ADUs & backyard cottages in Renton
What HB 1337 actually allows in Renton — how many ADUs you can build, how big, how tall, the permit timeline, the fees, and the local quirks that decide whether your backyard cottage pencils out.
What HB 1337 lets you build in Renton
Washington's HB 1337 took effect June 30, 2025 and overrode local zoning across King County. In Renton that means up to 2 ADUs on a single lot, no owner-occupancy requirement, and parking handled as 1 space (waived near transit). Detached units (DADUs) can run to 1000 sf of heated space; attached units follow a different rule — up to 1,000sf.
| Rule | Renton |
|---|---|
| ADUs per lot | 2 |
| Owner occupancy | Not required (abolished by HB 1337) |
| Parking | 1 space (waived near transit) |
| Max DADU size | 1000 sf heated |
| Max AADU size | Up to 1,000sf |
| Max height | 24ft |
Pre-approved plans in Renton
PRADU (Pre-Reviewed ADU) is live and free to use. Picking a pre-reviewed plan instead of a custom design is the single biggest lever on your permit timeline in Renton. The plans are already vetted against city code, so review collapses to weeks instead of months.
Permit timeline
Renton permit timeline: Expedited, 1-on-1 staff review. A pre-approved plan is the fastest route — custom designs add review cycles. Run the numbers for your own lot in the feasibility tool before you commit.
Fees & what you'll pay
Fee waivers offered historically.
Statewide, HB 1337 caps ADU impact fees at 50% of what a full house would pay — but Renton layers its own fees and exemptions on top. The line item that surprises people most here is in the fee note above; price it before you fall in love with a floor plan.
Sewer & septic
Standard utility connection.
If your property is on septic (an on-site system, or OSS), that's the single biggest hidden risk in an ADU budget — many older septic systems are at or past their design life. The feasibility tool asks about your utilities so the estimate reflects connection or drainfield costs.
Renton quirks worth knowing
Every city has local rules that don't show up in the headline numbers. In Renton:
- PRADU program: 8 free pre-reviewed plans
- Converts non-conforming garages
- Historical fee waivers
Up to 2 ADUs on a single residential lot — that can be an attached unit (AADU/MIL) plus a detached backyard cottage (DADU). HB 1337 (effective June 30, 2025) preempted Renton's old one-ADU limits and removed owner-occupancy.
