Accessory Dwelling Units (ADUs) have exploded in popularity across Washington State. Recent legislation has made it easier than ever to add a backyard cottage, convert a garage, or build above your detached garage. This guide covers everything you need to know to plan, permit, and build your ADU.
What We'll Cover
Types of ADUs
ADU is an umbrella term for several types of secondary dwelling units. Understanding the options helps you choose the right approach for your property.
DADU (Detached ADU)
A standalone structure in your backyard. Most flexibility for design and privacy. Often called a "backyard cottage."
$250,000 - $450,000+
Attached ADU
Built as an addition to your existing home, with a separate entrance. Shares a wall with the main house.
$150,000 - $350,000
Garage Conversion
Convert an existing garage into living space. Often the most affordable option if you don't need the garage.
$100,000 - $200,000
Above-Garage Unit
Living space built above a new or existing detached garage. Efficient use of space on smaller lots.
$300,000 - $500,000
Zoning and Regulations
Washington's 2023-2024 housing legislation dramatically expanded ADU allowances statewide. Here's what you can generally build:
Two ADUs allowed on most single-family lots (one attached, one detached)
No owner occupancy requirement in most jurisdictions
No off-street parking required for ADUs within ½ mile of transit
Reduced setbacks for ADUs (often 5 feet)
Height limits typically 24-28 feet depending on jurisdiction
Important: Check Your Specific Jurisdiction
While state law sets minimums, cities can be more permissive. Seattle, Bellevue, and other cities have their own ADU codes with specific size limits, design standards, and lot coverage rules. Always verify requirements with your local building department before designing.
Size Limits by Jurisdiction
| City | Max DADU Size | Notes |
|---|---|---|
| Seattle | 1,000 sq ft | Pre-approved plans available |
| Bellevue | 900 sq ft | New code as of 2024 |
| Kirkland | 1,000 sq ft | Or 60% of main house |
| Redmond | 1,000 sq ft | Lot coverage limits apply |
Costs: What to Budget
ADU costs vary widely based on size, design, site conditions, and finishes. Here's what to expect in the Seattle area:
Cost Breakdown for a Typical 800 sq ft DADU
Design & permits: $25,000 - $50,000
Site work & foundation: $40,000 - $80,000
Construction: $180,000 - $280,000
Utilities connection: $15,000 - $40,000
Landscaping/restoration: $10,000 - $25,000
Cost Per Square Foot
For new construction DADUs in the Seattle area, expect $350 - $500 per square foot all-in. Higher for challenging sites, luxury finishes, or complex designs. Conversions and attached ADUs typically cost less.
Design Considerations
Good ADU design balances these factors:
Lot constraints: Setbacks, lot coverage, and height limits shape what's possible
Privacy: Window placement and entry location for both units
Utilities: Where will sewer, water, and electrical connect?
Parking: Do you need to maintain or replace garage space?
Future use: Rental, family, or home office—each has different needs
Pre-Approved Plans
Seattle offers pre-approved ADU plans that expedite permitting. These can save 2-4 months and $10,000-$20,000 in design costs, but they offer less customization. They work best for standard lots with good access.
The Permit Process
Expect permitting to take 3-6 months depending on:
Your jurisdiction (Seattle is generally faster than some Eastside cities)
Whether you use pre-approved plans
Complexity of your design
Any variances or design review required
Typical Permit Steps
Pre-application meeting
Optional but recommended to clarify requirements early
Submit plans
With site survey, engineering, and architectural drawings
Plan review
4-12 weeks depending on city
Corrections
1-2 rounds typical
Permit issuance
Ready to build!
Construction Timeline
Once permits are in hand, expect 5-8 months of construction for a new DADU:
Weeks 1-3: Site prep, utilities, foundation
Weeks 4-8: Framing, roofing, windows
Weeks 9-14: Rough mechanical, electrical, plumbing
Weeks 15-18: Insulation, drywall, paint
Weeks 19-24: Finishes, cabinets, flooring
Weeks 25-28: Final fixtures, punch list, landscaping
Conversions and attached ADUs typically take 3-5 months.
ROI: Is an ADU Worth It?
ADUs can provide return in multiple ways:
Rental Income
A well-designed 800 sq ft ADU in Seattle rents for $2,000 - $2,800/month. That's $24,000 - $33,600/year before expenses. At a $350,000 build cost, that's a 7-10% gross return—before appreciation.
Property Value
ADUs typically add 20-30% of construction cost to property value immediately, with potential for more as the rental market develops. A $350,000 ADU might add $70,000-$105,000 in appraised value.
Other Benefits
Housing for aging parents or adult children
Home office or studio space
Future flexibility as needs change
Helping address housing shortage (good for the community)
Thinking About an ADU?
We've built dozens of ADUs across King County and can help you understand what's possible on your specific property. Schedule a free consultation to discuss your lot, your goals, and realistic costs and timelines.
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