Snohomish County updates drainage rules for retaining wall projects

Snohomish County’s updated Drainage Manual took effect July 1, 2026, adding another layer to retaining wall planning in unincorporated areas. The change matters because drainage, grading and land disturbance can now shape permitting and design decisions for landscape and wall projects.

Why it matters: - Snohomish County’s updated stormwater rules can affect how retaining walls, grading and drainage projects are designed before construction starts. - The change is especially relevant for landscape contractors and property owners in unincorporated Snohomish County, where drainage review can be triggered by site disturbance and runoff changes. - Wall height alone no longer tells the full permitting story. Surcharge, soil movement and water management can also drive review requirements.

What happened: - Snohomish County put its updated Drainage Manual into effect on July 1, 2026. - The manual revised the county’s stormwater framework for development and redevelopment projects in unincorporated areas. - Ivan’s Landscape & Construction, a Snohomish-based contractor, said the update reinforces a jobsite reality: water behind a wall can matter as much as the wall itself.

The details: - Retaining walls hold back soil, but saturated soil creates extra pressure on the structure. - Drainage, backfill, grading and runoff direction need to be considered before the first block or concrete section is installed. - Depending on the project, contractors may use drainage rock, pipe, properly placed backfill, grading or other methods to manage water around a wall. - Ivan’s Landscape & Construction handles retaining wall installation along with excavation, grading and drainage work throughout Snohomish County. - Snohomish County does not use the July drainage update to create a new retaining wall permit requirement. - Separate county rules already determine when retaining walls need permits and engineered plans. - Land Disturbing Activity review can be triggered when a project clears or grades more than 7,000 square feet. - Review can also be triggered when a project moves more than 100 cubic yards of material within an 18-month period without an associated building permit. - Other triggers include changing an existing drainage pattern or disturbing a critical area or buffer. - Those triggers can apply when a retaining wall is part of a larger landscape project, major slope correction, drainage improvement or site redevelopment. - The new Drainage Manual works with the county’s drainage code and Engineering Design and Development Standards. - Snohomish County requires a building permit and engineered plans for walls over 4 feet tall, measured from the bottom of the footing to the top of the wall, 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 wall, such as a driveway, parked vehicles, a nearby structure or additional fill. - Two retaining walls of the same height may have different design and permitting needs. - Ivan’s Landscape & Construction installs and repairs retaining walls for residential and commercial properties, including concrete, block, rock, gravity and cantilever systems. - The company also helps property owners with permitting when a project requires county approval. - 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’s Landscape & Construction provided a Facebook page link: More information.

Between the lines: - The county’s drainage update appears to push more projects toward whole-site planning instead of treating a wall as a standalone feature. - For contractors, the practical shift is coordination: drainage, excavation and wall design need to be considered together to avoid permit surprises and structural problems. - For property owners, the main takeaway is that a short wall can still trigger engineering review if the site load or drainage conditions make the project more complex.

What’s next: - Property owners planning retaining walls, grading, drainage improvements or larger landscape projects will need to review the full site before construction begins. - Contractors working in Snohomish County will likely keep factoring drainage and land disturbance thresholds into early project planning. - Ivan’s Landscape & Construction says it is available to discuss retaining wall installation, repair, excavation, grading and drainage work throughout Snohomish County.

The bottom line: - In Snohomish County, retaining wall planning now goes beyond height. Drainage, slope conditions and site disturbance can determine whether a project needs more engineering, more review or both.