ADU Zoning: Where You Can Build an Accessory Dwelling Unit

Types of ADUs Zoning Codes Recognize

An accessory dwelling unit is a complete second home on the same lot as a main house, with its own kitchen, bathroom and sleeping area. Codes usually sort ADUs by how they're built:

• Detached: a separate backyard cottage
• Attached: an addition to the main house with its own entrance
• Conversion: an existing garage, basement or attic turned into a unit
• Junior ADU: a smaller unit carved out of the main house, which may share a bathroom

The type matters because size, setback and parking rules often differ by type. Conversions of existing space usually face the lightest rules, since the building already exists.

State Laws That Override Local ADU Rules

Several states limit how much cities can restrict ADUs. California is one example. According to the state housing department's ADU fact sheet, cities there generally must approve ADUs ministerially in residential and mixed-use zones, and a local ordinance may not:

• Impose a minimum lot size for ADUs
• Set a maximum ADU size below 850 square feet, or 1,000 square feet for an ADU with more than one bedroom
• Require more than one parking space per unit, or any parking for some ADUs, such as those within a half mile of public transit

The same fact sheet says a junior ADU in California can't exceed 500 square feet. Other states have their own versions, so check your state's housing agency before relying on a local ordinance that looks strict.

Standards Your Local ADU Ordinance Sets

Within whatever limits state law allows, the city or county ordinance usually sets:

• Maximum floor area, sometimes tied to lot size or the main home's size
• Side and rear setbacks and a height limit
• How many ADUs a lot may have
• Parking, if any
• Design rules, such as matching roof pitch or entry placement
• Whether the owner must live on the property

The setback calculator, lot coverage calculator and FAR calculator help you test a design against those numbers.

Renting an ADU

Long-term rental is the main reason many cities allow ADUs, so it's usually permitted. Short-term rental is a different story. California, for example, lets cities limit ADUs as short-term rentals and requires rentals of some ADUs and junior ADUs to be for more than 30 days. Many other cities ban or cap nightly rentals of ADUs; see short-term rental zoning for how those rules work.

What If Your Lot Isn't Zoned Residential?

ADU rules usually apply only where a home is already allowed. A house on commercial or industrial land may be a nonconforming use, which can block an ADU. Some mixed-use and agricultural zones do allow ADUs with an existing home. Search for your city's zoning map to confirm the district, then read its permitted uses.

Steps Before You Build

1. Confirm the zoning district and any overlays, such as historic or coastal zones, that add rules.
2. Read your state's ADU law and your local ADU ordinance side by side.
3. Pick an ADU type and check its size, setback and parking rules.
4. Ask the utility providers about separate meters and connection fees.
5. Submit plans for the ADU permit and building permits.

If you only need sleeping space for visitors, a guest house without a kitchen may follow simpler rules.