City of Sammamish

Accessory dwelling units go by many names, such as ADUs, in-law units, granny flats, and casitas.

An ADU is an attached or detached residence located on the same lot as a single-family home. ADUs are separate, complete residences with their own kitchens, bedrooms, bathroom, and entrances.

ADU Basic Information:

Up to two ADUs permitted on lots with a detached single-family residence, in the following configurations:
- Two attached ADUs, such as in a basement, above an attached garage, or home addition
- Two detached ADUs, which may be attached to each other or a detached garage
- One attached ADU and one detached ADU
- A conversion of an existing structure into an accessory dwelling unit
A detached ADU can be a maximum of 1,000 square feet.
An attached ADU can be up to 50% of size of the single-family residence or 1,000 square feet, whichever is larger.
ADUs do not count towards the Floor Area Ratio (FAR) of the single-family residence.
No additional parking is required when the property already has four or more parking spaces (which includes garage space and the driveway).

Option 1: Detached ADU Development Standards by Zone (only when ADU is not attached to another structure or improvement, such as garage or shed)

Development Standard	R-1	R-4	R-6	R-8	R-12	R-18
Front setback*	20 ft.	15 ft.	15 ft.	10 ft.	10 ft.	10 ft.
Rear setback	5 ft.	5 ft.	5 ft.	5 ft.	5 ft.	5 ft.
Side setback	5 ft.	5 ft.	5 ft.	5 ft.	5 ft.	5 ft.
Height	24 ft.	24 ft.	24 ft.	24 ft.	24 ft.	24 ft.

*Front setback along arterial streets is a minimum of 30 feet.

Option 2: Other Structure Development Standards by Zone (required when ADU is attached to other structure, like a garage)

Development Standard	R-1	R-4	R-6	R-8	R-12	R-18
Front setback*	20 ft.	15 ft.	15 ft.	10 ft.	10 ft.	10 ft.
Rear setback	10 ft.	10 ft.	10 ft.	10 ft.	5 ft.	5 ft.
Side setback	10 ft.	10 ft.	10 ft.	10 ft.	5 ft.	5 ft.
Height	35 ft.	35 ft.	35 ft.	35ft.	35 ft.	35 ft.

*Front setback along arterial streets is a minimum of 30 feet.