400 Sq Ft ADU Cost in Seattle, WA
A 400 sq ft ADU in Seattle, WA costs roughly $205,000–$351,000 all-in for 2026, including permits, utilities, and soft costs. Adjust the slider to see other sizes.
Cost Breakdown (Mid-Range)
| Category | Amount | Share of Mid Total |
|---|---|---|
| Hard construction (labor + materials) | $210,000 | 80% |
| Soft costs (design, engineering, PM) | $25,200 | 10% |
| Permit and impact fees | $16,000 | 6% |
| Utility hookups | $12,000 | 5% |
| Mid-range total | $263,200 | 100% |
Last updated April 2026.
Building a 400 Sq Ft ADU in Seattle: What's Specific Here
Seattle removed the owner-occupancy requirement for DADUs in 2019, and the city now allows up to two ADUs per lot (one attached, one detached). SDCI's pre-approved DADU catalog has grown to about 20 designs that cut plan review to 6-8 weeks if you build straight from the catalog. The Pacific Northwest rain climate is the dominant cost driver — moisture detailing, rain screens, and code-mandated whole-house ventilation per the Washington Energy Code each add real dollars. For a 400 sq ft build specifically, expect 14-22-week plan review through Seattle Department of Construction and Inspections (SDCI) and a build sequence shaped by the city's wet temperate climate and Seattle Residential Code, Energy Code, SMC 23.44.041 (DADU rules) requirements.
What a 400 Sq Ft ADU Actually Gets You
A 400 sq ft Seattle ADU is usually a studio DADU or a basement conversion AADU. Pre-approved catalog covers most at this size and the simplest permit path applies.
Cost by Build Method
| Build method | Low | Mid | High | Notes |
|---|---|---|---|---|
| Stick-built (custom) | $205,220 | $263,200 | $351,180 | Most common path; highest cost, most layout flexibility |
| Prefab / panelized | $180,594 | $242,144 | $337,133 | 15-25% faster, 5-10% cheaper; limited layout options |
| Modular (factory-built) | $184,698 | $250,040 | $351,180 | 20-30% faster install; needs crane access on site |
| Garage conversion | $112,871 | $163,184 | $252,850 | Only if existing garage is at least 400 sq ft; lowest-cost route |
Realistic Build Timeline for 400 Sq Ft in Seattle
| Milestone | Typical duration |
|---|---|
| Design + construction documents | 4-8 weeks |
| Seattle, WA permit submission + review | 14-22 weeks |
| Site prep + foundation | 2-3 weeks |
| Framing + rough-ins (MEP) | 4-6 weeks |
| Finishes + final inspection | 6-8 weeks |
| TOTAL (design → certificate of occupancy) | 30-47 weeks |
Financing a $263,000 400 sq ft ADU in Seattle, WA
Most Seattle, WA homeowners stack a cash or HELOC down payment with a construction-to-perm loan. Sample scenario for a mid-range 400 sq ft build:
| Total project cost (mid) | $263,200 |
| 20% down payment (cash or HELOC) | $52,640 |
| Loan principal (80%) | $210,560 |
| Estimated rate (May 2026) | 7.5% (30-year) |
| Estimated monthly payment | $1,840 |
| Estimated rent (400 sq ft in Seattle, WA) | $1,425–$2,550/mo |
| Break-even rent target | $1,840/mo |
Sample scenario only — your actual rate, term, and rent will vary. Use the loan calculator to plug in your numbers.
Seattle Permit Fee Detail
| Plan check | $1,000–$2,500 |
| Building permit | $2,000–$4,500 |
| Impact fees | $0–$8,000 |
| School fees | N/A |
| Utility hookups | $2,500–$7,000 |
Questions to Ask Seattle ADU Contractors
Before signing with a contractor for your 400 sq ft ADU in Seattle, WA, ask:
- How many studio and 1-bedroom ADUs have you completed in Seattle in the last 24 months?
- What's your average permit timeline with Seattle Department of Construction and Inspections, and how do you handle plan-check correction cycles?
- Do you handle Pacific Northwest rain-screen detailing and Washington Energy Code whole-house ventilation in-house or sub it out?
- What's specifically included vs. excluded in your turnkey quote — soft costs, utility tap fees, landscaping, driveway repair?
- What's your payment schedule, and what triggers a change order vs. an allowance overrun?
- Can you provide addresses for 2-3 completed Seattle ADU projects I can drive by?
How We Calculate This
These are 2026 all-in market estimates. Hard costs scale linearly with square footage using a low/mid/high per-sq-ft band calibrated to recent build data in each metro. Soft costs (design, engineering, project management) are applied at ~12% of hard costs. Permit and utility hookup fees are set to local base amounts and lightly adjusted for size. For an itemized estimate tied to your lot, finish level, and specific city jurisdiction, use our Cost Estimator.
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Frequently Asked Questions
How much does a 400 sq ft ADU cost in Seattle, WA?
A 400 sq ft ADU in Seattle, WA typically costs $205,000–$351,000 all-in for 2026. The mid-range estimate is $263,000, which includes hard construction ($210,000), soft costs ($25,200), permit fees ($16,000), and utility hookups ($12,000).
What is the price per square foot for ADUs in Seattle, WA?
For a 400 sq ft build in Seattle, WA, expect $513–$878/sq ft all-in. The mid-range is roughly $658/sq ft. Per-sq-ft cost falls slightly at larger sizes because permit and utility fees amortize over more square footage.
How long does a 400 sq ft ADU take to build in Seattle, WA?
Total timeline runs 30-47 weeks from design start to certificate of occupancy in Seattle, WA. Permits alone take 14-22 weeks through Seattle Department of Construction and Inspections (SDCI), followed by 12+ weeks of construction. Pre-approved plans and prefab paths can shorten the permit phase.
Can I finance a $263,000 ADU in Seattle, WA?
Yes. For a $263,000 mid-range build, common options are construction-to-permanent loans, HELOC, cash-out refinance, or RenoFi. At current ~7.5% rates, a fully-financed build is roughly $1,840/month on a 30-year term. Use the ADU loan calculator to compare products side by side.
What size of ADU is allowed in Seattle, WA?
Seattle, WA permits detached ADUs up to 1000 sq ft on most lots. A 400 sq ft unit is within the typical limit, so the standard ADU permit path applies.
What rent can I expect from a 400 sq ft ADU in Seattle, WA?
In Seattle, WA, 400 sq ft ADUs rent for approximately $1,425–$2,550/month based on 2026 market rents for comparable units. Newer construction with separate parking and a private entrance lands at the high end of that range.
What's the difference between a 400 sq ft and a 500 sq ft ADU in Seattle, WA?
A 400 sq ft Seattle, WA ADU comes in around $263,000 mid-range, while the next bucket up at 500 sq ft runs about $322,000 — roughly $59,000 more. The bigger size adds a bedroom or larger living area but still falls within the city ADU size cap.
What's the most common mistake when building a 400 sq ft ADU in Seattle, WA?
Underestimating the pacific northwest rain-screen detailing and washington energy code whole-house ventilation requirement — many homeowners get bids from contractors who don't price these into the quote, then face change orders mid-build. Always confirm in writing how pacific northwest rain-screen detailing and washington energy code whole-house ventilation is handled before signing.
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