2010)	2009	Three Los Angeles neighborhoods selected for high foreclosure counts	Field survey	“34% to 80% of single-family housing units in these areas were likely to have illegal ADUs”	Single-family housing units in the three surveyed neighborhoods. Not all of Los Angeles, not a random sample of the city, and not a share of ADUs.
Chapple and others (2012)	2011	San Francisco Bay Area	Homeowner survey	“More than 90% of secondary units lacked required building permits”	Secondary units reported by responding homeowners. A share of the units that exist, not a share of houses, and subject to whatever selection operates on who answers a survey about their own informal unit.
Wegmann & Nemirow (2011); Wegmann & Chapple (2012)	2011–2012	California	Named in Freddie Mac’s literature review as essentially the only prior work in the field	Cited for the existence and framing of the informal-housing literature; no headline share retrieved by us	Not applicable — we did not retrieve a quantitative finding from these and do not attribute one to them.
Freddie Mac (July 2020)	Data to 2019	National, MLS-covered properties	Text mining of listing descriptions	Does not estimate the unpermitted stock. States that “illegal units—also called ‘shadow housing’—will not be represented in” permits data.	Not applicable. Its own 1.4 million figure counts properties whose listing text matched, and can incidentally include unpermitted units without identifying them as such.

HyreADU note: both studies are good research being asked to do a job it was not designed for. Neither claimed to produce a state or national informal-unit count, and neither would survive being turned into one. The failure is in the citation practice, not in the papers.

The four ways these numbers get misused

We have seen each of these in circulation. They are listed here so that a reader can check any ADU informality claim — including ours — against them.

1. Denominator swap

Quoting “more than 90 per cent” as the share of ADUs in California that are unpermitted. It is not. It is the share of secondary units reported by surveyed Bay Area homeowners in 2011 that lacked required permits — a much narrower population, and one which by construction over-represents the pre-legalization informal stock.

2. Geography inflation

Quoting “34 to 80 per cent” as a Los Angeles figure, or a California figure.

The fieldwork covered three neighborhoods, selected because they had high foreclosure counts — that is, selected on a characteristic plausibly correlated with informal conversion.

A purposive sample is the right design for the question the authors were asking and the wrong basis for a citywide rate.

3. Time-travel

Applying a 2009 or 2011 ratio to today’s housing stock. Between 2018 and 2025 alone, California jurisdictions reported 178,495 ADU permit units and 102,356 completions. Those units enter the denominator as permitted.

Even with zero new informal construction and zero legalizations, the informal share falls. A ratio measured before a legalization wave cannot describe the stock after it.

4. Multiplying the two together

The most damaging one, and the most common. Taking a share-of-houses figure and a share-of-units figure and combining them to produce a count.

The two quantities have no arithmetic relationship: one is prevalence across properties in a specific place, the other is informality within a set of existing units in a different place, seven hundred kilometres and two years apart.

Every route to a count, and where each one fails

The route-by-route audit below is what we ran before concluding that we could not publish a figure. If a route is not listed here, we did not consider it; if you think we missed one, the source list at the foot is the place to start arguing.

Route	What it would measure	Why it fails, or what it would cost	Status on 2026-09-05
Permit records	Permitted units.	Cannot contain an unpermitted unit. This is a definitional exclusion, not a coverage gap.	Closed
Federal housing surveys (AHS, ACS)	Structure type and unit counts for a national sample.	No accessory category exists at all: the terms “accessory”, “ADU”, “granny” and “secondary unit” are absent from the 2023 AHS definitions document. A perfectly enumerated informal unit would appear as an ordinary one- or two-unit structure.	Closed
Assessor and property-tax records	Improvements the assessor knows about.	An owner who avoided a permit generally avoided a reassessment. Where the assessor does know, the unit is usually permitted. The overlap with the target population is small and not measurable.	Closed in practice
Utility connection records	Separately metered dwellings.	Many accessory units, permitted and not, share the main house’s meter — and state law in California limits when a separate connection can be required. A shared meter is common precisely in the internal units most likely to be informal.	Weak signal; not attempted
Aerial or streetview imagery against parcel records	Structures present but not recorded.	Detects new detached structures reasonably well. Detects a converted garage poorly and a converted basement or bedroom not at all — and internal conversion is the dominant informal type in dense stock. Systematically biased toward the visible.	Not attempted
Field survey	Observed units against recorded units in a defined area.	The Cuff, Higgins and Dahl method. Expensive, geographically narrow and, as a purposive design, not projectable. It is also the only method that has produced a prevalence figure at all.	Prior work only, 2009
Owner survey	Self-reported units and their permit status.	The Chapple method. Depends on homeowners disclosing an unpermitted unit to a researcher, which selects on trust, tenure and perceived enforcement risk in ways nobody has measured.	Prior work only, 2011
Legalization and amnesty applications	Units that existed unpermitted and came forward.	A genuine administrative record, and a hard floor rather than an estimate. It undercounts by exactly the amount that matters — units whose owners stay quiet — and it is only as old as the programs. We could not retrieve a statewide count on this date.	The best available route; not yet published at scale

Routes to an unpermitted-ADU count, audited on 2026-09-05. “Closed” means the route cannot in principle produce the quantity; “not attempted” means we judged it unlikely to be defensible and did not run it.

The legalization record: the floor that is being built now

California created a statutory route for legalizing unpermitted units built before 2020.

The provision was enacted as Government Code section 66332 and was renumbered to section 66311.7 by SB 543 (Stats. 2025, Ch. 520) with effect from 1 January 2026 — a detail worth carrying, because a great deal of city guidance, including some published by cities running these programs, still cites the old number.

Cities run their own tracks alongside it. Oakland’s published guidance sets out three separate eligibility routes with three different cut-off dates: a Planning Code amnesty for units built or occupied before 1 January 2021, a building-code enforcement delay for units before 1 January 2020, and the state pathway for units before 1 January 2020, with applications accepted until before 1 January 2030.

Long Beach operates an unpermitted dwelling unit amnesty program under its own municipal code chapter.

Why this is the route that will eventually work. Every completed legalization converts an unrecorded unit into a recorded one, with a date, a jurisdiction and a unit type attached.

Aggregate those and you have a lower bound on the informal stock built from an administrative record rather than from a sample.

It will always understate — the whole population of interest is defined by not coming forward — but a floor you can audit is worth more than a range you cannot.

What we could not do. We could not retrieve a statewide count of legalization applications or approvals on this date, and HCD’s Annual Progress Report unit categories do not isolate them: Table A2 has no separate flag for a unit legalized under the amnesty.

So the floor exists in principle and is not yet published in practice. We have not estimated it, and a figure we could not retrieve is not a figure we will print.

What we could not verify

We did not retrieve the original Cuff, Higgins and Dahl (2010) or Chapple and others (2012) papers. Both are reported here as summarized in Freddie Mac’s July 2020 literature review, and the findings are attributed to their authors with that route stated.

We could not retrieve any statewide or national count of unpermitted-ADU legalization applications, approvals or completions. HCD’s Table A2 unit categories do not isolate legalized units, and we located no separate state return that does.

We could not verify the ordinance numbers or adoption dates for the Long Beach amnesty program; the municipal code host returned an access error on every attempt. The program’s existence is confirmed from the city’s own pages, but its eligibility terms are not published here because we could not read the code text.

We have no measurement of unpermitted ADUs outside California at all, of any vintage, and we make no claim about any other state.

Questions

How many unpermitted ADUs are there in California?

Nobody knows and no agency publishes it. The two figures usually quoted are field research from 2009 and 2011 with different denominators: Cuff, Higgins and Dahl found that 34 to 80 per cent of single-family housing units in three surveyed Los Angeles neighborhoods were likely to have illegal ADUs, and Chapple and others found that more than 90 per cent of secondary units reported by surveyed Bay Area homeowners lacked required permits. Neither is a statewide count, and neither can be converted into one.

Is it true that 90 per cent of ADUs are unpermitted?

That is a misquotation of a real finding. Chapple and others surveyed San Francisco Bay Area homeowners in 2011 and reported that more than 90 per cent of secondary units among their respondents lacked required building permits. It is a share of the units those respondents already had, in one region, before California’s ADU legalization wave. Since 2018, California jurisdictions have reported 178,495 permitted ADU units, all of which sit in the denominator today. The 2011 ratio does not describe the 2026 stock.

Why can’t you estimate it from the housing surveys?

Because the surveys have no accessory category. The words “accessory”, “accessory dwelling”, “ADU”, “granny” and “secondary unit” do not appear anywhere in the 2023 American Housing Survey definitions document, and the structure definitions place a detached ADU in the same class as the main house and an internal ADU in the same class as a duplex. Even a perfectly enumerated informal unit would be invisible as such.

Could you count them from aerial imagery?

Partially, and with a bias that defeats the purpose. Imagery detects new detached structures reasonably well, converted garages poorly, and converted basements or bedrooms not at all. Internal conversion is the dominant informal type in dense older housing stock, so an imagery-based count would systematically miss the units most likely to be unpermitted while appearing comprehensive.

Do amnesty programs tell us how many there are?

They give a floor, not a total, and only for the units that come forward. That is still the most promising route available: every legalization is an administrative record of a unit that previously had none. California’s statutory pathway sits at Government Code section 66311.7 — renumbered from section 66332 by SB 543 with effect from 1 January 2026 — and cities such as Oakland run local tracks alongside it. We could not retrieve a statewide count of applications on this date.

Are unpermitted units included in the Freddie Mac 1.4 million figure?

Some may be, incidentally, and the study cannot tell you which. Freddie Mac identified properties by text-mining MLS listing descriptions, and a unit that was never permitted can still be described in a listing. What Freddie Mac does say directly is that permits data cannot represent illegal or “shadow housing” units at all. The number itself counts properties, not units, and is discussed in full in our national count study.

Does an unpermitted ADU affect the property’s value or sale?

It is a real and material issue in valuation, lending and insurance, but the answer is specific to the property, the lender, the appraiser and the jurisdiction, and it is not something this page can generalise. Our value study covers what the appraisal evidence supports for permitted units, and is explicit about where the evidence stops. Nothing here is financial, legal or valuation advice.

Why does HyreADU not publish its own estimate?

Because we could not build one that would survive scrutiny. Every route we audited is either definitionally closed, systematically biased in a direction we cannot measure, or dependent on records that are not yet published. Publishing a range anyway would give the number a false provenance — our name — and that is exactly the practice this page exists to argue against.

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 Prior estimates as reported in the Freddie Mac Economic & Housing Research Insight of July 2020, retrieved 2026-09-05; California HCD Annual Progress Report Table A2 extract, HCD file last updated 4 September 2026; California Government Code Chapter 13 retrieved from leginfo 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

Freddie Mac, “Granny Flats, Garage Apartments, In-Law Suites: Identifying Accessory Dwelling Units from Real Estate Listing Descriptions Using Text Mining” , Economic & Housing Research Insight, July 2020. Source of the verbatim statement that “illegal units—also called ‘shadow housing’—will not be represented in” permits data, and of the literature review in which the Cuff, Higgins and Dahl (2010) and Chapple and others (2012) findings, and the Wegmann and Nemirow (2011) and Wegmann and Chapple (2012) work, are reported. The prior findings are attributed to their own authors throughout this page. Retrieved 2026-09-05.
California Department of Housing and Community Development, Housing Element Annual Progress Report — Table A2 , Full Table A2 extract, HCD file last updated 4 September 2026. 178,495 ADU permit units and 102,356 completions for 2018–2025, deduplicated by tracking identifier across 511 reporting jurisdictions. Used here as the permitted denominator that has grown since the prior field estimates were made. Table A2 carries no flag identifying a unit legalized under an amnesty program. Retrieved 2026-09-05.
California Government Code, Title 7, Division 1, Chapter 13 — Accessory Dwelling Units , Chapter text retrieved from the Legislative Counsel’s codes service. Confirms the current chapter structure, §§ 66310–66342, created by SB 477 (Stats. 2024, Ch. 7) effective 25 March 2024. The unpermitted-unit provision formerly at § 66332 was amended and renumbered to § 66311.7 by SB 543 (Stats. 2025, Ch. 520), operative 1 January 2026. Retrieved 2026-09-05.
City of Oakland, “Legalize Your Unpermitted Accessory Dwelling Unit (ADU) or Junior ADU (JADU)” , The city’s own guidance page. Sets out three eligibility tracks — Planning Code amnesty for units built or occupied before 1 January 2021, building-code enforcement delay for units before 1 January 2020, and the state pathway for units before 1 January 2020 — with applications accepted until before 1 January 2030. The page cites the state provision by its former number, § 66332. Retrieved 2026-09-05.
U.S. Census Bureau, 2023 American Housing Survey — Definitions , Searched for “accessory”, “accessory dwelling”, “ADU”, “granny” and “secondary unit”: none appears. Establishes that the national housing survey has no category in which an informal accessory unit could be identified even if it were enumerated. Retrieved 2026-09-05.
U.S. Census Bureau, Building Permits Survey — item definitions (items.txt) , Item 434 records additions and alterations with a permit count and valuation but no units field; items 101–105 classify buildings by unit count with no accessory item. Establishes the definitional exclusion of unpermitted units from the national permit series. Retrieved 2026-09-05.

When the honest answer is “nobody knows”, the useful thing is to say why

HyreADU publishes the shorter page rather than the confident one. If a number here cannot be traced to a document with a retrieval date, it is not on the page.

Legalizing an unpermitted ADU All HyreADU research

HyreADU does not design, permit or build accessory dwelling units, and does not practice law. This page is informational and is not legal, tax, insurance or financial advice. It publishes no estimate of the unpermitted ADU stock at any geography.

The prior findings described are the findings of their own authors — Cuff, Higgins and Dahl (2010) and Chapple and others (2012) — retrieved through the Freddie Mac literature review of July 2020 rather than from the original papers, and are not restated as HyreADU research.

Amnesty eligibility, enforcement posture and the consequences of an unpermitted unit vary by jurisdiction and by property; the useful next step is the permitting agency and a licensed professional.