Zoning & Size Limits
| Requirement | Seattle, WA | Bellevue, WA |
|---|
| Allowed Zones | All residential zones (Neighborhood Residential and other re | Any lot in a land use district that allows a single-family d |
| Max Size (Detached) | 1,000 sq ft of floor area above grade (size also constrained by FAR and lot coverage) | 1,200 sq ft (Director may approve more for single-floor units in the primary structure or conversions/additions to existing accessory structures) |
| Max Size (Attached) | 1,000 sq ft | 1,200 sq ft, or 40% of the structure, whichever is greater |
| Max Height | Up to 32 ft in Neighborhood Residential and most r | Detached ADUs: 24 ft to the highest point (28 ft w |
| Parking Required | Not required (no parking minimum for ADUs) | None under 1,000 sq ft; 1 space for 1,000-2,000 sq ft; none |
| Owner Occupancy | Not required | Not required (removed by Ordinance 6746 in 2023; prohibited |
Setback Requirements
| Setback | Seattle, WA | Bellevue, WA |
|---|
| Rear Setback | 5 feet | Subject to middle-housing dimensional standards (LUC 20.20.538) — Confirm with Bellevue Development Services |
| Side Setback | 5 feet (reduced setbacks allowed along an alley/easement) | Subject to middle-housing dimensional standards (LUC 20.20.538) — Confirm with Bellevue Development Services |
| Front Setback | Per zone; DADU generally behind primary structure | Per underlying district / middle-housing standards |
Permits & Timeline
| Detail | Seattle, WA | Bellevue, WA |
|---|
| Permit Timeline | Several weeks to a few months depending on review queue | Several weeks to a few months depending on review queue — Confirm with Bellevue Development Services |
| Permit Fees | $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) | Permit and plan review fees based on project valuation — Confirm with Bellevue Development Services |
| Impact Fees | ADUs exempt from MHA affordability contributions and most im | School/transportation impact fees may apply; HB 1337 caps AD |
| Pre-Approved Plans | Yes — ADUniverse pre-approved DADU plan library | No city pre-approved plan library — Confirm with B |
Construction Costs
| Cost | Seattle, WA | Bellevue, WA |
|---|
| Avg Build Cost | $160,000-$320,000 | $180,000-$380,000 |
| Cost Per Sq Ft | $130-$270 | $200-$350 |
| Garage Conversion | $75,000-$170,000 | $90,000-$190,000 |
| Prefab ADU | $110,000-$260,000 | $120,000-$280,000 |
Rental Income Potential
| Rental | Seattle, WA | Bellevue, WA |
|---|
| 1BR Avg Rent | $1,600-$2,100/mo | $2,500-$2,900/mo |
| 2BR Avg Rent | $2,000-$2,800/mo | $3,100-$3,900/mo |
| Short-Term Rentals | Allowed with a Seattle STR operator license ($75/yr); a 'sho | No citywide STR ban identified — Confirm current short-term |
| Est. ROI | 6-8% annual return | 6-8% annual return |
Which City Is Better for ADUs?
Seattle scores 9/10 while Bellevue scores 8/10 on our ADU-friendliness index.
Seattle edges ahead overall. Both cities offer viable ADU development opportunities — the best choice depends on your specific property, budget, and goals.
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