Key Concepts

The following concepts will provide a basis for understanding the new development types that will be permitted in Mercer Island under the new interim development regulations.

Middle Housing

Middle Housing includes the housing types that fall in between detached single family homes and midrise apartment buildings. In the interim development regulations, Mercer Island has permitted six middle housing types in the Residential zones: Duplex, Triplex, Fourplex, Townhome, Courtyard Apartment and Stacked Flat.

The permitted middle housing types will be subject to the existing development standards for single-family development in the Residential zones as well as the Unit Density standards, described further below. Thus, on most residential lots, two middle housing units will be allowed. The development regulations do not include minimum or maximum size limits for middle housing units. Rather, the size is governed by the dimensional standards in the development regulations, including building heigh, setbacks and gross floor area. Any middle housing units (as well as single family or ADU units) on the lot must fit within these limits.

Accessory Dwelling Units

Accessory Dwelling Units (ADUs) are a small housing unit (between 220 and 1,000 square feet) that is either attached or detached from a primary dwelling unit. The interim development regulations permit up to two ADUs per Residentially-zoned lot. These ADUs are limited, along with other accessory buildings (e.g. garages, sheds), to 25% of the gross floor area (GFA) allowed on the lot. So, for example, on a lot that permits 4,000 square feet of GFA, up to 1,000 square feet (total) can be used for ADUs, garages, sheds and other accessory buildings.

Unit Density

HB 1110 requires cities to establish a new system for regulating housing density that has not been commonly used in the past. This system regulates density based on the number of housing units allowed per lot, regardless of lot size. This is referred to as “unit density”. Any of the permitted middle housing types can be combined on residential lots to meet the unit density as follows:

• Allow at least 2 middle housing units per lot.
• Allow at least 4 middle housing units per lot if:
  1. The lot is located within a quarter mile walking distance of the light rail station (see Map 2: Distance from Light Rail, on the following page), or
  2. At least one unit is affordable (more information is provided below on this affordability incentive).

Unit Lot Subdivision

Unit lot subdivision allows the land beneath detached single family housing, ADUs or middle housing where no units are stacked on another unit, to be divided for individual sale. Unit lot subdivision is defined in state law as a type of short subdivision. The Mercer Island City Code defines a short subdivision as “a subdivision consisting of four or less lots on four or less acres.” Together these requirements will enable parent lots to be divided into up to four unit lots for individual sale and fee-simple ownership.