Mercer Island
ADU Builder.

Frequently asked

• How much does an ADU cost in Mercer Island, WA in 2026?

  Turn-key ADUs in Mercer Island run roughly $295–$465 per square foot all-in for 2026 contracts, depending on lot access, foundation type, and finish package. A 600 sqft one-bedroom DADU typically closes between $215K and $310K including permits, side-sewer connection, and electrical service work. The biggest swing factors in Mercer Island are soils (pier-and-beam adds $18–32K over a slab), required stormwater detention on lots over 2,000 sqft of impervious, and whether a panel upgrade to 200A is needed. Our Mercer Island feasibility report gives you a ±15% number in two weeks for $1,500, credited to the build contract.

• How long does the ADU permit take in Mercer Island?

  Median Mercer Island ADU permit issuance is currently 8–16 weeks from a clean submittal in 2026, faster on pre-engineered catalog plans. The bottlenecks are almost never the zoning review — they are side-sewer capacity letters, drainage review on lots with steep slopes, and addressing assignments. We pull a pre-application meeting with Mercer Island planning before drawings start, which typically saves one full review cycle (4–6 weeks). RCW 36.70A.681 requires most WA jurisdictions to permit ADUs ministerially when zoning is met, which has trimmed median timelines roughly 30% versus 2022.

• What size DADU can I build on my Mercer Island lot?

  Under HB 1337 / RCW 36.70A.681, Mercer Island must allow at least one DADU up to 1,000 sqft on any residential lot zoned for single-family, with height to 24 ft and standard 5 ft side/rear setbacks unless local code is more permissive. Many Mercer Island zones allow a second ADU (a stacked DADU or AADU + DADU combo). Your ceiling is usually lot coverage, not the 1,000 sqft cap — once you add the primary house footprint plus the new DADU plus impervious driveway and walks, you frequently hit the lot-coverage ratio before the size cap. Our zoning lookup checks both.

• Do I need to live on the property to build a Mercer Island ADU?

  No. Owner-occupancy requirements were preempted statewide by RCW 36.70A.681 — Mercer Island cannot impose an owner-occupancy condition on either the primary or accessory unit. That means you can build a DADU and rent both units, build a DADU for a family member while living off-site, or build to sell. Short-term rentals (≤30 days) are still governed by local STR codes, which vary; long-term rentals (30+ days) face no special restriction beyond standard landlord-tenant law (RCW 59.18).

• Can I subdivide and sell the ADU separately in Mercer Island?

  Yes, in most Mercer Island zones, since the 2023 unit-lot subdivision and condominiumization changes paired with HB 1110 middle-housing reform. The two common pathways are: (1) unit-lot subdivision — carving a new fee-simple lot around the DADU, requires a short plat (~4–6 months) and shared-driveway easements; (2) condominium declaration under RCW 64.34 — faster (~2 months) but lenders sometimes resist financing a 2-unit condo. We model both at feasibility because the right answer depends on your hold-vs-sell horizon and current lender appetite.

• What is the rental income from a Mercer Island ADU?

  2026 Mercer Island ADU rent comps for a 1-bedroom DADU built to current finish standards land in a wide band depending on submarket — generally $1,950–$3,200/mo for a long-term lease, with mid-term (30–90 day) commanding 10–25% premium. Net operating income after vacancy (5%), management (8%), maintenance reserve (5%), and property tax/insurance increment typically hits 62–70% of gross. At today's construction cost and rent comps the unleveraged cap on a new Mercer Island DADU pencils 4.8–6.4%, with leveraged cash-on-cash 7–11% using a HELOC or renovation loan.

• Will an ADU raise my Mercer Island property taxes?

  Yes, but predictably. The county assessor adds the marginal improvement value (typically 55–70% of construction cost, not 100%) to your assessed value, and the levy rate applies to that increment under RCW 84.40.0301. For a $270K-cost DADU in Mercer Island, expect roughly $1,900–$3,100/yr of additional property tax depending on your levy code area. The increment is partially offset because King and Pierce County both classify newly created ADUs into a single building-permit assessment cycle, not a full reassessment of the primary house.

• What's the first step to build an ADU in Mercer Island?

  Start with a paid feasibility study — not a plan set. The feasibility deliverable is a 12–20 page report covering Mercer Island zoning analysis, setbacks and lot-coverage math, water meter sizing, side-sewer capacity, electrical service amperage, critical-areas screening, three layout options, and a ±15% budget. It costs $1,500 and is credited to the construction contract. This step eliminates the two most expensive surprises — an undersized water service and an unmappable side sewer — before you spend $15K+ on full design. Schedule a free 20-minute scoping call to confirm fit.