What Is an ADU and Why Do You Need an Architect?
An Accessory Dwelling Unit (ADU), also called a DADU (Detached Accessory Dwelling Unit), mother-in-law suite, backyard cottage, or granny flat, is a secondary housing unit on a single-family residential lot. In Seattle and across King County, ADUs have become one of the fastest-growing housing solutions as the region grapples with a significant housing shortage and rising property costs.
For Kirkland, Bellevue, Redmond, and Seattle homeowners, adding an ADU to your property can provide rental income, housing for aging parents, space for adult children, or a dedicated home office. With the right architectural design, an ADU can also meaningfully increase your property value.
But ADU design is more complex than it looks. Zoning codes, setback requirements, height limits, owner-occupancy rules, and utility connections all require careful navigation. That is why working with a licensed architect, one familiar with the specific codes of your jurisdiction, is essential.
Seattle and King County ADU Rules (2026 Update)
Washington State HB 1337, passed in 2023, significantly liberalized ADU rules across the state. As of 2026, here is what Kirkland, Seattle, and surrounding Eastside cities generally allow:
Up to 2 ADUs per lot
one attached (AADU) and one detached (DADU)
No owner-occupancy requirement
thanks to HB 1337, you no longer need to live on the property to rent an ADU
DADU size limits
typically up to 1,000 sq ft (Kirkland), 1,000 sq ft (Seattle), or 1,200 sq ft in some jurisdictions
Height limits
generally 24 to 28 ft for DADUs; varies by city and lot location
Setbacks
typically 5 ft from side and rear property lines for DADUs
Parking
most jurisdictions no longer require additional parking for ADUs within half a mile of transit
Important: Rules vary by city. Kirkland, Seattle, Bellevue, Redmond, and Mercer Island each have their own codes. Your architect will confirm exactly what is allowed on your specific parcel before design begins.
Planning a project?
Wondering what an ADU on your lot would cost?
Send David the address and a few lines about what you want. The first feasibility check is free and usually takes about a day: what the site and the code are likely to allow, and what to check first.
Architect since 1996. Three Masonry Excellence Awards for commercial projects; the Carr residence was featured in Above The Lake and Lux magazines.
The ADU Design Process with Piper Cole Architects
At Piper Cole Architects, we have guided dozens of Eastside homeowners through the ADU design and permitting process. Here is what to expect:
Step 1: Feasibility Assessment
Before you invest in drawings, we assess your property: lot size, existing structures, zoning designation, setbacks, utility connections, and any covenant or HOA restrictions. This takes about 1 to 2 weeks and confirms what is actually buildable on your site.
Step 2: Schematic Design
We develop initial design concepts, floor plan layouts, exterior massing, window placement, and entry options. We present 2 to 3 options and refine based on your feedback. For most ADUs, this phase takes 4 to 6 weeks.
Step 3: Design Development
We finalize the design: exact dimensions, materials, interior layout, kitchen and bath placement, and connection to existing utilities. We coordinate with structural engineers at this stage for anything requiring structural review.
Step 4: Construction Documents and Permit Application
We produce permit-ready drawings: site plan, floor plans, elevations, sections, and details. We submit to the relevant city or county and manage the permitting process through approval. Permit timelines in the Seattle area range from 6 to 16 weeks depending on jurisdiction and project complexity.
Step 5: Construction Administration
Once your contractor begins building, we are available for site visits, RFI responses, and inspection coordination to ensure the build matches the design intent.
How Much Does an ADU Cost in Seattle? (2026)
ADU construction costs in the greater Seattle area have stabilized somewhat after the post-pandemic surge, but are still high relative to national averages:
Basic detached ADU (400 to 600 sq ft)
$180,000 to $280,000 all-in
Mid-range DADU (600 to 800 sq ft)
$280,000 to $400,000 all-in
High-end DADU (800 to 1,000 sq ft)
$400,000 to $600,000 or more
Attached ADU (basement conversion)
$80,000 to $180,000 depending on scope
Garage conversion ADU
$60,000 to $150,000
Architecture fees for ADU projects typically run 10 to 15% of construction cost. On a $300,000 DADU, that is $30,000 to $45,000 for full architectural services, less than the cost of a design mistake that gets flagged at permit review or during construction.
ADU Rental Income Potential in Kirkland and Seattle
A well-designed 600 to 800 sq ft DADU in Kirkland or the Seattle Eastside can command $1,800 to $2,800 per month in rent (2026 market), generating $21,600 to $33,600 annually. Over 10 years, that is $216,000 to $336,000 in rental income, often exceeding the total cost of construction.
Beyond income, ADUs are increasingly popular as homes for aging parents (allowing multi-generational living while maintaining privacy), spaces for adult children returning from college, or premium home office setups for remote workers.
ADU Design Considerations for Seattle Climate
Seattle marine climate, mild temperatures, high humidity, significant rainfall, shapes every ADU design decision we make:
Roof pitch and drainage
Adequate slope and properly detailed eaves prevent water intrusion
Vapor barriers
Critical for below-grade or slab-on-grade construction in the Puget Sound region
Natural ventilation
Operable windows placed for cross-ventilation reduce reliance on mechanical cooling
Durable exterior materials
Fiber cement siding, metal roofing, and aluminum-clad windows hold up better in Seattle wet climate than wood alternatives
Passive solar orientation
South-facing windows maximize natural light and solar heat gain during gray winter months
Common ADU Design Mistakes to Avoid
After 25 years and 800 projects across the Eastside, we have seen homeowners run into the same ADU pitfalls:
- Designing before confirming zoning, spending money on drawings for an ADU that is not buildable on your lot
- Underestimating utility costs, new water and sewer connections can add $20,000 to $50,000 to ADU costs
- Ignoring solar access, placing the ADU where it shades the main house or where existing trees block natural light
- Maxing out square footage, a well-designed 500 sq ft ADU rents better than a cramped 800 sq ft one
- Skimping on sound insulation, especially important for attached ADUs; acoustic separation affects habitability and tenant satisfaction
Frequently Asked Questions: ADU Design in Seattle
Do I need an architect for an ADU in Seattle?
For new detached ADUs, yes, a licensed architect is typically required to stamp the permit drawings in the City of Seattle and most King County jurisdictions. Garage conversions and basement conversions may qualify as minor alterations in some cities, but most require architectural drawings for permit.
Can I build an ADU if I have an HOA?
HOA CC&Rs can restrict ADU construction even where city zoning allows it. However, as of 2024, HB 2071 limits HOA ability to prohibit ADUs on lots where they are otherwise permitted by local government. Consult your HOA documents and legal counsel, we can review design implications once you have confirmed approval.
How long does ADU permitting take in Kirkland and Seattle?
Permit timelines vary: City of Kirkland typically runs 8 to 12 weeks for DADU permits; City of Seattle runs 10 to 20 weeks for new DADUs. We submit complete, code-compliant packages to minimize back-and-forth with plan review staff.
Explore Our Related Services
Residential Architecture
Custom homes, additions, and ADUs across Seattle and the Eastside
From Our Blog
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- Complete Guide to Custom Home Design in the Seattle Area
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- Residential vs. Commercial Architecture: Key Differences
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:
