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Phase 0 · Feasibility · Step 0.1

Confirm DADU is allowed on your lot

Drop your address into SDCI's free ADUniverse tool and you'll know in about a minute whether a DADU is allowed on your specific lot.

Who
Homeowner
How long
1 day
Cost
Free
You end up with
ADUniverse confirmation screenshot

If you skip this: A small number of lots sit in the shoreline overlay or an Environmentally Critical Area and aren't ADU-eligible. ADUniverse flags that in about a minute, before you've spent a dollar on design.

Start here

SDCI runs a free per-address tool called ADUniverse that tells you exactly what's allowed on your lot — zoning, size cap, yes/no on whether an ADU or DADU is permitted at all. Type your address, get the rules. It's the cheapest, fastest move on the whole project, and it's the right place to start.

What ADUniverse tells you after the 2025 reforms

The Seattle ADU rulebook changed twice in 2025-2026. Two ordinances took effect 2025-06-30 (HB 1337 and HB 1110 compliance, Ord. 127211 and 127219), and Phase One of the broader One Seattle Plan zoning update took effect 2026-01-21. ADUniverse reflects the current rules for your specific lot — and they're looser than older blog posts describe, so trust it over anything you find from before mid-2025.

Here's the shape of what's allowed today, per the City's own Building Connections post:

  1. Zoning. ADUs are allowed across all Neighborhood Residential zones. The old NR1/NR2/NR3 designators are being folded into a single "NR" zone under the One Seattle Plan; older guidance still applies in transition. Lots in the shoreline overlay get extra review.
  2. Number of units. Up to two ADUs per lot on top of the primary unit (per the 2025 compliance ordinances).
  3. Unit size cap. Gross floor area of 1,000 sq ft for most ADUs (1,200 sq ft for units with three or more bedrooms). The old 650 sq ft carve-out for townhouse / rowhouse zones is gone.
  4. Owner-occupancy. Not required. Both units can be rented.
  5. Lot size. State law (HB 1337) blocks cities from setting minimum lot sizes for ADUs in most NR zones. ADUniverse will tell you if your lot has any remaining size or dimension constraint.
  6. Parking. Usually not required.

How to do it

  1. Open the ADUniverse lookup tool.
  2. Enter your address.
  3. Save or screenshot the result. You'll attach it to the eventual permit submittal.

What "qualified" doesn't mean

ADUniverse confirms zoning eligibility. It doesn't tell you the cottage will physically fit (setbacks, trees, ECA), that utilities can support it (steps 0.2 and 0.3), or that a specific design will pass review. Those are the steps that follow.

Where this information came from

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