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FAQs - Development Services - Building & Planning
Development on Waterfront Lots

Q: How close to Lake Washington can I construct a structure?

A: There is a 25-foot setback from the OHWM for all structures.


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Q: Where is the Shoreline Critical Area?

A: This is the area within 50 feet landward of the OHWM, which directly impacts water quality as surface and subsurface waters are filtered back into the lake. Additionally, shorelines are valuable fish habitat areas characterized by lake bottom conditions, erosion tendencies and the proximity to watercourse outfalls. These may combine to provide a suitable environment for spawning fish.


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Q: Do I need a special Shoreline permit to construct a single-family residence on my waterfront lot?

A: A substantial development permit (SDP) is required for construction of or an addition on to a single family residence, unless you are the owner, lessee or contract purchaser and are constructing or altering the residence for your own use and are complying with all requirements of the City’s shoreline master program and the ULDC. In this case, the single family residence is categorically exempt from applying for a shoreline substantial development permit. Construction authorized under this exemption shall be located landward of the ordinary high water mark. There is an affidavit available from the City that the owner, lessee or contract purchaser signs to verify that the development is for his/her own use, which must be filed with the City at time the building permit is applied for.


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Q: What is the OHWM?

A: OHWM stands for the Ordinary High Water Mark and is some times written as OHW. This line is usually identified by examining the bed and banks of the water along the shore to determine where action of the water has created a distinct mark upon the soil with respect to upland vegetation. <p>If there are any questions at all, regarding the location of the OHWM, contact the Washington Department of Fish and Wildlife for assistance.


For more information, contact Fish and Wildlife at 425.649.7042.


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Q: Are there special requirements for impervious surfaces for development located landward from the OHWM?

A: Yes, the maximum impervious surface coverage requirement between 0 and 25 feet from the OHWM is 10%. Between 26 feet and 50 feet from the OHWM structures are allowed; however there is a maximum impervious surface limit of 30%.

Distance from OHWM Impervious Surface Limitations
0 – 25 feet 10% – No structure(s) allowed
26 – 50 feet 30% – Structure(s) allowed


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Q: What is the Shoreline Jurisdiction?

A: The shoreline jurisdiction is geographically defined as:

  • All lands extending landward 200 feet in all directions as measured on a horizontal plane from the ordinary high water mark and all associated shorelands. (RCW 90.58.030)
  • All lands under Lake Washington extending waterward to the line of navigability/inner harbor line as established in 1984 by the Board of Natural Resources by Resolution No. 461.
The City’s Shoreline Master Program (ULDC 19.07.080) applies to all development within the shoreline jurisdiction and shoreline uses are established by Shoreline Designated Environments. There are three shoreline environments designated along Mercer Island’s shoreline: Conservancy Environment, Urban Park and Urban Residential. A map of the designation of the various shoreline environments is available for review at City Hall.

The shoreline jurisdiction and the shoreline designated environments are superimposed upon the existing zoning classifications. All uses, activities, and developments must comply with both the Mercer Island development code and the shoreline master program. If there is a conflict between the two, the more restrictive regulation applies.



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