ADUs & backyard cottages in Auburn
What HB 1337 actually allows in Auburn — 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 Auburn
Washington's HB 1337 took effect June 30, 2025 and overrode local zoning across King County. In Auburn 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 | Auburn |
|---|---|
| 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 Auburn
No pre-approved plan program yet. Every Auburn ADU goes through a custom permit review. You (or your designer) submit a full plan set, so budget for the longer timeline below and the design work up front.
Permit timeline
Auburn permit timeline: Varies. Because there is no pre-approved track here, plan for a full custom review. Run the numbers for your own lot in the feasibility tool before you commit.
Fees & what you'll pay
3-year property-tax exemption on the assessed-value increase from the ADU.
Statewide, HB 1337 caps ADU impact fees at 50% of what a full house would pay — but Auburn 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.
Auburn quirks worth knowing
Every city has local rules that don't show up in the headline numbers. In Auburn:
- 3-year property-tax exemption on the ADU value delta
- Garage conversions must replace garage doors with matching exterior finish
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 Auburn's old one-ADU limits and removed owner-occupancy.
