Construction on Slopes
The City of Mercer Island is very concerned with the safety of any development on sloping sites and with the impacts of new construction or subdivisions on undeveloped hillsides. The City has regulations that either limit construction on slopes or require more detailed application submittals or additional permits.
What is considered a "steep slope"?
A steep slope is considered a slope of 40 percent or greater calculated by measuring the vertical rise over any 30-foot horizontal run. If you are planning construction or subdivision of a property that contains slopes of 40 percent or more on any portion of the site, you should read the regulations related to Geological Hazards and Critical Slopes in the MICC 19.07.060.
Click here to also view our building page which provides more information on Geologic Hazard areas.
Can I build on a steep slope?
In short, most likely, but building can be restricted. Some lots on Mercer Island have designated building pad areas. This means that building construction is limited to that designated building pad area.
If there is no designated building pad area recorded for a single family lot, a building permit will only be issued if the applicant's site construction plan and supporting professional documentation demonstrate that the applicant has used reasonable best efforts to comply with the preferred practices identified in the development code. Structures and street surfaces should be located outside of the critical areas or, if that is not possible, minimize the adverse impacts on critical areas to the extent reasonably feasible.
Where critical slopes are greater than 50 percent, no development is allowed and no impervious surfaces are permitted, unless the applicant can demonstrate through professional reports that the public's health, safety and welfare will not be comprised.
What about impervious surfaces?
In addition to the impervious surface limitations set forth in MICC 19.02.020(D), on those geographical portions of a lot constituting a critical slope, impervious surfaces shall be limited to the following on critical slopes:
- 30-50% slope limited to 30% impervious surface
- Greater than 50% slope limited to 20% impervious surface
This information is summarized from Title 19 of the Mercer Island City Code, "Unified Land Development Code" and is provided for quick reference only. To determine exact requirements, and for other development regulations affecting a parcel or lot, Title 19 should be consulted.