As housing shortages continue to challenge communities across the United States, Accessory Dwelling Unit (ADU) state laws have emerged all over the nation. It’s clear to see why. ADUs offer a multitude of uses from passive rental income to in-home care for family members in need.

These regulations often address common barriers like zoning restrictions, parking requirements, and permitting challenges making it easier to build. However, not every state has these regulations in place.

If your state isn’t on this list, that doesn’t mean that you have to give up your ADU dreams. It just means that individual cities have complete free reign to set their ADU laws any way they see fit. Unfortunately, that means some have opted to outlaw ADUs all together.

Let’s take a look at which states have chosen to jump on the ADU bandwagon.

California State ADU Laws

California has some of the most progressive ADU laws in the nation due in part to the severe housing shortage.

These laws have been designed to make it easier and more affordable for homeowners to build ADUs. We could fill a whole blog about California ADU Regulations (actually we have), so for the sake keeping everything high level, here are the must-know ADU regulations that will set you off on the right foot.

  • Owner Occupancy: Owner occupancy is NOT required and cannot be enforced by your city.
    • The only exception is if you are building a JADU (junior accessory dwelling unit).
  • ADU Sizes: All California homeowners are entitled to build at least an 800 sq ft ADU on their property regardless of floor area ratio, property size, or other factors. Larger sizes are allowed as follows:
    • 850 sq ft for 1 bedroom ADUs
    • 1,000 for 2+ bedroom ADUs
    • Up to 50% of the main residence for attached ADUs (unless this figure equates to less than 800 sq ft)
    • 1,200 sq ft is the max size for detached ADUs in cities that have not passed an ADU ordinance.
    • No size restrictions for a garage conversion as long as you stay within the existing footprint
  • Setbacks: Setbacks requirements may not exceed 4 ft for the side and rear
    • For garage conversions, setbacks requirements do not apply, as long as the ADU stays within the existing footprint.
  • ADU Heights: 16 ft is allowed for every ADU under any circumstance. Higher heights are allowed under the following scenarios:
    • 18 ft is allowed if the ADU is within half a mile walking distance from public transit.
    • 18 ft is also allowed if the ADU is to be built on a property that already has a two-story multi-family dwelling.
    • 25 ft is allowed if the ADU is going to be attached to the main residence, regardless of its height.
  • Parking: Off-street parking is not required if your ADU or property meet at least one of the following criteria:
    • The ADU is located within half a mile walking distance from public transit
    • The ADU is located within an architecturally or historically significant district
    • The ADU is converted from an existing space or accessory structure
    • On-street parking permits are required but not offered to the occupant of the ADU
    • A car share vehicle is located within one block of the ADU
  • Permitting:
    • Local planning departments are legally required to issue a decision on your ADU permit application within 60 days with ALL required revisions noted at one time.

If your city has passed an ADU ordinance, then your local regulations may be a bit more flexible than what’s listed here. Remember, these are just the baseline requirements that cities are allowed to set.

Colorado State ADU Laws

Colorado is new to the ADU state law game, having passed one of it’s most groundbreaking ADU bills in 2024. Let’s see what

House Bill 1152 in Colorado simplifies the design and size rules for ADUs in cities that are part of a metropolitan planning area with populations over 1,000. This means cities in these areas can no longer impose overly restrictive requirements, such as strict architectural styles or specific dimension limits, making it easier for homeowners to build ADUs. The goal is to standardize regulations and remove barriers that often make ADU construction complicated or expensive in cities where affordable housing is scarce.

This includes cities Denver, Boulder, Aurora, and Lakewood.

Homeowners with properties in cities that fit the criteria benefit from the following regulations:

  • By-right zoning: Permits ADUs between 500–750 square feet without requiring additional discretionary approval.
  • Flexible setbacks: Rear setbacks are capped at 5 feet or the same as those required for the primary dwelling.
  • Minimum lot size parity: ADU construction cannot be restricted by minimum lot size requirements that exceed those for primary residences.
  • Parking allowances: Local governments can require only one parking space for an ADU, but only where on-street parking is unavailable and an existing parking requirement was in place as of January 2024.
  • Owner-occupancy compromise: Requires owner occupancy only at the time of permitting. This innovative provision aims to deter investor speculation while protecting owners from being unable to rent their properties if circumstances change after permitting or construction.

This is just the beginning to Colorado’s ADU journey.

Connecticut State ADU Laws*

In 2021, the Connecticut government passed Public Act 21-29. This act is different from the other state’s regulations in that cities can vote to opt out. If a city decides to opt-in, they can benefit from the following ADU initiatives and clarifications:

  • Fair Housing and Equity: The act ensures zoning rules support fair housing and economic diversity, banning discrimination based on income or personal characteristics (except for specific age-restricted or disability-focused housing).
  • By-Right: Municipalities must now allow ADUs (both attached and detached) without requiring special permits or public hearings, streamlining the approval process. These units must stay proportional in size to the main home.
  • Parking Requirements: The law limits excessive parking mandates for new housing, reducing barriers to development.
  • Training for Officials: Zoning and planning commissioners are now required to undergo training on fair housing and affordable housing policies. Zoning enforcement officials must also be certified to ensure consistent application of the law.
  • Zoning Definitions: The term “character” in zoning now refers to physical features of a site, rather than subjective or exclusionary interpretations, encouraging inclusivity in housing development.

Hawaii State ADU Laws

state adu laws

Hawaii has enacted progressive ADU laws to address its housing crisis and high cost of living. Recent legislation, such as Senate Bill 3202 signed in 2024, guarantees the following regulations:

  • Mandatory ADU Allowance:
    • All urban residential-zoned lots must permit at least two ADUs by the end of 2026.
    • Applies uniformly across Hawaii’s four counties.
  • Prohibition of Restrictive Covenants:
    • Private land-sale covenants cannot block the development of ADUs, ensuring more flexibility for homeowners.
  • Adaptive Reuse of Commercial Properties:
    • Encourages converting commercial spaces into residential housing to increase inventory.
  • Addressing Housing Affordability:
    • Focuses on reducing the housing cost burden, as over half of Hawaii households spend more than 30% of their income on rent

Maine State ADU Laws

state adu laws

Maine’s ADU laws, updated through the passage of LD 2003, aim to address the state’s significant affordable housing shortage. These regulations make it easier to build ADUs by reducing zoning restrictions and increasing flexibility for homeowners. Here are the key provisions:

  • Unit Limits: On single-family zoned lots, up to two dwelling units are allowed. On undeveloped parcels, up to four units are permitted.
  • Location Flexibility: ADUs can be placed within existing homes (e.g., basements or upper floors), attached to homes, or created from accessory structures like barns. They can also be standalone structures.
  • Size Requirements: While municipalities can set maximum sizes, the state mandates that ADUs must be at least 190 square feet.
  • Setbacks and Zoning: ADUs must adhere to local setback and size requirements for single-family homes unless municipalities opt for more permissive rules.
  • Parking: Additional parking is not required for ADUs under the new law.
  • Local Adaptation: Cities and towns can implement less restrictive policies, allowing more flexibility, such as multiple ADUs on a single property.

New Hampshire State ADU Laws

state adu laws

New Hampshire’s ADU laws, established under RSA 674:71-73, aim to increase affordable housing options. It prohibits municipalities from imposing restrictions such as:

  • Requiring fewer than two bedrooms in an ADU
  • Limiting the size of the ADU to less than 750 square feet
  • Mandating that a relative of the owner must live in the added unit

However, local communities still have some authority. They can:

  • Create parking requirements
  • Limit the number of ADUs to one per home
  • Require that the ADU design fits with the neighborhood

Additionally, towns and cities can choose to require the property owner to live in one of the units.

Oregon State ADU Laws

state adu laws

Oregon was the first state to pass major statewide ADU laws. Like California, the statewide ADU regulations are meant to be a guideline for municipalities to follow, but cities can still pass their own ordinances to make regulations more flexible.

  • Zoning: ADUs are allowed in all residential zones
  • ADU Sizes: Oregon does not have static max sizes for ADUs, but the following guidelines apply:
    • Detached ADUs are not to exceed a floor area of 800-900 sq ft
    • Attached or interior ADUs are not to exceed 75-85% of the floor area of the primary dwelling
  • Height: ADUs are not to exceed 20ft or the height of the primary dwelling, whichever is lower
  • Setbacks: ADU setbacks must adhere to the setback regulations for the primary dwelling
  • Parking: Parking is not required for ADUs
  • Owner Occupancy: Owner occupancy cannot be enforced

A large portion of ADU cities in Oregon have passed their own ADU ordinances, especially Portland. Make sure to check your local regulations to see what’s possible for your ADU.

Utah State ADU Laws

Utah’s state ADU laws apply to internal accessory dwelling units only, but it’s a step in the right direction nonetheless. An internal ADU is a living space built within the existing space of the primary dwelling. In 2021 Utah lawmakers passed House Bill 82 which makes having an internal ADU a permitted use, as long as the owner lives on the property. This means that the ADU can be rented out. Many Utah cities previously allowed ADUs, but set high restrictive ADUs that made building an ADU almost impossible. HB 82 requires jurisdictions to reduce these restrictions.

Washington State ADU Laws

In July 2023, HB 1337 came into effect, requiring all local governments in Washington State, under the Growth Management Act (GMA), to update their regulations for accessory dwelling units (ADUs). Here are the key requirements for ADUs:

  • Minimum number of ADUs: Two ADUs per lot must be allowed in urban growth areas, in addition to the primary residence, as long as the lot meets size requirements.
  • Size standards: ADUs cannot be smaller than 1,000 square feet.
  • Dimensional standards: ADU requirements must be equal to or less restrictive than those for the primary unit (e.g., setbacks and yard coverage).
  • Street improvements: No street improvements can be required as part of ADU permits.
  • Owner occupancy: Local governments cannot mandate that the owner live in either the primary home or the ADU.
  • Condominium sales: ADUs cannot be prohibited from being sold separately from the main house.
  • Design review: ADUs cannot face stricter aesthetic standards than the primary dwelling.
  • Parking requirements: Parking requirements are relaxed for ADUs, especially near major transit stops.
  • Impact fees: Fees for ADUs must be capped at 50% of the impact fees for the primary home.

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