Your back fence looks like the edge of the lot, and the county map seems to agree. For a backyard cottage, that guess is risky. A DADU property survey gives the designer measured lot lines, ground elevations and recorded easements, so the cottage lands where the rules allow. Here, DADU means detached accessory dwelling unit, a separate home on the same lot as a house.

Seattle does not require a survey for every project. It does require one in certain cases, and the measured data helps on almost any small lot.

Details below reflect official Seattle pages checked in October 2026. Confirm with city staff for your property.

Why Fences and Online Maps Fall Short

Fences usually go up for convenience, not precision. An older fence can sit a foot inside or outside the true line, and nobody may remember why. Online parcel maps show approximate shapes rather than measured corners.

Small errors matter when a cottage fits within tight margins. Setbacks, lot coverage and height all depend on exact measurements. A few feet of doubt can move the building, as the post on Seattle ADU setbacks shows.

What Does a Survey Drawing Show?

Licensed land surveyors measure the site and prepare a drawing. Depending on the scope, it can show:

• Boundary lines and lot corners, based on the legal description.
• Existing buildings, sheds, decks, retaining walls and trees.
• Elevations and contour lines across the ground.
• Easements and other recorded rights on the lot.

SDCI Tip 103 lists other places to learn about boundaries. These include the surveyor’s map that often comes with a deed, older site plans in city microfilm records and King County Assessor records. The tip also says a licensed surveyor can prepare a topographic survey to serve as the base for the site plan.

Boundary Surveys Versus Topographic Surveys

Each product does a different job. A boundary survey locates the lines and corners. The topographic version adds the ground surface, with elevations, contours and existing features.

Plenty of cottage projects need both. Boundary data controls setbacks, while elevation data supports height and drainage design. It helps to tell the surveyor where the cottage might go before field work starts, since a narrow scope may skip the very corner you care about.

When Does Seattle Require a Survey?

Some projects need one by rule. SDCI Tip 303 calls for a topographic survey with corner stakes and all property lines when:

• The building comes within 2 feet of the maximum allowed height.
• Grading plans are required for an environmentally critical area site.
• The site sits in a steep slope, potential landslide or known landslide area.

SDCI can waive the slope requirement when it grants a modification or an exemption for a simple project. Tip 303 also asks for a recorded copy of any short plat or boundary line adjustment if the boundary was created or changed since 1974. Under Tip 103, critical area site plan items must be prepared and stamped by a Washington licensed surveyor.

Elevations and Height Limits

Height limits are measured from grade, so elevation data matters even on a lot that looks flat. A designer working close to the limit needs measured numbers, not an estimate. That is exactly the case where Tip 303 asks for a survey.

Elevations also shape drainage, foundations and access. Tip 103 notes that a site without a steady, gradual slope may need a licensed surveyor to plot the contour lines. The post on DADU height limits explains how the height rules work.

Easements and Recorded Rights

An easement can limit where a cottage goes, even on an open yard. A utility line, shared driveway or access right may cross the back of the lot. Tip 103 asks for the location of any easements on an enhanced site plan, along with the King County recording number.

Surveyors work from title documents to show these rights. Pulling recorded agreements and the title report early saves a second trip. The post on DADU easement restrictions covers how they affect a design.

A Hypothetical Beacon Hill Lot

Picture an invented example: a Beacon Hill lot where an old cedar fence runs along the rear line. The owner sketches a cottage 5 feet from that fence. Measurements show the true line sits 18 inches closer to the house, and a recorded sewer easement crosses one corner.

Because the designer learns this before drawing plans, the cottage shifts and shrinks slightly on paper. Nothing here describes a real property, but it shows how cheaply a survey catches what a foundation crew would find too late.

Common Survey Surprises

• Fences or sheds that sit over the property line.
• Easements nobody mentioned at purchase.
• Steeper grades than the yard seems to have.
• Boundary changes from an old short plat.
• Missing or disturbed corner markers.

Property Survey FAQ

Is an old survey from my purchase good enough?

Sometimes, as background. Tip 103 lists a deed map or older site plan as a starting source. Older drawings may not show later changes, such as a new shed or a boundary adjustment. A surveyor can say whether the old work still fits your permit path and building height.

Does every Seattle DADU need a topographic survey?

No. Tip 303 requires one in specific cases, such as a cottage designed close to the height limit or a site in a steep slope or landslide area. Even when it is optional, a survey often makes setbacks and drainage easier to design.

Who can stamp a critical area site plan?

A Washington licensed surveyor. Tip 103 reserves the extra items on that type of plan for a licensed surveyor’s stamp. A designer or homeowner cannot substitute their own measurements for those items.

Checklist Before Ordering a Survey

• Gather the deed, title report and any old surveys.
• Look up King County Assessor records for the parcel.
• Sketch the likely cottage footprint on a site photo.
• Ask the surveyor about boundary and topographic scope.
• Check whether the lot has critical area mapping.

Order the DADU property survey before design starts, and confirm with SDCI whether your height, slope or critical area status makes it mandatory. The post on DADU permit documents lists what else an application needs, and the site preparation section covers the other early steps.