Do you need a survey for a DADU?
A site plan and a survey do different jobs. Confirm what property measurements and professional work your permitting authority needs before placing the cottage against a boundary or committing to a foundation elevation.
Sources checked November 5, 2026. Confirm the requirements for your parcel with its permitting authority.
Distinguish measured records from the proposed design
A survey can establish property information needed for design; a site plan shows the existing conditions and proposed work for review. The required survey scope and the person permitted to prepare each document depend on the project and authority.
Gather what already exists before commissioning new work: recorded surveys, prior permits, relevant site drawings and easement documents. Ask the project team which information can be used, what needs field verification and what is missing. An old drawing may be useful without being sufficient for the new application.
Check the actual site-plan requirements
King County’s August 2025 residential site-plan handout describes building, drainage and critical-area information together with a separate temporary erosion-control plan for new residential applications. It lists property lines, easements, existing and proposed structures, utilities, elevations and other applicable site elements.
The handout says professional preparation depends on the level of review; small, simple projects may use properly scaled hand-drawn plans where a licensed professional is not required. Confirm the required scope rather than assuming every project needs the same document package.
Official guidance: King County residential site-plan requirements, August 2025.
Use mapping as an early research tool
The same county handout cautions that online map contours are approximate and may need correction to match site conditions. That distinction matters when a proposed cottage depends on a particular grade or relationship to the existing home.
For planning, label the source of each measurement on the working sketch. Identify which dimensions are approximate and which have been verified. Ask the appropriate professional to resolve boundary, elevation and easement questions before those assumptions become part of the final design.
Official guidance: King County residential site-plan requirements, August 2025.
Coordinate the survey scope with the design questions
Explain what you are trying to establish: a potential building envelope, access route, elevation relationship, utility path or another specific constraint. Give the surveyor and design team the same project brief so the field information supports the decisions ahead.
At a free TimberworX consultation, bring the records you have and identify the questions still open. Detailed site research, design and budgeting are paid preconstruction services. Agree which professional services are included and which will be separately commissioned before proceeding.
A few things to know
Is an online parcel map enough for final DADU placement?
Use it for initial research. Confirm which measurements and records the permitting authority and design professionals require for final placement.
Must every DADU site plan be prepared by an engineer?
Not necessarily. King County’s handout ties professional preparation to the review involved. Your jurisdiction and project determine the requirements.
Keep planning
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