Backyard Cottage Design in Seattle: DADUs, Permits and What to Expect

Backyard cottages — formally called Detached Accessory Dwelling Units (DADUs) — have become one of the most in-demand residential projects in Seattle. They generate rental income, house extended family, add significant property value, and make use of underutilized backyard space. But designing and permitting a backyard cottage in Seattle involves more than most homeowners expect. This guide covers what you need to know before you start.

What Is a Backyard Cottage in Seattle?

A backyard cottage is a fully independent dwelling unit located in the rear yard of a property that already has a primary home. It has its own entrance, kitchen, bathroom, and living space. Seattle’s zoning code calls these Detached Accessory Dwelling Units (DADUs). They are distinct from attached ADUs (basement apartments, garage conversions) because they stand as entirely separate structures.

What Seattle Allows for Backyard Cottages

Seattle’s 2020 ADU reforms significantly expanded backyard cottage rights:

• Size: Up to 1,000 sq ft or 60% of the primary home’s gross floor area, whichever is greater
• Height: Up to 24 feet — enough for two full stories on most lots
• Setbacks: 5 feet from rear and side property lines (in most zones)
• Two ADUs allowed: One attached and one detached can coexist on the same lot
• No owner-occupancy required: Both the main house and the cottage can be rented
• No replacement parking required: Removing a garage for a DADU does not require replacing parking

Backyard Cottage Design Options

Studio Cottage (Under 500 sq ft)

A compact, well-designed studio cottage can provide everything needed for comfortable living — open sleeping/living area, full kitchen, full bath. Studios are the most cost-effective option and the easiest to permit. They work well for rental income on smaller lots where a larger DADU is not feasible.

One-Bedroom Cottage (500–750 sq ft)

The most popular category. A one-bedroom cottage at this size commands premium rental rates in Seattle, typically $1,800–$2,400/month. The program fits comfortably on most Eastside lots and can be designed with a loft to maximize usable area within the height limit.

Two-Story Cottage (750–1,000 sq ft)

For lots where the maximum footprint allows, a two-story cottage provides one- or two-bedroom layouts with distinct living and sleeping floors. Two-story cottages command the highest rents and property value premiums — a well-designed 900 sq ft cottage on a Kirkland lot adds $250,000–$350,000 to property value in current market conditions.

Site Considerations That Shape Design

Access

The cottage needs access — either from a side yard, through a shared gate, or with its own entrance from a rear alley. Lots with rear alley access have the most design flexibility. Lots where the only access is through the front yard require careful planning for privacy between primary residence and cottage.

Lot Coverage

Adding a DADU counts toward lot coverage. Seattle residential zones cap coverage at 35–50% depending on zone and lot size. We calculate the remaining allowable coverage before designing the cottage footprint — running out of coverage mid-design is a common and preventable mistake.

Critical Areas

Properties near streams, wetlands, or with steep slopes may have critical area setbacks that constrain the buildable area in the rear yard. We assess critical areas before design begins on every project.

Permitting a Backyard Cottage

DADUs in Seattle require a full building permit through SDCI. Timeline for a DADU permit is typically 6–10 weeks. Key submittals include architectural drawings, structural engineering, and energy calculations. A well-prepared application from an experienced architect minimizes correction rounds and keeps the timeline on track.

Cost of a Seattle Backyard Cottage

Construction costs in Seattle for a well-built DADU typically run $350–$500 per square foot all-in. A 600 sq ft cottage costs $210,000–$300,000 including site work, permits, and landscaping. Quality architecture, not minimum-code design, maximizes the rental income and property value return.

Working With Piper Cole Architects on Your Backyard Cottage

Our residential architecture team designs backyard cottages across Seattle and the Eastside. We handle design, structural coordination, permitting, and construction support. We serve Seattle, Kirkland, Bellevue, Redmond, and all Eastside cities. Learn more about our design and permitting process.