The purpose of this chapter is to allow for accessory dwelling units to be established which are incidental to the primary residential use of a single-family residence or middle housing residence, while ensuring compatibility with surrounding residential uses. The accessory dwelling unit(s) must be clearly subordinate to the primary use. Accessory dwelling units may not be established until the principal residence is constructed on the property.
(Ord. 3351 § 6 (Exh. B), 2025; Ord. 3367 § 2 (Exh. B), 2025)
In the zones in which an accessory dwelling is listed as a permitted use, the community development director or designee shall review all proposals to establish an accessory dwelling unit. The following standards and regulations shall apply to all proposed accessory dwelling units:
(1) 
On each lot developed with a single-family residence, accessory dwelling units may be constructed subject to the standards set forth in Table 1 below. An accessory dwelling unit may not be located on a lot on which a temporary dwelling, as defined in Chapter 22C.110 MMC, is located.
Table 1
Principal Dwelling Unit(s)
Single-Family Residence
Middle Housing
Number of accessory dwelling units allowed per lot 1
Two
Two if the lot allows for a density of four dwelling units per lot pursuant to MMC § 22C.010.080. Note: The accessory dwelling unit(s) apply to the total allowed unit density for the lot.
Owner occupancy requirement for principal dwelling or accessory dwelling units
None
Allowed accessory dwelling unit configurations
(a) One attached unit and one detached unit;
(b) Two attached units; or
(c) Two detached units, which may be comprised of either one or two detached structures.
Use as short-term rental
Prohibited
Minimum size per accessory dwelling unit 2
200 SF gross floor area
Maximum size per accessory dwelling unit 2, 3
1,000 SF gross floor area or 50 percent of the total floor area of the single-family residence, whichever is greater; provided, that in no case shall an accessory dwelling unit exceed 1,400 square feet 1
Maximum bedrooms per accessory dwelling unit
Two
Front setback 4
See MMC § 22C.010.080 for standard lots and MMC § 22G.080.080 for PRD lots.
Side setback 4
Five feet
Side street setback 4
10 feet along nonarterials; 15 feet along arterials
Rear yard setback 4
15 feet; provided, that the rear yard setback may be reduced to 10 feet for one-story structures, or for structures located in a PRD 5
On-lot structure separation
A minimum of five feet of separation is required between structures.
A five-foot-wide paved pathway that is free of obstructions must also be provided between the front entrance of the accessory dwelling unit and the property line from which the property is addressed to ensure adequate access for emergency services.
Building and impervious coverage 4
See MMC § 22C.010.080 for standard lots and MMC § 22G.080.080 for PRD lots.
Height
30 feet
Parking 6
See parking for middle housing in MMC § 22C.130.030, Table 1.
Notes:
1 On any lot that meets the minimum lot size required for the principal dwelling unit.
2 Floor areas shall be exclusive of garages, porches, or unfinished basements.
3 The community development director is authorized to allow an accessory dwelling unit greater than the maximum size limit in the following circumstances:
(a) For an attached accessory dwelling unit within an existing structure, when a denial of such an increase would result in an unreasonable division of interior space between the ADU and the primary residence;
(b) An existing residence is converted to an accessory dwelling unit, provided the size of the existing residence to be converted does not exceed 100 square feet more than the allowance for a new accessory dwelling unit; or
(c) A minor revision is needed to accommodate industry standards for building dimensions.
4 Existing structures, including but not limited to detached garages, may be converted to accessory dwelling units, even if they do not comply with current code requirements for setbacks or building coverage.
5 Detached accessory dwelling units may be built at a lot line, if the lot line abuts a public alley.
6 No parking areas other than driveways may be in front yards. When the property abuts an alley, parking areas must be accessed from the alley.
(2) 
The architectural character of the single-family dwelling shall be preserved. Exterior materials, roof form, and window spacing and proportions shall match that of the existing single-family dwelling. In no case shall a detached accessory dwelling unit have axles or be on a chassis.
(3) 
The city may not prohibit the sale or other conveyance of a condominium unit independently of the principal structure solely on the grounds that the condominium unit was originally built as an accessory dwelling unit.
(4) 
The provisions of this section do not apply to portions of lots designated with critical areas or their buffers as designated in RCW 36.70A.060, or to lots in a watershed serving a reservoir for potable water if that watershed is or was listed, as of July 23, 2023, as impaired or threatened under section 303(d) of the Federal Clean Water Act (33 U.S.C. Section 1313(d)).
(5) 
In addition to the conditions that may be imposed by the community development director, all accessory dwelling units shall also be subject to the condition that the use shall be discontinued if:
(a) 
The accessory dwelling unit is substantially altered and no longer conforms with the plans approved by the community development director and the building official; or
(b) 
The subject lot ceases to maintain the required parking spaces outlined in MMC § 22C.130.030, Table 1.
If the deficiencies outlined in subsections (5)(a) and (5)(b) of this section are remedied, the community development director may allow the use to be reestablished.
(Ord. 3351 § 6 (Exh. B), 2025; Ord. 3367 § 2 (Exh. B), 2025)