How Much Does It Cost to
Build an ADU in Washington?
Get an instant, detailed cost estimate for your ADU or DADU project in the Seattle area. Our calculator includes 2026 permit fees, WA energy code requirements, sales tax, and all construction costs.
ADU Cost Calculator
Washington State · Updated March 2026
Mid-range finishes, solid-surface counters, LVP flooring
Average Market Price
$309,582 – $395,577
Based on current WA contractor rates · $573/sf
With Giant Builders
$257,985 – $275,184
You save $68,796 – $85,995 compared to market average
Based on savings from our previous 100+ ADU projects
* Market estimate is based on current average WA contractor rates for planning purposes. Giant Builders consistently delivers 20-25% below market through efficient project management, established supplier relationships, and streamlined permitting. Final pricing depends on site conditions and material selections. Get your personalized quote above.
ADU Construction Costs in Washington State
Understanding the full cost of building an ADU in Washington requires accounting for construction, permits, design, utilities, and state-specific requirements. Here's what you need to know.
New DADU
$300K - $600K+
600-1,000 sq ft detached unit
$400 - $650/sf
Attached ADU
$200K - $400K
Addition or conversion
$300 - $500/sf
Garage Conversion
$80K - $250K
Reuse existing structure
$200 - $350/sf
Basement ADU
$100K - $350K
Finish existing space
$150 - $300/sf
What Goes Into the Cost of Building an ADU
Permits & Fees
- Seattle SDCI: $20K-$40K
- King County: $15K-$30K
- Impact fee exemption for ADUs
- Fees increased ~14% in 2026
Design & Engineering
- Architectural plans: $5K-$15K
- Structural engineering: $2K-$4K
- Site survey: $2K-$3.5K
- Geotech/arborist reports: $2K-$5K
Foundation & Structure
- Slab on grade: $25K-$35K
- Framing & structural: $35K-$65K
- WA seismic code: concrete footings required
- Sloped lots add $10K-$25K+
Mechanical Systems
- Plumbing: $12K-$25K
- Electrical: $10K-$22K
- HVAC (heat pump): $8K-$18K
- Utility connections: $10K-$30K
Exterior & Roofing
- Siding: $8K-$20K
- Windows (energy code): $8K-$18K
- Roofing: $10K-$25K
- Exterior doors: $2K-$7K
Interior Finishes
- Drywall & paint: $8K-$15K
- Flooring: $5K-$15K
- Kitchen (full): $13K-$28K
- Bathroom: $4K-$12K
Washington State ADU Regulations You Need to Know
HB 1337: Washington's ADU-Friendly Law
Effective July 2025, HB 1337 made Washington one of the most ADU-friendly states in the country. The law allows up to two ADUs per lot in urban growth areas, eliminates owner-occupancy requirements, and sets minimum size and height standards that cities cannot restrict below.
- Two ADUs per lot — attached, detached, or a combination
- No owner-occupancy requirement — rent both units
- Minimum 1,000 sq ft allowed — cities cannot set lower limits
- Minimum 24-foot height — enables two-story DADUs
- ADUs can be sold as condos — condominiumization allowed
WA Energy Code Requirements
Washington has one of the strictest energy codes in the nation (2021 WSEC-R, effective March 2024). ADUs under 1,500 sq ft need 5 energy credits, which affects insulation, air sealing, and HVAC choices. Heat pumps are strongly encouraged as they earn more credits. This can add $5,000–$15,000 in energy-efficient upgrades, but reduces long-term operating costs significantly.
Seismic Requirements
Western Washington falls under Seismic Design Category D/D1, requiring concrete footings (no crushed stone), rebar in stem walls, and a complete load path from roof to foundation. These requirements are factored into our calculator's foundation costs.
ADU Return on Investment in Seattle
Building an ADU is one of the best investments Seattle-area homeowners can make.
$200K - $350K
Property Value Increase
Homes with ADUs sell for 15-35% more
$1,500 - $3,500
Monthly Rental Income
Depends on size, finish & neighborhood
6 - 12 Years
Break-Even Period
Through rental income alone
26% Faster
Time to Sell
Properties with DADUs sell faster
How to Finance Your ADU in Washington
HELOC (Home Equity Line of Credit)
Draw funds as needed during construction. Variable rate, interest-only on amount used. Best for owners with solid equity who want flexibility.
Cash-Out Refinance
Replace your current mortgage with a larger one and receive the difference as a lump sum. Fixed rate for payment predictability.
Construction Loan
Purpose-built for ground-up construction. Draws tied to construction milestones, converts to permanent mortgage upon completion.
FHA 203k Loan
Government-backed loan ideal for garage conversions and renovations. Requires FHA-approved lender and 203k consultant.
WA State Tax Exemption
Three-year property tax exemption available if your ADU improvement represents 30% or less of your home's pre-remodel assessed value (RCW 84.36.400).
Frequently Asked Questions About ADU Costs
How much does it cost to build an ADU in Washington state?
In 2026, building an ADU in Washington state costs between $150,000 and $600,000+ depending on the type of construction, size, finish level, and location. A new detached ADU (DADU) in Seattle typically costs $300,000-$500,000 for a 600-800 sq ft unit. Garage conversions are the most affordable option at $80,000-$250,000. These figures include construction, permits ($16,000-$40,000), design ($12,000), utility connections ($10,000-$30,000), and Washington sales tax (10.25% in Seattle).
How much does a DADU cost in Seattle?
A detached accessory dwelling unit (DADU) in Seattle costs $300,000-$600,000+ in 2026. The cost per square foot ranges from $400-$650 all-in. Seattle has some of the highest permit fees in the state at $20,000-$40,000. A typical 600 sq ft DADU with standard finishes costs approximately $350,000-$420,000 total, including all soft costs.
What is the cost per square foot to build an ADU?
The all-in cost per square foot varies significantly by construction type: new detached ADUs average $380-$500/sf, attached ADUs $300-$500/sf, garage conversions $200-$350/sf, and basement conversions $150-$300/sf. Keep in mind that smaller units have a higher cost per square foot because fixed costs like permits, design, and utility connections are spread over less area.
What permits do I need to build an ADU in Seattle?
You'll need permits from SDCI (Seattle Department of Construction & Inspections), costing $20,000-$40,000. This includes building, plumbing, electrical, and mechanical permits. Under HB 1337, Seattle allows two ADUs per lot with no owner-occupancy requirement. ADUs are exempt from impact fees in Seattle. Permit review typically takes 2-8 weeks.
How big can I build an ADU in Washington?
Under Washington HB 1337 (effective July 2025), cities cannot restrict ADU size below 1,000 square feet. Height cannot be limited below 24 feet. You can build up to two ADUs per lot in urban growth areas. Setbacks cannot be more restrictive than for the principal unit — in Seattle, that's typically 5 feet from property lines.
Can I rent out my ADU?
Yes. Under HB 1337, there is no owner-occupancy requirement in Washington. Both your main house and ADU can be rented. New construction rental properties are exempt from Seattle's rent cap for the first 12 years. A DADU in Seattle can generate $1,500-$3,500/month in rental income depending on size and neighborhood.
How long does it take to build an ADU?
Typically 12-18 months from design to completion. This includes 2-4 months for design and permitting, and 6-12 months for construction. Garage conversions may be completed in 4-8 months. Timeline depends on project complexity, site conditions, permit review speed, and material availability.
Is building an ADU a good investment?
ADUs are one of the best investments for Seattle-area homeowners. Properties with ADUs sell for 15-35% more (typically $200K-$350K increase in value) and sell 26% faster. Rental income of $1,500-$3,500/month provides a 6-12 year payback period. Combined annual returns from rent and appreciation average 8-10%.
Ready to Build Your ADU?
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