You have three cottage plans open on the table, and they all look fine until you try to picture breakfast in each one. DADU plan reading is a skill any owner can learn in an afternoon. A DADU, or detached accessory dwelling unit, is a separate home on the same lot as a house. Once you know what the lines and numbers mean, comparing designs gets much easier.

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

What Does the Scale Tell You?

Every plan shows the building at a reduced size, and the scale says how much. SDCI Tip 106, updated in July 2022, says permit plans must state the scale clearly and show it as a graphic. For single family projects the minimum scale is a quarter inch per foot.

Dimensions carry the real information. Look for strings of numbers along the walls, which give exact lengths. A plan with no dimensions is a sketch, not a design.

Plans, Elevations and Sections

Floor plans show the layout from above. Elevations show each outside wall, and sections slice through the building to reveal ceiling heights and roof shape.

Good DADU plan reading means checking all three. A window or a loft may only make sense once you see it in an elevation or section.

How Big Are the Rooms Really?

Marketing plans often list room sizes as a width and a length. Ask whether those figures measure to the wall surface or to the wall center, since that changes usable space.

Code sets a floor. SDCI Tip 303A says habitable rooms other than kitchens need at least 70 square feet and at least 7 feet in any horizontal direction. A room that barely meets the minimum can feel tight once a bed and dresser go in.

Reading Door Swings and Windows

Each door shows as a line with an arc that traces its swing. That arc tells you how much floor the door claims when it opens. A bedroom door that hits a closet door becomes a daily annoyance.

Windows appear as thin breaks in the wall, often with a tag that points to a schedule. Each bedroom needs an emergency escape window or door, so check that every bedroom has one on an outside wall. Tip 303A also calls for at least one 3 foot wide swinging exit door per unit.

Where Are the Stairs?

Stairs show as parallel lines with an arrow pointing up or down. A break line often cuts the stair where the floor above takes over. Look for the stair on both floor plans, because it uses space on each level.

Code limits stair width, step size and headroom. A stair that looks slim on a marketing plan may grow during design, so ask whether it already meets the residential code.

Tracing Paths Through the Home

Circulation means the routes people take every day. Trace a few with your finger, such as entry to kitchen, bedroom to bathroom and living room to yard.

Short, clear paths make a small home work. A route that cuts through the middle of the living room or past the stove makes both spaces harder to use. Long halls that serve only one room waste area.

Will Your Furniture Fit?

Furniture drawn on plans is often undersized. Measure your own bed, sofa, table and desk, then draw them to the same scale on a copy of the plan. Paper cutouts work well, and you can slide them around to test different arrangements in each room.

Leave room to move around each piece, especially beside beds, in front of closets and around the dining table. The post on DADU bedroom count explains how bedroom choices shape the rest of the plan.

A Hypothetical Plan Comparison in Fremont

Picture an invented Fremont lot where an owner compares two 650 square foot plans. On paper, the first looks roomier. Its owner draws her queen bed and sofa to scale on both.

In the first plan the bed blocks the closet, and the path to the bathroom crosses the kitchen. The second plan has a smaller living room but clear paths and a full closet. She picks the second one. A look at its elevations confirms that the bedroom window faces her garden, not the neighbor’s deck.

Common Surprises in Plan Reading

• Room sizes measured to wall centers.
• Furniture symbols that shrink real pieces.
• Door swings that collide.
• Bedrooms without an outside wall for an escape window.
• Stairs that grow to meet the code.

Frequently Asked Questions

Are standard plans good for practice?

They are a good place to practice. Seattle’s pre‑approved DADU plans come as complete drawing sets with plans, elevations and details. Each one needs to fit your lot, as the post on the Seattle pre‑approved DADU program explains.

What should a plan show at minimum?

A stated scale, dimensions on every room and labels for each space. Elevations and sections should come with it. If a seller offers only a single floor plan image, ask for the rest of the set before comparing it with other options.

Can you judge cost from a floor plan?

Only roughly. Complex rooflines, many corners and long plumbing runs tend to add work, while simple shapes reduce it. Costs depend on the full design, the site and the builder. Treat any figure drawn from a plan alone as a rough guess.

Questions to Ask About Any Plan

• Does the plan state its scale and dimension every room?
• Where do the water heater, heating equipment and laundry go?
• Does every bedroom have a compliant escape window?
• How would the plan change to fit the setbacks and height limit on your lot?
• Which changes would need new structural design?

Scale and dimensions come first, and furniture drawn to scale tells you the rest. Ask your questions before you commit, since early changes cost less, as the post on DADU design cost notes. The floor plans and space planning section has more on layout.