DADU stormwater and drainage planning
Drainage planning needs to consider the site work around the DADU as well as the cottage roof. Establish the applicable review with the permitting authority before choosing a drainage system or assuming the project is exempt.
Sources checked November 5, 2026. Confirm the requirements for your parcel with its permitting authority.
Draw all the proposed site changes
Put the cottage, entry paths, parking changes, access work and any replaced paving on one plan. Identify what remains, what is removed and what is added. This makes the drainage discussion more useful than starting with the building footprint alone.
Keep a site-area schedule tied to that drawing. If the proposed access or outdoor layout changes, update the schedule at the same time. Ask the design team which surfaces and disturbed areas the authority includes in its review calculations.
Confirm the current manual and review category
King County’s residential drainage handout describes several review categories and a sequence starting with a site plan and site-area worksheet. It explains that project characteristics and site conditions affect the review path, and states that the Surface Water Design Manual controls if the summary conflicts with it.
The handout is dated March 2023 and includes references to different manual editions. Use the county’s current surface-water design resources and staff confirmation to establish the governing manual and thresholds for a new application. Avoid using an older summary as the sole exemption test.
Official guidance: King County residential drainage-review process, March 2023, King County current surface-water design resources.
Review soil and discharge conditions before choosing a system
The county handout describes soil information and site constraints as part of evaluating drainage measures. Infiltration or dispersion is not simply a device selected from a catalog; its suitability needs to be established for the actual location.
Ask the appropriate designer what investigation is needed and how the proposed solution relates to slopes, neighboring property and other site features. Keep drainage areas and service routes visible during layout discussions so a later adjustment does not displace space already needed for another function.
Official guidance: King County residential drainage-review process, March 2023.
Include construction and maintenance in the scope
Separate the permanent drainage work from temporary measures needed while the site is disturbed. Confirm who designs, installs and inspects each part. Keep any maintenance information with the final project records so the owner understands how the completed system should be cared for.
Bring drainage questions into a free TimberworX consultation alongside the cottage design. Detailed research, design and budgeting are paid preconstruction services. The next scope should identify the unresolved site work and professional input needed before the construction plan is finalized.
A few things to know
Does a small DADU automatically avoid drainage review?
No. Ask the authority to evaluate the full project and site conditions under its current requirements.
Can I choose a drywell before reviewing the soil?
Confirm suitability and design requirements with the appropriate professional and permitting authority before selecting a drainage measure.
Keep planning
Do you need a survey for a DADU?
Understand how a DADU site plan differs from a survey and which property information supports design and permit review.
Can a DADU use septic or existing utilities?
Check septic capacity, sewer availability, water service and utility routes before committing to a DADU design in King County.
Eastside DADU Permit Costs: Duvall and Bothell Examples
Compare published DADU permit charges in Duvall and Bothell, Kirkland plan licensing and the separate scope behind a construction estimate.