Research study

Who actually adopted the model ADU ordinance?

Name: Provision convergence across seven state accessory dwelling unit statutes, coded September 2026
Creator: HyreADU Research Desk
Published: 2026-09-05
License: https://creativecommons.org/licenses/by/4.0/

Model codes are supposed to propagate. This page tests whether they do — and is explicit about the comparison we set out to run, could not run, and refused to fabricate.

Updated September 2026 · Data as of Seven state statutes retrieved from official legislature or code sites 2026-09-05; AARP publication inventory read from AARP’s own ADU resource page 2026-09-05

Written by HyreADU Research Desk Primary-source research and statutory coding

Audited by HyreADU Research Desk Coding-scheme and retrieval-date audit

7 of 7 state statutes require ADUs to be allowed by right or ministerially HyreADU coding of the statutory text of California, Oregon, Washington, Montana, Maine, Colorado and Arizona, each retrieved directly on 2026-09-05.

3 of 7 apply statewide with no population or geographic trigger California, Montana and Maine. Oregon, Washington, Colorado and Arizona each restrict application by population threshold, urban growth boundary or metropolitan planning organization geography.

Not retrieved the operative text of the AARP model ordinance The publication is named on AARP’s own ADU resource page as a 56-page guide for state and local elected officials. Its text could not be retrieved on 2026-09-05, so no comparison against it is made anywhere on this page.

The finding

What travels between jurisdictions is a provision set, not a text. We coded seven state ADU statutes — California, Oregon, Washington, Montana, Maine, Colorado and Arizona — against a five-provision fingerprint, reading each statute directly.

Four of the five substantive provisions recur almost everywhere: all seven require ADUs to be allowed by right or through a ministerial or administrative process, six bar a local owner-occupancy requirement outright, and five bar an additional parking requirement outright with two more restricting it in a narrower form.

The provision that almost nobody copies is the one that decides who the statute applies to. Only California, Montana and Maine apply their ADU statute across the state without a population or geographic trigger.

Oregon uses city and county population thresholds inside an urban growth boundary; Washington uses urban growth areas; Arizona applies only to municipalities over 75,000; Colorado invented a bespoke “subject jurisdiction” definition built from municipal population and metropolitan planning organization boundaries.

That pattern is the finding. The substantive rules look like a shared template.

The applicability clause looks like seven separate political negotiations, because that is what it is — it is the clause that decides which local governments lose authority, and it is therefore the clause a legislature writes itself.

The comparison we could not run, stated plainly. This page was commissioned to compare adopted local codes against AARP’s published model ordinance.

AARP’s own resource page names the publication — “Accessory Dwelling Units: Model State Act and Local Ordinance”, described there as a 56-page guide for state and local elected officials — but we could not retrieve the model’s operative text on this date.

We therefore make no claim about what the model says and no claim about whether any statute derives from it. A text comparison against a document we have not read would be a fabrication, and we would rather publish the smaller true finding.

Nothing here uses HyreADU’s own store, which is Californian, and nothing here asserts that any state copied any other.

Read this first — including what we could not do

We did not read the AARP model ordinance, and we make no claim about its contents
We confirmed the publication exists and how AARP describes it, from AARP’s own resource page. We could not retrieve the document itself on this date.
Every sentence on this page about provisions is a statement about statutes we read directly, and none of it is a statement about the model.
If you see a source elsewhere claiming a state “adopted the AARP model”, ask whether they read both documents.
Convergence is not derivation
Seven statutes sharing a provision does not mean one copied another, or that all copied a model. Legislatures facing the same problem, advised by overlapping advocacy and research networks, converge on similar language for reasons that have nothing to do with a specific document. We measure convergence. We do not, and cannot, measure influence.
This is state law, not local ordinance text
The contract for this page envisaged comparing adopted local codes. We coded state statutes, because those are what we could retrieve completely and verify. Local ordinance text remains largely inaccessible to us: municipal code hosts returned access errors on every attempt during this research program, a limitation we also record in the ordinance tracker.
The coding is ours and is a judgment
Deciding whether Colorado’s application-time residence demonstration counts as an owner-occupancy requirement, or whether Washington’s transit-proximity parking rule is the same provision as Montana’s flat prohibition, is a coding decision.
We publish the scheme and the reasoning for every non-obvious cell so that a reader can disagree with a specific call rather than with the table as a whole.

Why the model-code question matters

Model ordinances are one of the main mechanisms by which land-use policy is supposed to spread.

An organization with subject expertise writes a clean, defensible text; jurisdictions that lack the drafting capacity to write their own adopt it or adapt it; the policy propagates faster and more consistently than it would if every planning department started from a blank page.

It is a genuinely important pathway, and it is very rarely evaluated.

The evaluation is hard for an unglamorous reason: it requires both halves of the comparison to be machine-readable, and local ordinance text mostly is not.

Municipal codes sit behind commercial hosting platforms that block automated retrieval, are published as scanned PDFs, or are amended by ordinance documents that never get consolidated.

Anyone who tells you they have systematically compared hundreds of adopted ADU ordinances against a model text has solved a retrieval problem that we did not solve.

What we could do instead. State statutes are published as structured text by legislative counsel offices and are retrievable. Seven of them, read directly, are enough to test a narrower but still meaningful version of the question: is there a common provision set in circulation, and which parts of it are stable?

HyreADU analysis: the answer turns out to have a clean shape. The provisions that constrain the design and approval of an ADU are near-universal. The provision that determines which jurisdictions are bound at all is unique in almost every state.

That is exactly what you would expect if a common technical template is circulating while the political question — how much local control to remove, and from whom — is being settled separately each time.

The provision fingerprint, seven states

Each cell is coded from the statutory text we retrieved directly. “Variant” means the provision exists in a materially different form, and every variant is explained in the notes below the table rather than hidden in a symbol.

Five ADU statutory provisions against seven states. A filled cell means the provision is present in the statutory text we retrieved; a half-filled cell means it is present in a materially different variant; an empty cell means it is not in the text we retrieved. The bottom row — statewide application with no trigger — is the one that does not converge. Chart: HyreADU Research Desk. Coded from Gov. Code §§ 66310–66342 (CA); ORS 197A.425; RCW 36.70A.681; MCA 76-2-345; 30-A M.R.S. § 4364-B; C.R.S. 29-35-101 et seq.; A.R.S. § 9-461.18. All retrieved 2026-09-05.

Provision	California	Oregon	Washington	Montana	Maine	Colorado	Arizona
ADU allowed by right / ministerially	Yes	Yes	Yes	Yes	Yes	Yes	Yes
No local owner-occupancy requirement	Yes	Yes	Yes	Yes	Yes	Variant	Yes
No additional parking requirement	Yes	Yes	Variant	Yes	Yes	Variant	Yes
Statutory floor on permitted size	Yes	No	Yes	No	No	No	No
Statewide application, no population trigger	Yes	No	No	Yes	Yes	No	No

The five-provision fingerprint across seven state ADU statutes, coded 2026-09-05. Every cell is reproducible from the statutory text cited in the source list.

The variant cells, explained. Washington, parking: the statute bars a parking requirement within half a mile of a major transit stop rather than everywhere, so it is a geographically conditioned version of the provision.

Colorado, parking: the statute bars requiring a new off-street parking space but carries express exceptions, so it is neither a flat prohibition nor an absence.

Colorado, owner-occupancy: the statute bars an owner-occupancy requirement while permitting a jurisdiction to require the applicant to demonstrate residence at the time of application — narrower than the prohibition in the other six, and a genuinely different rule.

The clause nobody copies: who the statute binds

Who the statute binds is the divergent row of the matrix, set out in full. Seven states, six different answers to the same question, and no two of the triggers are constructed the same way.

State	Who the ADU statute binds	Mechanism used	Statute
California	All local agencies, expressly including charter cities.	No trigger at all. The chapter states that it applies to a charter city notwithstanding the general home-rule provision.	Gov. Code Chapter 13, §§ 66310–66342 (§ 66312 on charter cities)
Montana	All jurisdictions with single-family zoning, as far as the retrieved text shows.	No population trigger in the text retrieved.	MCA 76-2-345
Maine	Any lot with a single-family principal structure in any area where residential uses are permitted.	No population trigger; a single named geographic carve-out for the Lewiston and Auburn watershed.	30-A M.R.S. § 4364-B
Oregon	Cities over 2,500 and counties over 15,000, inside an urban growth boundary, in areas zoned for detached single-family dwellings.	Population threshold and a growth-boundary geography and a zoning condition — three conditions stacked.	ORS 197A.425
Washington	All zoning districts inside urban growth areas.	Growth-area geography alone, with no population threshold.	RCW 36.70A.681
Arizona	Municipalities with a population of more than seventy-five thousand.	A single population threshold, set high — which excludes most Arizona municipalities.	A.R.S. § 9-461.18
Colorado	A “subject jurisdiction”: a municipality of 1,000 or more inside a metropolitan planning organization, or the portion of a county inside a census designated place of 40,000 or more and inside an MPO.	A bespoke composite definition combining municipal population, MPO boundaries and census designated place population. No other state uses anything like it.	C.R.S. 29-35-102(21), 29-35-103(1)

Applicability provisions in seven state ADU statutes, retrieved 2026-09-05. The substantive rules converge; this clause does not.

HyreADU analysis. There is a practical reading of this pattern and a political one, and we think both are true. Practically, the substantive provisions are technical: how big, how tall, how many parking spaces, how the application is processed.

A drafter can lift those from anywhere because they are the same everywhere.

The applicability clause is not technical — it is the answer to “which of our cities and counties are we overriding”, and that is the negotiation that gets the bill passed or killed.

Colorado’s composite definition in particular reads as the residue of exactly such a negotiation, and no model text would have produced it.

The AARP ADU publications, and what we could establish about each

AARP is unusually active on ADU policy and publishes a family of documents in this area. This table records what its own resource page says about each, and — separately — what we were able to retrieve. The two columns are not the same thing, and we have kept them apart deliberately.

Publication	As described on AARP’s ADU resource page	Did we retrieve the document text?
“Accessory Dwelling Units: Model State Act and Local Ordinance”	A 56-page guide, described as being for state and local elected officials. This is the model text the contract for this page asked us to compare adopted codes against.	No. Not retrieved on 2026-09-05. No claim about its contents appears anywhere on this page.
“The ABCs of ADUs”	Described as a 24-page, image-filled primer for local leaders.	No. Existence and description confirmed from the resource page only.
“Accessory Dwelling Units: A Step-by-Step Guide to Design and Development”	Described as a 113-page design catalog, credited to the AARP Public Policy Institute and Blink!LAB Architecture.	No. Existence, length and credit confirmed from the resource page only.
“Expanding ADU Development and Occupancy”	Described as a 104-page guide created by AARP with the American Planning Association.	No. Existence, length and joint credit confirmed from the resource page only.
“Fact Sheet: ADUs”	Described as a one-page, easily sharable PDF.	No. Existence and description confirmed from the resource page only.

AARP ADU publications as named and described on AARP’s own ADU resource page, retrieved 2026-09-05. The third column records our retrieval outcome, not a judgment about the documents.

To be unambiguous: the descriptions in the middle column are AARP’s descriptions of AARP’s own publications, read from AARP’s website. They are not our assessment of the documents, which we have not read.

We record them because the existence, scale and intended audience of this publication family is itself relevant to the model-code question — five documents, one of them 113 pages, aimed at local leaders, is a serious propagation effort by any standard.

How we coded the matrix

Published so that a reader can reproduce or contest any individual cell. The coding is a judgment, and a judgment that is not documented is not a finding.

1
Retrieve the operative statutory text, not a summary
Each statute was retrieved from the state legislature’s or the official code publisher’s site: leginfo for California, the Oregon public law text of ORS 197A.425, the Washington legislature’s RCW service, the Montana Code Annotated, the Maine legislature’s statute service, the enrolled Colorado bill text, and the Arizona Revised Statutes. No state was coded from a secondary description.
2
Code only what the text says
A provision is coded present only if we can point at statutory language for it.
Where a provision is widely reported but we could not find it in the text we retrieved, the cell reads “No” and means “not in the text retrieved” — not “the state does not do this”.
That distinction is stated in the chart legend as well as here.
3
Use a variant code rather than forcing a binary
Washington’s parking rule and Colorado’s parking and owner-occupancy rules are neither the same as the flat prohibitions elsewhere nor absent. Coding them as either would misrepresent the statute. They are coded as variants and each is explained in prose, so no information is lost inside a symbol.
4
Do not infer influence from similarity
The matrix measures whether provisions co-occur. It cannot establish that any state drew on any model or on any other state, and we make no such claim. Establishing derivation would require drafting records, committee analyses and, ideally, the model text itself — which we did not have.

What we could not verify

The AARP model ordinance text. Not retrieved. The publication is named and described on AARP’s own resource page; its operative provisions are not reproduced or characterized anywhere on this page, and no state statute is described as deriving from it.

Local ordinance text at scale. The comparison the contract envisaged was against adopted local codes. Municipal code hosts returned access errors throughout this research program, and we did not obtain a corpus of local ADU ordinance text sufficient to code.

Influence and derivation. We have no drafting records, committee analyses or legislative histories for any of the seven statutes, and therefore no evidence about where any provision came from.

The twelve other states in the Mercatus count. Hamilton and Peterson of the Mercatus Center count 18 broadly legalizing states as of July 2025. We retrieved seven statutes. The other states are neither coded nor characterized here.

Questions

Is there a model ADU ordinance?

Yes — AARP publishes one, described on its own resource page as “Accessory Dwelling Units: Model State Act and Local Ordinance”, a 56-page guide for state and local elected officials, alongside four other ADU publications. We could not retrieve the model’s text on the date of this study, so this page makes no claim about what it contains and does not compare any statute against it.

Did states copy the model ordinance?

We cannot tell you, and neither can anyone who has not read both the model and the drafting record. What we can show is that seven state ADU statutes converge strongly on four substantive provisions — by-right or ministerial approval, no owner-occupancy requirement, no additional parking requirement, and in two cases a statutory size floor — while diverging almost completely on which jurisdictions the statute binds. Convergence is evidence of a shared problem and a shared advisory network. It is not evidence of copying.

Which provision is most consistent across states?

By-right or ministerial approval: all seven statutes we coded require the ADU to be allowed without a discretionary hearing, whether the statute calls it by right, ministerial or an administrative approval process. The removal of discretionary review is the one thing every one of these laws does.

Which provision is least consistent?

The applicability trigger. Only California, Montana and Maine apply their statute without a population or geographic threshold. Oregon stacks a population threshold, an urban growth boundary and a zoning condition; Washington uses urban growth areas; Arizona applies only above 75,000 population; and Colorado uses a bespoke “subject jurisdiction” definition combining municipal population, metropolitan planning organization boundaries and census designated place population.

Does a state ADU statute override a city’s zoning?

Where it applies, that is generally the point of it — but the extent varies and the answer for any specific jurisdiction is a legal question about that state’s statute and that city’s code. California’s chapter states expressly that it applies to charter cities notwithstanding the general home-rule provision; the others use their own applicability formulations. Nothing here is legal advice.

Why did you code state statutes instead of local ordinances?

Because state statutes are published as retrievable, structured text by legislative counsel offices and local ordinances largely are not. Municipal code platforms blocked automated retrieval throughout this research program. Coding a corpus we could not read would have meant coding secondary summaries, and a matrix built from summaries is not evidence.

Is Colorado’s “subject jurisdiction” definition unusual?

On the evidence of the seven statutes we coded, yes — no other state constructs applicability that way. It combines a municipal population floor of 1,000 with metropolitan planning organization membership, and separately reaches the portion of a county inside a census designated place of 40,000 or more that is also inside an MPO. It is the clearest single illustration on this page of an applicability clause being written from scratch rather than adopted.

What would it take to answer the original question properly?

Three things we did not have: the model text itself; a retrievable corpus of adopted local ADU ordinance text; and a comparison method robust to paraphrase, since a jurisdiction that adapts a model rewrites its sentences. That is a real research project and a worthwhile one. It is not a project that can be faked with a plausible-sounding table, which is why this page publishes the narrower thing we could actually verify.

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 Seven state statutes retrieved from official legislature or code sites 2026-09-05; AARP publication inventory read from AARP’s own ADU resource page 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

AARP, “Accessory Dwelling Units (ADUs)” resource page , Source of the publication inventory table: “Accessory Dwelling Units: Model State Act and Local Ordinance” (described as a 56-page guide for state and local elected officials), “The ABCs of ADUs” (24-page primer for local leaders), “Accessory Dwelling Units: A Step-by-Step Guide to Design and Development” (113-page design catalog, AARP Public Policy Institute and Blink!LAB Architecture), “Expanding ADU Development and Occupancy” (104-page guide, AARP with the American Planning Association), and a one-page ADU fact sheet. Descriptions are AARP’s own. The model ordinance text itself was not retrieved. Retrieved 2026-09-05.
California Government Code, Title 7, Division 1, Chapter 13 — Accessory Dwelling Units , Sections 66310 to 66342, created by SB 477 (Stats. 2024, Ch. 7) effective 25 March 2024 and amended by SB 543 (Stats. 2025, Ch. 520) operative 1 January 2026. Coded for by-right approval, the owner-occupancy prohibition, the parking provisions, the 850 sq ft size floor and the express application to charter cities. Retrieved 2026-09-05.
Oregon Revised Statutes § 197A.425 , Formerly ORS 197.312(5)–(6); enacted by SB 1051 (2017). Coded for the population and growth-boundary trigger and for the express exclusion of owner-occupancy and additional off-street parking from “reasonable local regulations relating to siting and design”. Retrieved 2026-09-05.
Revised Code of Washington § 36.70A.681 , 2023 c 334 (HB 1337). Coded for two ADUs per lot inside urban growth areas, the 1,000 sq ft floor on any maximum gross floor area cap, the owner-occupancy prohibition, and the transit-proximity parking rule coded here as a variant. Retrieved 2026-09-05.
Montana Code Annotated § 76-2-345 , SB 528, “En. Sec. 1, Ch. 502, L. 2023”. Coded for by-right approval on single-family lots, the flat prohibitions on occupancy restriction, additional parking, design-match mandates and impact fees, and the $250 application fee cap. Retrieved 2026-09-05.
Maine Revised Statutes, 30-A M.R.S. § 4364-B , PL 2021 c.672 § 6 (LD 2003), amended through PL 2025 c.385. Coded for statewide application with the Lewiston and Auburn watershed carve-out, and for the prohibitions on additional parking and owner-occupancy. Retrieved 2026-09-05.
Colorado Revised Statutes 29-35-101 to -105 (HB24-1152), enrolled bill text , Source of the “subject jurisdiction” definition at § 29-35-102(21), the 30 June 2025 administrative-approval duty at § 29-35-103(1), the parking provision with exceptions at § 29-35-103(2)(a) and (3), and the owner-occupancy provision with its application-time residence demonstration at § 29-35-103(2)(b). The statute also defines “universal design” and “visitable unit”. Retrieved 2026-09-05.
Arizona Revised Statutes § 9-461.18 , Coded for the 75,000-population applicability threshold, the requirement to allow at least one attached and one detached ADU, the prohibition on required familial, marital or employment relationships, the parking prohibition, and the 75 per cent or 1,000 sq ft size limit. Retrieved 2026-09-05.
Emily Hamilton and Kol Peterson, “A Taxonomy of State Accessory Dwelling Unit Laws 2025”, Mercatus Center , August 2025. Cited as the authors’ own count: 18 states with broad ADU legalization as of July 2025, 10 classified strong and 8 weaker. Referenced here to make clear that our seven-state matrix is a subset and not a census of state ADU law. Retrieved 2026-09-05.

We publish the comparison we could run, not the one we were asked for

The AARP model text was not retrievable on the date of this study, so no claim about it appears here. The seven-statute matrix is reproducible from the sources listed, cell by cell.

ADU laws by state 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 advice; it describes statutory text as retrieved on 5 September 2026 and does not tell you what is permitted on any particular lot.

The publication descriptions in the AARP inventory are AARP’s own descriptions of AARP’s own documents, read from AARP’s website; we did not retrieve any of those documents and make no claim about their contents.

The provision matrix measures co-occurrence of statutory provisions and does not establish that any state adopted, copied or was influenced by any model text or any other state.