How much do ADU permits cost in Seattle and King County? The total pre-construction budget -- including permit fees, utility connections, and professional services -- ranges from $20,000 to $50,000 for Seattle city properties and $35,000 to $80,000 for rural King County lots requiring septic and well work. The building permit itself is just one piece; utility hookups and site-specific studies often cost more than the permit fees.
Below is the complete fee-by-fee breakdown so you can budget accurately before breaking ground. If you want us to handle the entire permit process, our Full Permit Package covers all submissions, coordination, and approvals — and your investment is applied as a build deposit when you sign the Letter of Intent.
Good news: Seattle ADUs have been fully exempt from impact fees since the 2019 ordinance (CB 119544/Ord 125791). HB 1337 also caps any future impact fees at 50% of the principal unit's fees.
ADU Permit Fees & Government Charges
| Fee Type | Typical Range | Notes |
|---|---|---|
| Building permit | $5,000–$15,000 | Based on project valuation per Seattle DCI fee schedule. |
| Plan review fees | $2,500–$7,500 | Typically 65% of building permit fee for Seattle DCI. |
| SEPA review (if triggered) | $500–$2,000 | Usually not required for standard ADU projects. |
| Impact fees | $0 | ADUs are exempt from impact fees in Seattle per 2019 ordinance (CB 119544/Ord 125791). HB 1337 caps future fees at 50% of principal unit. |
| Stormwater/drainage review | $1,000–$3,000 | Required for projects with new impervious surface. |
| SPU sewer connection | $5,000–$15,000 | Seattle Public Utilities new or upgraded sewer connection. |
| Seattle City Light electrical | $2,000–$6,000 | New or upgraded electrical service connection. |
| Total government fees | $4,000\u2013$12,000 | Varies by jurisdiction and project scope |
Utility Connection Costs
| Utility | City (Sewer) | Rural (Septic/Well) |
|---|---|---|
| Sewer (SPU) | $5,000–$15,000 | $15,000–$35,000 (new/upgraded septic) |
| Water (SPU) | $3,000–$8,000 | $5,000–$20,000 (well upgrade or new well) |
| Electrical (Seattle City Light) | $2,000–$6,000 | $3,000–$12,000 |
| Gas (if applicable) | $1,000–$3,000 | Propane tank install: $2,000–$4,000 |
| Total utilities | $8,000\u2013$21,000 | $25,000\u2013$59,000 |
Rural utility costs are the single biggest differentiator between city and county ADU projects.
Professional Service Costs
| Service | Typical Cost | When Needed |
|---|---|---|
| Architectural design | $5,000–$15,000 | Always (custom design) or $0–$2,000 (pre-approved plans) |
| Site survey | $1,000–$3,000 | When property boundaries are unclear or required by jurisdiction |
| Structural engineering | $2,000–$5,000 | Required for most new construction and major remodels |
| Geotechnical report | $2,000–$5,000 | Required for sites with slope, fill, or questionable soil |
| Critical areas consultant | $3,000–$10,000 | Required when critical areas are present on or near the property |
| Energy code consultant | $500–$1,500 | Often included in architectural services |
| Permit expediting / management | Varies | Our Full Permit Package investment is applied as a build deposit |
| Total professional services | $10,000\u2013$35,000 | Highly variable by project |
Total Pre-Construction Budget
How much does it cost to permit an ADU in total? When you add up government fees, utility connections, and professional services, here is what you should budget before construction begins:
Seattle (City Sewer)
Before construction begins
Rural King County (Septic/Well)
Before construction begins
What About Construction Costs?
Pre-construction costs are separate from the actual build. Total ADU construction in Seattle and King County ranges from $150,000 to $450,000+ depending on type and size. For a complete cost picture, see our full ADU cost breakdown or our pricing packages.
Not sure what your specific property will require? Get a free feasibility study -- we'll identify your jurisdiction, check utility access, and flag every potential cost factor before you commit.
Ways to Reduce Permit Costs
Check if your neighborhood qualifies for the transit parking exemption — properties within ½ mile of Link Light Rail or RapidRide stops require no off-street ADU parking, saving $5,000–$15,000 in site work.
Convert an existing structure — garage conversions or basement ADUs avoid some utility connection fees and reduce permit scope.
Submit a complete application the first time — resubmission fees and delays add up. See our Seattle permit guide for the full document checklist.
Bundle permit work with construction — our Full Permit Package investment is applied as a build deposit when you sign the LOI.
Take advantage of the 30% federal energy efficiency tax credit through sustainable ADU construction.
Seattle ADUs are fully exempt from impact fees (2019 ordinance, CB 119544) — no impact fee budgeting required.
Last updated: June 2026. Costs are estimates based on typical Seattle and King County projects. Actual costs vary by project scope, jurisdiction, and site conditions.
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