ADU Construction Planning in Burien, WA

Burien’s fresh Maps results retained low-review businesses for both principal ADU queries. A useful city page should therefore make local rules and site questions easier to compare, not mimic a provider landing page.

This independent guide is not the City of Burien or a contractor. Confirm the adopted code and property with Community Development.

What does Burien’s current code say?

Burien’s 2025 code amendments include BMC 19.17.070. The published text says ADUs are allowed with a residential primary use, permits a maximum of two per lot and generally limits internal, attached and detached ADUs to 1,000 square feet. It also describes a possible size exception for better use of existing space in a building at least five years old.

Read the whole current section and confirm the lot. A summary of unit count and size does not answer placement, height, parking, utilities or building-code requirements.

How should a Burien property be screened?

Collect the parcel, zoning, existing dwelling and survey or site-plan information. Review:

• proposed number and combination of ADUs;
• unit type and floor area;
• yards, coverage, height and building placement;
• trees, slope, drainage and retaining walls;
• driveway, alley and construction access;
• sewer, water and electrical routes;
• existing garage or basement condition; and
• easements or title restrictions.

Burien includes varied topography and established neighborhoods. Tight access or drainage work can control the project even when the unit type is permitted.

Use the feasibility guide to record property facts before plan selection.

Which ADU path should be compared?

A detached unit provides separation but needs a complete foundation, building shell and utility route. An attached addition changes the main home. A garage or basement conversion may reuse space, but existing structure, moisture, ceiling height, fire separation and building systems need investigation.

If the property could support two units, do not design the first in a way that unintentionally blocks the second without considering the long-term plan. Access, utilities and outdoor space should be evaluated as a whole.

The detached, attached and conversion guides provide scope checklists.

What permits and design work belong in the proposal?

Ask who confirms zoning, prepares the site plan, documents existing conditions, completes architecture, structure and energy work, and coordinates mechanical, electrical and plumbing permits. The proposal should name the person or firm responding to City corrections.

If a provider suggests a standard or prefab plan, confirm its code basis, modification limits and Burien review path. A design marketed across Washington is not property approval.

Use the design and permits guide to assign each task.

What utilities and external work need a separate line?

Map sewer, water, electrical and drainage routes. Ask whether the existing services have capacity and whether trenching will cross paving, trees or landscaped areas. Include right-of-way work, hauling, erosion control and property restoration where applicable.

A 1,000-square-foot cap does not mean site costs are capped. The utilities and site-work guide helps prevent external work from becoming a late allowance.

How much does a Burien ADU cost?

No citywide price applies to every unit type and lot. Use the Seattle-area cost guide as a budget schedule, then replace broad ranges with the Burien site and design.

Separate feasibility, survey, design, engineering, permits, demolition, access, foundation, building work, utilities, finishes, restoration and contingency. Require written allowances and owner-supplied-item lists from every provider.

How should Burien builders be compared?

Verify the legal entity and registration through Washington L&I. Request recent Burien or south King County ADU work and ask which authority reviewed it. Confirm whether the proposal entity performed the work shown in the portfolio.

Give providers the same site facts, plan basis and finish schedule. Use the builder comparison guide for registration, project proof, design, permits, utilities, payment, schedule assumptions and written warranty terms.

Plan the lot for the long term

If the current rules and property could support more than one accessory unit, show both the immediate project and a possible later unit on a concept plan. Check whether the first footprint, trench, electrical equipment, sewer route, outdoor area or construction access would make the later option impractical. The owner does not need to build both, but should understand the trade-off before fixing the first design.

Also record existing legal units and structures. Confirm with Burien how the proposed configuration is counted and which size or existing-space provisions apply to this parcel.

What should a Burien bid packet contain?

Include current parcel and zoning information, site plan or survey, selected ADU concept, access and tree notes, utility routes and a finish schedule. Require every bidder to identify the same design, engineering, permit, site, utility and restoration responsibilities.

Ask who answers City corrections and who coordinates external utility or right-of-way work. Mark unknown conditions and the investigation needed to convert them into fixed scope. This prevents a simple unit-size comparison from hiding a difficult site or incomplete service work.

What should be checked before work starts?

Compare the issued Burien permit set with the contract drawings and allowances. Confirm the legal contractor, current L&I registration and proof of insurance. Record permit conditions, inspection milestones, utility approvals and any work that remains owner-direct.

Give the site supervisor the same approved tree, drainage, access and utility information used for pricing.