ADUs went from "maybe allowed in your zone" to "yes, by right" in most Eastside cities. Washington State's HB 1337 (effective 2024, fully implemented across Eastside jurisdictions through 2025) reset the math on backyard cottages, attached units, and garage conversions. Here's what's actually different in 2026 — and what didn't change.
What HB 1337 did. Three big shifts. First, it eliminated minimum lot size requirements for ADUs in most residential zones — meaning your standard 5,000–8,000 sqft Eastside lot now qualifies regardless of city. Second, it removed off-street parking mandates for ADUs in many jurisdictions, freeing up site planning. Third, it allows BOTH a detached ADU (DADU) AND an attached ADU (AADU) on a single lot in most cities — meaning you can do a backyard cottage AND convert your basement, on the same property.
What it didn't do. Didn't change setback requirements — most Eastside cities still require 5–10 ft from property lines. Didn't override HOA covenants — if your CC&Rs restrict ADUs, the state law doesn't supersede them. Didn't make permits faster or cheaper. And it didn't change utility capacity rules: most older Eastside lots still need a new electrical service to support an ADU.
Bellevue specifically. Permitted DADUs up to 1,000 sqft. AADUs (including basement conversions) up to 1,200 sqft. Both allowed on single-family lots regardless of size. Permit review currently runs 4–8 weeks for a complete application — the variance depends on whether your site triggers tree removal, critical-area review, or design review for sloped lots.
Kirkland, Redmond, Sammamish. Similar by-right framework, slight variations. Kirkland allows DADUs up to 1,200 sqft. Redmond permits them in all R-zones with no special review. Sammamish has specific design standards for plateau lots and shoreline-adjacent properties. Permit timelines across all three: 4–8 weeks once a complete application is submitted.
The real timeline. Most homeowners hear "4–8 weeks for permits" and assume that's the whole timeline. It isn't. Here's the realistic sequence: Pre-application research (zoning verification, setback measurement, utility feasibility, soil and drainage check) — 1–2 weeks. Preconstruction (architectural drawings, structural engineering, energy compliance package, site plan) — 6–8 weeks. Permit submission to issued permit — 4–8 weeks. Construction — 5–8 months for a detached unit. Total time from "I want an ADU" to certificate of occupancy: 9–14 months.
What drives variance. Site complexity is the biggest swing factor. A flat lot with established utilities and no critical-area overlay can land at the fast end. A sloped lot near a stream buffer, with old electrical service, can add 2–4 months. Story count matters too — single-story DADUs are faster and cheaper than two-story. And whether you need a tree removal permit (most Eastside cities require one for trees over 6" diameter) can add weeks.
Cost reality in 2026. Eastside ADU pricing runs $350K–$650K+ for full design-build scope — preconstruction, structural engineering, permits, foundation, framing, separate utility connections, full finish. (See our pricing page for the full breakdown.) The widely-circulated $250K ADU figures online reflect pre-2023 pricing and finish-only estimates. They no longer apply — Seattle DADU builders publicly debunked them in 2025.
Common misconceptions. *"I can just convert my garage and call it done."* Garage conversions still require full permitting, insulation, heating, egress windows, and occupancy classification — typically $200K–$350K all-in. *"It'll pay for itself in rent quickly."* Eastside ADU rents in 2026 run $2,200–$3,800/month for a 600–900 sqft unit. Payback period is typically 8–14 years. *"I don't need an architect."* Sometimes true (most design-build firms handle the drawings in-house) but always check who's stamping your structural drawings — that has to be a licensed Washington engineer.
Where ERC fits. We handle the full ADU permit + build process in-house — site evaluation, zoning verification, drawings, structural engineering, submissions, and construction. One team across both phases. If you're considering an ADU on your Eastside lot, the free consultation includes walking the property and telling you what's actually feasible (including pointing you to a different builder if scope or budget don't match).
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