Interim Development Regulations

In February and March 2025, the Mercer Island City Council enacted a series of interim development regulations and a fee schedule amendment intended to comply with several new state legislative requirements related to bills adopted in 2023 (HB 1110, HB 1337, and SB 5258). These interim development regulations were adopted quickly in order to comply with state-mandated deadlines and avoid state regulations superseding the City's authority. The City plans to adopt permanent regulations and will undertake additional public outreach and analysis to assist in developing these regulations, beginning in 2026.

The interim development regulations permit middle housing and expand allowances for Accessory Dwelling Units (ADUs) throughout the city's residential zones (Ordinance No. 25C-02). They also permit a new type of land division, Unit Lot Subdivision, that enables the individual sale of these housing units, separate from any primary unit located on the property (Ordinance 25C-06). Finally, the City's fee schedule has also been amended to apply the existing impact fee rates to middle housing and ADU units (Resolution 1671). These fees help to cover the costs of infrastructure improvements that are needed to accommodate additional growth and development within the city.

Additional details on each of these actions are summarized below, and links are provided to the full text of the adopted ordinances and resolution.

Ordinance No. 25C-02

MICC 19.02.010 Single-family:

·Permits middle housing as a use

·Allows more than one ADU per lot

MICC 19.02.020 Development Standards:

·Modifies the GFA incentive for ADUs to specify that the incentive applies to single family homes with attached ADUs when an additional off-street parking space is provided for the ADU

MICC 19.02.025 Middle Housing (new section):

·Permits the following housing types in the R-zones: Duplex, Triplex, Fourplex, Townhouses, Courtyard Apartments, Stacked Flats

·Unit density

·2 units per lot

·4 units per lot within ¼ mile walking distance from light rail, or if one unit is affordable

·Affordable housing

·60% AMI for rental, 80% AMI for ownership

·Parking requirements

·No parking required within ½-mile walking distance of the light rail station

·1 parking space per unit for lots smaller than 6,000 square feet

·2 parking spaces per unit for lots larger than 6,000 square feet

MICC 19.02.030 Accessory Dwelling Units:

·Removes the owner occupancy requirement

·Permits up to two ADUs per lot

·Increases the maximum gross floor area to 1,000 sq ft

·Adds parking requirements (same as for Middle Housing)

MICC 19.16.010 Definitions: revises and add definitions related to amendments

Ordinance No. 25C-06

MICC 19.08.080 Fee Simple Unit Lot Subdivisions (New Section):

·Enables the creation of unit lot subdivisions using the City’s existing short plat process

·Applies existing development standards to the parent lot

·Dimensional standards including setbacks, gross floor area, lot coverage, etc.

·No setbacks required on unit lots

·No prescribed requirements for unit lot length, width or area

·Required parking may be provided on another unit lot or tract

·Final plat must indicate that unit lots are not separate buildable lots and additional development of units lots may be limited by the application of development standards to the parent lot

MICC 19.15.030: adds Preliminary Unit Lot Subdivision and Final Unit Lot Subdivision to the City’s Land Use Review Type table

MICC 19.16.010: adds definitions for Parent Lot and Unit Lot

Resolution 1671

Fee Schedule Update

·Adds impact fees for middle housing and ADUs based on existing rates