Accessory Dwelling Units (ADUs) and Detached ADUs (DADUs)—often called backyard cottages—are one of the most popular ways Seattle homeowners add space, create rental income, or build a place for family. The city’s policy direction is clear: more small housing options. But on real lots, ADU feasibility depends on a stack of rules that don’t always show up on the first zoning lookup.
If you’re asking, “Can I build an ADU on my property?” you’re asking the right question. The frustrating part is that the answer often hinges on details like where your utilities run, whether a tree is protected, how steep your yard is, and what overlays apply to your parcel—not just your neighborhood’s base zoning.
1) Zoning says “possible,” but your lot decides “practical”
Zoning is the first filter (and it matters), but it’s not the final verdict. Two homes on the same block can have totally different ADU outcomes because lot shape and constraints change what’s buildable.
- Setbacks and buildable area: Irregular lots, corners, and narrow yards can shrink the usable footprint.
- Height limits: DADU height rules interact with roof forms, slopes, and measurement points.
- Lot coverage / FAR: A property can “allow” a DADU but hit size limits once you count existing structures.
2) Utilities are the #1 surprise cost for Seattle DADUs
Utility work is where budgets get ambushed. Many homeowners assume a DADU is like a big shed with plumbing. In reality, water, sewer, and power requirements can trigger upgrades that add both cost and time.
- Water service: Some projects require a new or upgraded water meter/service connection.
- Sewer capacity and routing: The path from backyard to main line can force long trenches and restoration.
- Electrical service: Adding a unit may require panel upgrades or service changes, especially on older homes.
Even when upgrades are “doable,” scheduling crews, coordinating inspections, and trenching around landscaping can add weeks to months. If you discover these requirements after you’ve paid for drawings, you may be paying to redesign.
3) Tree protection can stop an ADU project cold
Trees are a major reason Seattle ADU plans get delayed or abandoned. It’s not just whether a tree exists—it’s whether it’s protected, where its root zone extends, and how construction impacts it.
- Protected tree zones: You may not be able to build, trench, or even stage construction within certain areas.
- Access and staging: A DADU might fit, but the route for excavation equipment might not.
- Utility trench conflicts: The shortest path for sewer/water lines may be the worst path for root protection.
4) Overlays and critical areas add extra reviews
Some parcels fall under additional regulations beyond base zoning. These can trigger extra review layers, specialized reports, and more corrections—especially for steep slopes, drainage, and environmentally sensitive areas.
5) Why Seattle ADU permits feel slow
The ADU process can feel slow because reviews are sequential and interconnected. A change for one issue (like moving the building to avoid a tree) can ripple into setbacks, utility routing, and drainage plans. That’s how “small adjustments” become multiple correction cycles. Each cycle costs time, and time costs money.
How to avoid costly ADU delays (pre-check list)
Use this quick checklist before you hire a designer or pay for engineered plans:
- Confirm zoning + constraints: setbacks, height, lot coverage/FAR, and lot shape realities.
- Map utilities: where water/sewer/power run and what upgrades might be required.
- Identify trees: location, protection status, and whether access/trenching conflicts exist.
- Check overlays: steep slope, drainage, shoreline/critical areas, and anything that triggers extra review.
- Validate access: construction access and staging can be as limiting as the build footprint.
Want a fast “is this doable?” read on your property?
Use the Seattle Permits report to flag common ADU/DADU permit triggers and delay risks early—before you spend thousands on plans.