ADU Construction Planning on Mercer Island, WA

Mercer Island has its own permit material and important slope, critical-area and access questions. Those conditions need a local guide rather than a generic Seattle page.

This independent guide is not the City of Mercer Island or a contractor. Confirm the current form, code and property with Community Planning and Development.

What does Mercer Island publish?

The City’s Permit FAQ says an ADU permit is required for a new living unit. It may be added to, built inside or detached from a single-family home. The unit includes living, sleeping, eating, cooking and bath space.

The Forms, Permits and Applications page lists a current Accessory Dwelling Unit submittal document dated 2026. Use that form and the live code rather than an old project narrative or company summary.

Why can critical-area review matter?

Mercer Island describes levels of critical-area review and points to MICC 19.07.090. Slopes and mapped conditions can affect the unit’s location, soil work, drainage, access and review.

Before plan selection, check:

• slope and potential landslide-hazard mapping.
• watercourse, shoreline or other critical areas.
• trees and root protection.
• retaining walls and excavation.
• stormwater and roof drainage.
• driveway and work access.
• neighboring-property protection.

Do not choose a review level from photos. Ask the City and a qualified expert what applies to the site.

What belongs in the lot site check file?

Collect the parcel, zoning, current building, survey and title details. Record the planned unit type, floor area and relationship to the main home. Add utility and access facts.

The early file should include:

1. Current City ADU form and code source.
2. Site plan or survey.
3. Slope and critical-area details.
4. Tree and drainage observations.
5. Sewer, water and electrical routes.
6. Work access and staging.
7. unresolved questions for City staff.

Use the early site-check guide to avoid purchasing a plan before these constraints are understood.

Which ADU type may fit?

A detached ADU offers privacy but may need special foundation and access work on a slope. An attached unit may shorten utilities but change the main home. An internal conversion avoids a new footprint but must address height, moisture, fire, sound, escape and systems.

Compare the complete detached, attached and conversion paths rather than choosing by floor area alone.

What permit and design work needs a named owner?

Ask who prepares the site plan, building plans, structure, energy records, drainage and trade plans. If soil or critical-area work is needed, name the expert. State how the advice enters the permit and building plans.

The quote should name the party responding to City corrections and planning utility approvals. Use the design and permits guide to assign documents and fees.

What utilities and site work should be priced?

Map sewer, water, power and rainwater. Check whether the site needs service changes, pumps, long trenches or hard machine access. Include protection and repairs for driveways, plants and retaining walls.

A compact DADU can carry significant external work on a constrained lot. The utilities and site-work guide helps compare these items separately from the building shell.

How much does a Mercer Island ADU cost?

Do not use property values as a building-price method. Use the Seattle-area cost guide as a full category schedule. Then price the actual slope, access, design, utility and finish work.

Ask for separate prices for site checks, survey, experts, design and permits. Add digging, foundation, construction, utilities, finishes and repairs. Keep money for surprises. Put allowances and owner choices in writing.

How should Mercer Island builders be compared?

Check the legal builder through Washington L&I. Ask for recent Mercer Island ADUs or other projects on difficult lots. Record the permit office, critical-area work and site access for each.

Give each company the same property file and design basis. Use the builder comparison guide to normalize registration, consultants, permits, utilities, exclusions, schedule, payment and written warranty terms.

Prepare a constraints plan before choosing architecture

Start with a survey or reliable plan showing lot lines, current buildings, slopes, walls, access, trees and known critical areas. Add utilities, drainage and room for digging, deliveries and site setup. Mark questions that still need a soil, tree, civil or other expert.

Use that site plan to compare detached, attached and conversion options. A smaller detached unit can still be harder to build. Machine access, retaining work or a long utility route may control the project.

How should consultant work be priced?

Ask the lead company to list each expert, task and choice that depends on it. Separate survey, soil, structural, civil, critical-area, tree and energy work when needed. State whether each fee is fixed, an allowance or paid by the owner. Name who updates the permit plans.

If an expert is excluded because the need is unknown, name the review and its date. A clear choice point is more useful than a broad allowance.

What should be checked before construction?

Recheck the builder’s Washington L&I record and compare the issued plans with the contract. Record permit conditions, expert advice, inspections, utility approvals and open owner choices. Have the expert and City review changes to slopes, drainage, structure or critical areas before work begins.