New Snohomish County Drainage Manual Adds Another Layer to Retaining Wall Planning

Snohomish County makes a large change with it's drainage manual which is adding another layer to retaining wall planning for landscaping companies.

SNOHOMISH, WA, UNITED STATES, September 24, 2026 /EINPresswire.com/ -- A retaining wall has one main job: hold soil in place. But in Snohomish County, the amount of dirt being moved, where water travels across the property, and what sits behind the wall can all affect how that project needs to be built.

Those details are getting more attention after Snohomish County put its updated Drainage Manual into effect on July 1, 2026. The update revised the county’s stormwater framework for development and redevelopment projects in unincorporated areas.

For Ivan’s Landscape & Construction, a landscape contractor based out of Snohomish, Washington stated the change connects to something retaining wall contractors already deal with on the job: water behind a wall can be just as important as the wall itself.

Water Has to Go Somewhere

Retaining walls hold back soil, but soil changes when it becomes saturated.

When water builds up behind a wall, it can increase pressure against the structure. That makes drainage, backfill, grading, and the direction of runoff part of the project before the first block or section of concrete is installed.

“People usually notice the part of the wall they can see,” said Ivan of Ivan’s Landscape & Construction. “What matters just as much is what is behind it. If water has nowhere to go, that becomes part of what the wall has to deal with.”

Depending on the project, contractors may use drainage rock, pipe, properly placed backfill, grading, or other methods to help control water around the structure.

Ivan’s Landscape & Construction handles retaining wall installation along with excavation, grading, and drainage work throughout Snohomish County, allowing those parts of the project to be considered together rather than separately.

Larger Projects Can Trigger County Review

The July drainage update does not create a new retaining wall permit requirement. Snohomish County already has separate rules that determine when retaining walls need permits and engineered plans.

Retaining wall projects often involve the same land disturbance covered by county drainage regulations.

Snohomish County says Land Disturbing Activity review can be triggered when a project clears or grades more than 7,000 square feet, moves more than 100 cubic yards of material within an 18-month period without an associated building permit, changes an existing drainage pattern, or disturbs a critical area or buffer.

That becomes relevant when a retaining wall is part of a larger landscape project, major slope correction, drainage improvement, or site redevelopment.
The new Drainage Manual works alongside the county’s drainage code and Engineering Design and Development Standards to manage stormwater as properties are changed.

Height Is Not the Only Thing That Matters With a Retaining Wall

Snohomish County also looks at what a retaining wall is supporting.

Walls over 4 feet tall, measured from the bottom of the footing to the top of the wall, require a building permit and engineered plans in unincorporated Snohomish County.

A wall 4 feet tall or shorter may still need the same level of review when it supports a surcharge.

A surcharge is extra weight or pressure behind the retaining wall. A driveway, parked vehicles, a nearby structure, additional fill, or other loads can change what the wall needs to support.

That means two retaining walls of the same height may not have the same design or permitting needs.

Ivan’s Landscape & Construction installs and repairs retaining walls for residential and commercial properties, including concrete, block, rock, gravity, cantilever, and other wall systems. The company also assists property owners with permitting when a project requires county approval.

With Snohomish County’s updated drainage standards now in effect, property owners planning retaining walls, grading, drainage improvements, or larger landscape projects have another reason to look at the whole property before construction starts.

Contact Ivan’s Landscape & Construction to discuss retaining wall installation, repair, excavation, grading, and drainage work throughout Snohomish County.

About Ivan’s Landscape & Construction

Ivan’s Landscape & Construction is based in Snohomish, Washington, and serves residential and commercial properties throughout Snohomish, King, and Skagit counties. The company provides retaining wall construction and repair, excavation, grading, drainage, land clearing, landscaping, hardscaping, irrigation, and other outdoor construction services.

Ivan Diaz
Ivan's Landscape & Construction
+1 425-239-1339
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