Five Key ADU State Bills

With housing scarcity worsening in 47 states and Washington, D.C., increasing housing density is critical for ensuring people of all income levels have access to diverse, quality housing options. However, legalizing certain housing types can present political, regulatory, procedural, and financial hurdles within rural and suburban communities. One measure is increasingly gaining acceptance in communities of all sizes and types: accessory dwelling units (ADUs), or residential structures on the same property as detached single-family homes.

State-level ADU Legalization Efforts Increase

On the state level, there have been several efforts from legislators to legalize and accelerate the creation of ADUs. This year's legislative activity builds on a steady increase in state ADU legislation in recent years. ADU provisions are often included as part of a broader suite of zoning reform bills. States like California, Oregon, and Maine are places where by-right ADU development has received national attention in recent years.

Here are five key ADU bills that caught our attention out of the 100 that were introduced this year.

1. Montana SB 528

The new law will revise municipal zoning codes to allow for a minimum of one ADU on a lot that contains a single-family dwelling. This includes attached, detached, or internal ADUs. Once the law takes effect, a municipality in Montana may not require additional parking to accommodate the ADU or require fees instead of additional parking. The law will also require municipalities to permit ADUs that do not match the exterior design, roof pitch, or finishing materials of the single-family dwelling. The law does not require the single-family home or the ADU to be occupied by the owner or require a familial, marital, or employment relationship between the occupants of the single-family home and the occupants of the ADU.

The Montana bill was signed into law on May 17, 2023, by Governor Gianforte.

2. Washington HB 1110/SB 5190

In Washington, increasing housing supply and density is a top priority. HB 1110 includes several provisions that pave the way for local jurisdictions to create more housing options that are affordable and benefit residents at all income levels. The bill states that a city with a population of at least 75,000 must allow at least six of the nine types of middle housing to achieve the unit density required. To fulfill these requirements, a city may allow ADUs or other types of middle housing.

The bill was enacted into law by Governor Inslee on May 8, 2023.

3. Rhode Island HB 6082

The bill was introduced to accelerate the creation of ADUs in the state. It would allow homeowners to develop an ADU on any lot larger than 20,000 square feet as long as the design meets specified criteria. The bill also prohibits the use of these units as short-term rentals and would streamline the permitting process. Another key component is the removal of owner occupancy requirements.

The Rhode Island legislature introduced the bill along with a package of bills that aim to create more housing options for state residents. Though not yet law, the bill has momentum, passing the House earlier this month. To become law, HB 6082 will need to make it through the Senate before June 30 when the Rhode Island legislative session ends.

4. Colorado SB 23-213

The provision would have required the executive director of the Colorado Department of Local Affairs to create an ADU model code that would allow ADUs in any part of a municipality where detached single-family homes are permitted. Even if a municipality did not adopt the ADU model code, the municipality would have to adhere to ADU minimum standards established in the bill.

The bill failed in early May shortly after the Senate did not agree with the House amendments. The bill included several pro-housing initiatives, including middle housing and transit-oriented development as well as an ADU by-right provision.

5. Arizona SB 1117

The bill would have required municipalities with a population of more than 30,000 people to allow ADUs in all residential zones. The bill would have also prohibited municipalities from requiring owner-occupancy regulations.

After passing a Senate committee, SB 1117 failed a Senate reading and has not moved since there was a vote to reconsider. After a similar bill failed last year, Senator Kaiser (R-Phoenix) formed a bipartisan study committee with outside parties to determine what caused the lack of traction. Despite efforts to keep the bill alive this session, it will need more support to pass in the sessions to come.

Results counts per legislature.

This graphic shows the number of ADU bills introduced by the state in 2023. Graphic by Fiscal Note.

ADU Legislation Trends Towards Future Growth

While ADU legislation did not pass in every state highlighted, we expect these bills to return in future sessions. The trajectory of ADU legislation is growing throughout the country as more states grapple with providing diverse, attainable housing options.

Vermont, which passed zoning reform legislation in 2020, is using American Rescue Plan Act (ARPA) funds to develop a housing improvement program to finance ADU construction. With another key zoning reform bill (S 100) awaiting the governor's signature, the Vermont legislature continues to take steps to make quality housing options more affordable and readily available.

Internal, attached, and detached ADUs all have the potential to make housing more affordable for both renters and homeowners, create a wider range of housing options within the community, enable seniors to stay near family as they age and facilitate better use of the existing housing fabric in established neighborhoods.

Resources like AARP's ADU Model Legislation can help planners and communities tailor ADU proposals to meet the needs of individual states and communities. APA's Equity in Zoning Policy Guide provides policy solutions for removing barriers to ADU development. The guide points to establishing lot and building standards that accommodate less expensive "missing middle" housing which includes ADUs, duplexes, and other housing options.

While ADUs are just one of many viable housing solutions, we urge planners to proactively embrace them and to find ways to work with legislators to ensure that state-level policies amplify local efforts to increase housing supply. We expect to see more states taking action to permit ADU development in future state sessions.

How to Overcome ADU Barriers

Expanding ADU Development and Occupancy

APA and AARP partner to provide planners a guide to expanding local housing supply and legalizing ADUs.

Top image: Exterior image of an ADU. Image credit -

About the author
Karla Georges is APA's state government affairs manager.

May 31, 2023

By Karla Georges