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Council directs interim rules to allow duplexes, triplexes and limited ADUs under state law
Summary
To comply with new state housing laws, the council directed staff to draft interim development regulations permitting six middle-housing types, capping unit density on most lots at two (with four allowed near transit or for affordability), setting ADU size at 1,000 sq ft and applying impact fees to middle housing and ADUs.
The Mercer Island City Council Tuesday directed staff to prepare interim development regulations to implement Washington's recent middle-housing and ADU laws, and set follow-up hearings for ordinance adoption.
Allison Van Kork, deputy director of Community Planning and Development, summarized the state requirements contained in HB 1110 (middle housing), HB 1337 (ADUs) and related state provisions. "HB 1110 requires us to permit moderate density middle housing on all residential lots," Van Kork said, adding the legislation allows a menu of housing types and limits local parking and owner-occupancy restrictions.
The nut graf: to meet state deadlines and ensure local code compliance, council members approved a package of interim directives specifying allowed housing types, density rules and ADU standards; staff will draft ordinances for first reading on Feb. 4 and second reading on March 4, with an effective date set for June 30 (to align with state requirements).
Council actions and staff recommendations (summary)
- Allowed housing types: Staff recommended, and council directed, interim allowance of six state-defined types: duplex, triplex, fourplex, townhouse, courtyard apartment and stacked flat. Council vote: motion passed unanimously.
- Density approach: Council adopted the staff-recommended "standard density" option: a maximum of two units per lot in R-8.4, R-9.6, R-12 and R-15 zones, with a maximum of four units per lot permitted when the property is within one-quarter mile walking distance of a major transit stop or when one unit is provided as an affordable unit. The council also directed that accessory dwelling units count as a unit for calculating unit density.
- ADU size and bonus: Council directed staff to set a 1,000-square-foot maximum for ADUs and to tie the city's existing 5% gross-floor-area (GFA) bonus for ADU creation to an at-least-one-per-ADU parking requirement under the interim regulation. (Under state law, ADUs up to 1,000 sq ft must be permitted.)
- Impact fees: Council instructed staff to prepare a March 4 resolution to apply the city's existing impact-fee rates to middle housing and to apply an ADU impact-fee cap consistent with HB 1337 (ADU fees not to exceed 50% of single-family rates). Staff said middle-housing entries of one or two units would follow single-family fee rates; three or more units would trigger multifamily rates.
Process and next steps
Van Kork told the council the city will proceed with a two-phase approach: interim regulations now (phase 1) to achieve minimum legislative compliance and a phase 2 process over the next two years for more tailored rules with expanded community engagement. Councilmembers requested clearer guidance for critical-area lots and said staff should avoid excluding parcel types without site-specific analysis. Public hearing and ordinance first reading are scheduled for Feb. 4; second reading and adoption would follow on March 4 under staff's timetable.
