Design for an Accessory Dwelling Unit (ADU)(known as a Mother-In-Law Apartment)

By Alan Ness, Principal, Ten Directions Design

An Accessory Dwelling Unit or ADU is a second living space located on your property in a single-family zone.

To understand the background of the ADU, you need to know what is Zoning and why do we have it? Zoning is the segregation of functions in a city. In simple terms, a single-family area should look like a collection of single-family house, not apartment buildings or anything else. Although many cities allow ADUs to increase density and livability, they do not want to lose the "look and feel" of a single-family zone. This explains some of the rules that follow.

What are the Ten Golden Rules for ADU's?  From the City of Seattle:

  1. Owner Occupancy - Owner must occupy either unit for at least 50% of year and own at least 50% of house. Exceptions apply, see below.
  2. Total number of residents - may not exceed 8 people, unless related.
  3. Location - in the principal structure, or as a detached structure.
  4. Number of ADU's - no more than one ADU per property (total of 2 living units per property).
  5. Off-street parking: two spaces required. Minimum size: 8x16, or 9x16 by a wall or structure. If there is an alley, nominal property line must be 12' from alley centerline. Exceptions apply.
  6. Maximum area - if attached, 1000 SF; if house built by 1999 may occupy an entire level over 1000 SF. If detached, not more than 800 SF total of all floors.
  7. Entrance location - only one entrance on street or front side per property. Second entrance may not be on front side. Exceptions apply.
  8. Minimum ceiling height - in pre-1979 house, 6'4" to bottom of all beams and ducts, with a hard-wired smoke detector. House built after 1979 must have 7'-0" headroom to lowest point of any beam or duct.
  9. Bedroom egress windows - size to be 5.7 SF, with a maximum 3'-8" high sill (7" step OK). Exceptions for bedrooms existing before 1972 (with proof).
  10. . Full code compliance - Residential, Building, Mechanical, Electrical & Energy codes.

What are the Three Exemptions?

  1. Off-street parking - not needed in a RPZ; not needed if there is steep topography or location of existing structure blocks access.
  2. Owner Occupancy - may need to show good cause for up to 3-year absence.
  3. Entrance location - topography exemption or screening/design solution de-emphasizing.

Where can I put an ADU in my house? 

Anywhere your imagination can think of.

  • Basements
  • Main and Upper Floors
  • Attached garages.

What are the newest trends in ADU's ? 

Detached ADUs are the newest trend.

Basement Pitfalls:

  • Toilets - locating sewer line depth for connection. Sewer cards at DPD.
  • Head Height - can be very difficult, especially at stairs. Digging down may be needed.  Or, raise up your house, if you have foundation problems.
  • Mildew and Mold - Avoid, Isolate, Ventilate and Filter. Molds and dust problems. 
  • Also, energy costs: insulating the walls, better windows, extending ducts.

How much will it cost?

  1. Construction costs - SF costs: baths, kit, rooms. Line items: sewer; window wells.
  2. Design and engineering fees - 10% to 20% of construction costs. $4-8/SF +/-.
  3. Permit fees - $2800.00 fee on $100,000 cost.

For More Information from the City of Seattle, go to TIP #116.