Mercer Island adopts new residential development code

September 25, 2017 | by Erin Sirianni

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At last week’s meeting, the City Council adopted a new residential development code that will place new limits on house size and lot coverage, provide more protection for trees, and reduce construction impacts to neighborhoods. The changes will go into effect on November 1st.

The code revision was the result of a year-long process conducted by the Planning Commission and the Mercer Island City Council and was initiated in response to community concern about the changing character of neighborhoods as large new construction homes have replaced modest mid-century era homes throughout Mercer Island. According to our data analysis last year, Mercer Island has gotten about 60 new homes per year since 2013.

We’ve been following the residential standards review process throughout the months since a residential development moratorium was proposed unsuccessfully in November 2015. The update to the code has been controversial. Community members expressed their concern that a more restrictive code will impact their plans for building and home renovations as well as their property values when they decide to sell. Builders were also a strong voice in the debate and circulated mailers opposing the updates, especially in regard to caps on house size and lot coverage reduction.

Our evaluation of the new code is that the biggest impact to the average home owner or anyone looking to develop a property will be in regard to tree protections. As of November 1st, a permit will be required to remove any tree with a diameter of more than 10 inches; replacement trees will be required for trees removed; and “exceptional trees,” including any tree with a diameter of more than 36 inches, are protected.

To find out what trees are characterized as “exceptional,” see page 85 of the new code. Sixty-eight species are listed, along with their threshold diameters. Basically, if you have a healthy, mature tree that is not creating any hazards on your property, there’s a good chance that you may not be able to remove it, according to the new code.

In addition to tree protection, we’ve outlined the basic code changes below, based on this City of Mercer Island info sheet:

New Limits on House Size and Bulk

  • Reduction in gross floor area from 45% to 40% of the lot
  • Implementation of a maximum house size for each residential zone
  • Height limitation of 30′ on downhill side (reduced from 35′)
  • Side setback increased to 17% of lot width for lots wider than 90 ft, with additional increases for homes with a “tall wall”

New Tree Protection

  • A minimum of 30% of large trees must be retained when a lot undergoes major construction
  • Priority retention of large, healthy trees
  • Additional protection for exceptional trees (including any tree with a diameter of more than 36 inches), including requirements to re-design
  • Tree removal on any lot requires a permit (exceptions for trees that are hazardous, very small or noxious/invasive)

New Limit on Hardscapes

  • Up to 9% of the net lot area may be used for hardscape surfaces (decks, patios, etc.)

Construction Impacts

  • Construction hour reduction to 7 am – 7 pm on weekdays and 9 am – 6 pm on Saturdays
  • Requirement of construction management plans and a construction schedule for larger projects and permit renewals
  • Limitation of permit renewals and extensions to a total of 3 years

Fewer Deviations and Variances

  • Impervious surface and fence height deviations no longer allowed
  • Tougher standards on all other variances and deviations

For the finer details, be sure to read the final new code document with the full-text adopted changes.

The City will be providing additional outreach materials on the new changes and will also be hosting information sessions to educate builders, architects, permit applicants, and any interested residents about the new code.

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