Mukilteo City Zoning Code
CHAPTER 17
52A GEOLOGIC SENSITIVE AREA REGULATIONS
§ 17.52A.010 Purpose.
The purpose of this chapter is to regulate development activities in or near geologically hazardous areas to reduce risks to life and property and prevent erosion at both the parcel and system scale.
(Ord. 1533, 3/23/2026)
§ 17.52A.020 Applicability.
This chapter applies to any development activity within 50 feet of steep slopes or landslide hazard area, unless specifically exempted in Chapter 17.52.
(Ord. 1533, 3/23/2026)
§ 17.52A.030 Required reports and plans.
A.
Geotechnical report.
1.
Required. When proposing structures within 50 feet of a steep slope or landslide hazard area, applicants must submit a geotechnical report prepared by a qualified professional. The Building Official has discretion to waive or modify report requirements if the information is not needed to accomplish the purpose of this chapter.
2.
Contents. The report must include:
a.
Project description.
b.
Site plan showing the location of the soil investigations.
c.
Previous landslide activity in the vicinity, an assessment of the overall slope stability, and the anticipated effect of the development on the slope over time.
d.
Grading details, including: total amount of cut and fill, recommended procedures, fill placement, compaction criteria, temporary and permanent slope support, erosion control measures, wet weather limitations, and design criteria for any recommended corrective measures.
e.
A description of surface and subsurface hydrology together including any wetland, streams, springs, seeps, or ground water features, together with recommendations for addressing the impacts of those features in a manner consistent with the city's critical areas regulations.
f.
All ICC geotechnical requirements, including documentation to support any alternate setback proposals.
3.
Expiration. Geotechnical reports are valid for five years. Reports older than five years at the time of permit submittal must be updated.
B.
Adherence to the recommendations required. All recommendations in required reports are considered conditions of approval. Failure to follow report recommendations will be considered a violation of permit conditions.
C.
Third-party review. The city may retain a qualified professional, at the applicant's expense, to review and confirm any findings in required reports.
(Ord. 1533, 3/23/2026)
§ 17.52A.040 Vegetation management.
A.
Altering or removing vegetation on a steep slope or within a landslide hazard area is prohibited, with the following exceptions:
1.
Pruning. Pruning methods consistent with Chapter 17.59 are allowed.
2.
Removal of hazardous trees. Removal of hazardous trees is permitted when a certified arborist determines removal is needed to resolve a safety issue that cannot be eliminated through pruning or other methods. Ten vertical feet of stump must be left in place when removing trees 15 inches or greater DBH.
3.
Removal of alder, willow, and bitter cherry trees. These tree species may be removed when the stumps and roots are left in place and three deep-rooted bushes or ground cover such as ocean spray, snow berry, salal or evergreen huckleberry are planted around each stump.
4.
Removal of invasive species. Himalayan blackberry, Scot's broom, thistle and other similar invasive plants may be removed manually, provided there is minimal soil disturbance and the area is immediately be replanted with deep-rooted native shrub species such as Oregon grape, salal and evergreen huckleberry.
5.
Other modifications consistent with the Department of Ecology's "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" may be approved by the director.
B.
Replanting required. Applicants proposing vegetation removal, other than authorized pruning or snagging of hazardous trees, must submit a replanting plan that includes plant quantities, locations, and the schedule for installation.
(Ord. 1533, 3/23/2026)
§ 17.52A.050 Development standards.
A.
Site design. Structures and improvements must be located to avoid geologically hazardous areas wherever possible, minimize alterations to the natural contours, preserve the most sensitive portions of the site, maximize vegetation retention, and avoid increased risk to neighboring properties.
B.
Minimum setbacks from steep slopes that are part of a ravine or shoreline.
1.
Human safety standard. Structures must be 40 feet from the top of a steep slope and 15 feet from the toe. Applicants proposing structures, including decks, closer than these standards must comply with the building setback and development recommendations in their geotechnical report.
2.
Erosion reduction standard. All structures, except decks, must remain a minimum of 25 feet from the top of steep slopes that are part of a ravine or shoreline.
3.
Exceptions.
a.
Retaining walls. Retaining walls and other slope stability infrastructure are permitted in geologically hazardous areas when needed to remedy a safety issue or prevent damage to an existing structure. To be eligible for this exemption, a geotechnical report from a qualified professional is required and must contain a clear finding that the construction methods and project minimize slope disturbance as much as possible and will improve the long-term stability of the slope.
b.
Utilities. When there is no practical alternative, utility lines and pipes may be constructed on steep slopes. Lines and pipes must either be located above ground and properly anchored and designed to continue to function in the event of an underlying slide or bored to a depth beneath potential slope failure. Stormwater pipes must be designed with high-density polyethylene and fuse-welded joints, or similar product that is technically equivalent.
C.
Stormwater. Runoff must be collected, detained, and released in accordance with the city's stormwater detention requirements. Concentrated stormwater runoff is not allowed to flow directly over a steep slope or impact a neighboring property.
D.
Seismic safety factors. Development must be designed to provide a landslide safety factor of at least 1.5 for static conditions and 1.1 for dynamic conditions, based on horizontal acceleration and the standards in the current adopted version of the International Building Code.
E.
Seasonal restrictions. Clearing and grading within the wet weather months is only allowed with director approval of a winter weather construction plan that uses current best management practices.
F.
Alternative designs. Development within geologically hazardous areas must meet the requirements above unless the director finds that an alternative design standard will provide greater long-term slope stability (without the need for regular or periodic maintenance).
(Ord. 1533, 3/23/2026)
