What is an ADU (Accessory Dwelling Unit)? Zoning Definition
An Accessory Dwelling Unit is a smaller, self-contained residential unit located on the same lot as a primary single-family home, such as a backyard cottage, garage conversion, or basement apartment.
What ADUs Are
An Accessory Dwelling Unit (ADU) is a secondary housing unit on a single-family residential lot. ADUs go by many names -- granny flats, in-law suites, backyard cottages, or casitas. They include their own kitchen, bathroom, and sleeping area, making them fully independent living spaces. ADUs can be detached (a separate structure in the yard), attached (an addition to the main house), or internal (a converted garage, basement, or attic).
Why ADUs Matter for Property Buyers
ADUs have become one of the most popular ways to add value to residential property. They can generate rental income, house aging parents, or provide space for adult children. Many states -- most notably California -- have passed laws requiring cities to allow ADUs and streamlining the approval process. If you are buying a home and considering building an ADU, check the local rules on maximum size, owner-occupancy requirements, parking, and setback reductions that may apply.
Key Considerations
- Size limits vary widely. Some cities cap ADUs at 800 square feet; others allow up to 1,200 square feet or more.
- Junior ADUs (JADUs) are a smaller variant (typically under 500 square feet) created within the existing home footprint, often with relaxed permit requirements.
- Impact fees, utility connection fees, and permit costs can add significantly to construction costs -- research these before committing.
- Short-term rental restrictions may apply. Many cities prohibit using ADUs as Airbnb-style rentals.
- State preemption laws are changing rapidly, so rules that applied a year ago may have been loosened since then. Always verify current regulations with your local planning department.
Related Terms
Floor Area Ratio (FAR)
Floor Area Ratio is the total building floor area divided by the lot area, controlling how much total space you can build on a given parcel.
Setback
A setback is the minimum required distance between a building and a property line, street, or other reference point, ensuring space between structures.
Conditional Use Permit
A conditional use permit (CUP) is an approval that allows a specific land use in a zoning district where that use is permitted only if certain conditions are met, such as limiting hours of operation or providing extra parking.