Accessory Dwelling Units (ADUs): Types, Local Rules and Questions to Ask First

An accessory dwelling unit (ADU) is a smaller, independent home on the same lot as a main house. The American Planning Association describes it as a self-contained residential unit that can be created inside an existing home, added onto one, or built as a separate structure. Whether you can build one, how big it can be and what it needs are set locally, and the differences between cities are large. Start with your planning department, not a floor plan.

The main types of ADU

The same idea goes by many names, including accessory apartment, secondary suite and granny flat. The American Planning Association groups them into three forms, and some places add a fourth:

• Internal. Converted space inside the existing house, such as a basement or attic.
• Attached. An addition built onto the house.
• Detached. A new freestanding building, or a converted garage or other outbuilding.
• Junior ADU. In San Francisco's program, a junior ADU converts up to 500 square feet inside a single-family home. Los Angeles County's ADU summary also caps junior ADUs at 500 square feet, says they can include an attached garage, and requires a separate entrance.

In all forms, the unit is meant to function independently. A city FAQ from Annapolis, Maryland describes ADUs as having their own kitchens, bathrooms, living areas and entrances.

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Why the rules differ so much

There is no single national ADU code. Zoning, size limits, height, setbacks and parking are decided by states and cities, and the examples below show how far apart they can be. They are examples, not rules for your address.

The California handbook is listed in the sources below. The Washington examples come from the Municipal Research and Services Center's ADU overview, and the Portland figures from Portland's zoning page. The takeaway is that a size or height you read about online may be forbidden, or required, in your town.

Questions to ask before you design

A short call or visit to your planning department can save months. These are the questions worth asking, drawn from what the examples above regulate:

• Are ADUs allowed on my lot, and which types: internal, attached, detached or junior?
• What are the maximum size, height and setbacks?
• Is parking required, and does it change if the ADU is near transit or in a fire hazard zone?
• Must the owner live on the property? San Francisco, for example, ties owner occupancy to certain junior ADUs.
• Are there deed restrictions, homeowners association rules or historic district standards that apply on top of city rules?
• Can the ADU be rented, and does renting bring extra requirements?
• How are utilities connected, and are there separate fees?

Choosing a type: what tends to matter

Which type suits a property depends on the house, the lot and local rules. The comparison below is our editorial summary of common trade-offs, not a ranking. One design firm, Building an ADU, notes that additions can benefit from shared walls and easier access to utilities, which is a common reason people consider attached units.

Some regulations also blur the line with other small homes. Port Townsend, Washington, for example, lets a tiny home on wheels serve as a detached ADU under its code, as the MRSC overview notes. If you are weighing that route, read our guide to tiny house building codes and zoning.

Budget and payback: start with your own numbers

We do not quote ADU costs because they swing widely with type, site and local fees, and no national figure is reliable. Get itemized local estimates, then test the totals. Our home addition cost calculator adds site work, fees and a contingency to your per-square-foot quotes, and the rental property calculator shows whether expected rent covers the operating costs. Neither replaces a quote from a licensed contractor.

Your takeaway: before you sketch anything, find your city's ADU page, note the size, height, setback and parking rules for your lot, and write down the three questions the page does not answer. Take those to the planning counter.