Frequently Asked Questions
- 1 or 2 ADUs - these can be attached or detached and are limited to 1,000 sq ft per unit
- 2 units of middle housing in the form of a duplex, townhomes, stacked flats or courtyard apartments
- A duplex unit that’s attached to the existing single-family home – convert a portion of the existing square footage into a duplex unit or add attached square footage that constitutes a second middle housing unit (conversion of the single-family home to a duplex)
Are there minimum or maximum size standards for SFR and duplex units aside from allowable FAR on the parcel? I see ADU's/ DADU's are capped at 1,000sf.
There are no explicit minimum or maximum size limits for SFRs and middle housing units. Gross floor area and the other dimensional standards (height limit, setbacks, etc) will be the main controlling factor on building size. ADUs have a minimum size of 220 sq ft and a maximum of 1,000 sq ft (MICC 19.02.030 (B)(4)) and are also limited to 25% of the allowed gross floor area for the lot (MICC 19.02.040 (C)(1)(a)).
Based on what I am reading it appears that all lots on Mercer Island will be permitted to have a SFR and two "units" which could consist of an ADU, Duplex, Triplex, Fourplex, Townhouse, Stacked Flat, or Courtyard Apartments.
Yes, all R-zoned lots will be permitted two middle housing units (duplex, townhouse, stacked flat or courtyard apartment) or two ADUs. In addition, on R-zoned lots within ¼-mile walking distance from the light rail station, 4 units of middle housing are permitted. Similarly, four units are allowed if one unit is affordable (covenant on the property restricts sale price or rent). Thus triplexes and fourplexes can also be allowed under these circumstances.
What I am trying to figure out is say for instance there is a 2,500sf house existing on a lot with allowable FAR to 4,000sf. Can you build a 1,500sf stand alone structure (effectively a cottage) that you can sell separately from the existing house?
No, Mercer Island did not include cottage housing as one of the allowed types of middle housing, so a stand-alone middle housing unit is not permitted.
Or can you only build ADU's (1,000sf max) or duplexes that can be sold individually? Take parking and lot coverage out of the equation and assume all of that would have to be complaint to code.
On R-zoned lots (that are more than ¼-mile from the light rail station) with an existing single-family home, the new regulations will allow for one of the following configurations of additional units:
For large lots that could be subdivided into 2-3 lots, would the short plat and the unit lot subdivision need to be processed separately, or can they be consolidated into one short plat application?
A short subdivision to create the parent lots must be processed first. Once the parent lots are established/recorded, then unit lot subdivisions can be processed for each parent lot. We will require a separate ULS application for each parent lot. This will provide the mechanism for documenting development standards on each parent lot, including yard determination, setbacks, building pads, etc as part of the ULS.
How does yard determination work for parent lots that contain unit lots, especially when the parent lot is a corner lot?
The yard determination standards will be applied to the parent lot, not the unit lot. The standards in MICC 19.02.020 (C)(2) will be applied to the parent lot to establish the front, rear and side yards and associated setbacks. With a corner lot, the narrowest dimension of the lot abutting a street is the front yard. The rear yard is opposite the front yard, and the side yard is adjacent to the widest dimension of the lot abutting a street. Unit lots do not have an additional yard/setback requirements, beyond those that apply to the parent lot.
Does the accessory building limit of 25% of GFA apply to ADUs?
Yes, the accessory building limit in MICC 19.02.040 (C)(1)(a) does apply to ADUs. It limits accessory buildings to 25% of the allowed gross floor area for the parent lot.
Can an existing single family home that is a single story be remodeled to add a second story/stacked duplex unit? Can an existing two-story house be remodeled into a stacked duplex? Could an ADU also be added to this property?
Yes, yes and yes. Any of these scenarios could be viable, subject to the applicable development standards and the building code.