Seattle ADU Cost: What Homeowners Should Budget For
An all-in detached ADU (DADU) on a typical 5,000–7,000 sqft Seattle lot generally plans in the high-$200k to high-$300k range, with design, permits, and side-sewer/utility work tracked as separate lines. Construction dominates total cost; SDCI permit and review fees, side-sewer permits, and Seattle City Light service work are real but smaller line items. The largest single budget variable in Seattle is site condition — slope, soil, alley access, and existing utility capacity move the bottom line more than finish level. Authoritative current city fees live in the SDCI fee subtitle and SDCI's permit-cost calculator; verify before budgeting.
Last verified 2026-05-15
| Cost category | Under 3,000 sqft | 3,000–3,999 sqft | 4,000–4,999 sqft | 5,000–6,999 sqft | 7,000–9,999 sqft | 10,000+ sqft |
|---|---|---|---|---|---|---|
| All-in (DADU) | $136k–$195k | $168k–$239k | $199k–$284k | $261k–$369k | $293k–$414k | $325k–$458k |
| DADU construction | $120k–$160k | $150k–$200k | $180k–$240k | $240k–$320k | $270k–$360k | $300k–$400k |
| AADU conversion | $90k–$120k | $120k–$160k | $150k–$200k | $180k–$240k | $210k–$280k | $240k–$320k |
| Design & engineering | $9,000–$20,500 | $10,000–$23,000 | $11,000–$25,500 | $12,000–$28,000 | $13,000–$30,500 | $14,000–$33,000 |
| Permit & city fees | $6,800–$14,200 | $7,600–$16,400 | $8,400–$18,600 | $9,200–$20,800 | $10,000–$23,000 | $10,800–$25,200 |
Numbers above are planning estimates from current bids and recent Golden State ADU projects on Puget Sound lots — they are not city-published fees. Verify current Seattle permit and utility fees against the official sources linked below before committing a budget.
Seattle ADU cost is driven by site condition first, finish level second, and city fees a distant third. Get a site-and-utility check before drawings — it's the cheapest correction you can make.
Seattle cost drivers
Construction is the dominant line item on every Seattle ADU we estimate. Foundation work, framing, MEP rough-in, insulation, drywall, finishes, exterior siding, and roofing typically make up 75–85% of all-in cost on a 5–7k lot. Design, permits, and utility connections fill out the rest.
The next biggest variable is what your lot actually allows. Seattle's older housing stock means many parcels have undersized electrical service, aging side sewers, and limited rear-yard access. Each of those conditions adds real dollars — and they aren't visible from a setback diagram.
Permit and review fees in Seattle are a real cost, but they are smaller than most homeowners expect. SDCI publishes the current numbers in the 2026 Fee Subtitle (SMC 22.900) and in the "How Much Will Your Permit Cost?" calculator. Treat the table on this page as a planning estimate and confirm current fees against those SDCI sources before committing a budget.
Design and engineering scale with project complexity, not just square footage. A simple stacked DADU on a regular lot drafts cheaper than the same footprint on a steep, irregular, or covenant-restricted lot. Structural, energy, and stormwater engineering tend to be the line items that move most.
City Light service work is separate. New 200A panels, service drops, or meter relocations are coordinated with Seattle City Light on their own schedule and pricing. It is not a building-permit line item, and it can move the calendar more than the budget.
What Seattle actually charges — and how to verify it
SDCI is the issuing department for Seattle ADU permits. Application, plan review, and issuance fees are published in the SDCI fee subtitle. They are billed at submittal, during review (per resubmittal cycle), and at issuance — not as one number at the end.
Side-sewer permits in Seattle are issued separately by SDCI's side-sewer permit track, with their own application fee and contractor requirements. If your project taps a new sewer connection, plan for that as a distinct cost line.
Plan-review corrections are not a separate city charge per cycle, but each correction round consumes designer hours and calendar time. The cheapest correction is the one prevented by a complete first submittal.
Numbers in our cost table are planning estimates from Golden State ADU project data, not city-published fees. Always confirm current fees against SDCI's published schedule before committing a budget.
Site condition and utility scope
Slope and soil dominate foundation cost on Seattle hill lots. A standard footing on flat ground compares poorly to a stepped foundation, retaining wall, or pin pile on a steep lot — the same DADU footprint can carry a five-figure foundation delta from one block to the next.
Side-sewer condition matters. Many Seattle parcels still run on cast-iron or concrete side sewers from the mid-century build-out; an ADU often forces an upgrade. Get a CCTV inspection before pricing.
Electrical service capacity is the single most under-budgeted item we see. If the primary house is on a 100A service, a new ADU typically triggers a 200A upgrade and a Seattle City Light coordination cycle — that's both a contractor cost and a separate utility cost.
Stormwater requirements in Seattle scale with impervious surface added. Drainage, infiltration, or detention can show up in the plan-review cycle and add construction cost downstream. Address it in design, not in corrections.
Picking the cost-efficient project type
A garage conversion (AADU within or attached to existing structure) is generally cheaper than a new DADU because the foundation, exterior shell, and one utility tap already exist — but it isn't automatic. If the existing garage was built without a permit, lacks code-compliant framing, or has structural rot, the conversion can cost more than a new build. Get a structural walk before committing to either path.
What can increase the budget
Service-panel upgrades, new sewer side-line, water-meter sizing, and gas re-routing can each add thousands. The cost is real but the line item rarely appears in early estimates because it depends on field conditions and the utility provider's current schedule.
On garage conversions and additions, foundation upgrades, rotted framing, or missing seismic ties are the most common scope shocks. A pre-design structural walk catches these before they hit a change order.
Each round of permit corrections costs designer hours and calendar weeks. Resubmittals that bounce back at intake compound fast. A pre-screened submittal package is the highest-leverage place to defend the budget.
Cabinetry, tile, lighting, and appliances are where allowance budgets quietly drift 20–40% above plan. Locking spec early — not later — is the cheapest control.
Tight alleys, mature trees in the build envelope, slope, or shared driveways add labor hours, crane time, and material handling. They are city-agnostic but show up most on Puget Sound urban lots.
What reduces budget risk
An hour walking the lot with someone who has built ADUs in your city is worth ten hours of revised drawings. It catches setback, height, FAR, parking, and tree-protection constraints before they force redesign.
A side-sewer call, panel-capacity check, and meter-sizing question to the utility before drawings start prevents the most expensive late-stage surprises.
Pricing finishes to the level you actually want — not the level the spreadsheet rounds to — keeps change orders predictable.
A complete, code-cross-referenced submittal package shortens review and reduces corrections. Most permit overruns are correction-driven, not city-fee-driven.
A 10–15% contingency is the difference between an unexpected condition becoming a story and becoming a stalled project. Plan for it explicitly — don't borrow it from finishes.
No promised ROI, no guaranteed rent figures. Financing assumptions should be confirmed with a lender; rental assumptions should be confirmed against current local market data.
Seattle ADU pricing questions
What does a typical detached ADU cost in Seattle?
On a 5,000–7,000 sqft lot in Seattle, WA, an all-in detached ADU generally plans at $261k–$369k including design, permits, site work, and construction. Larger or constrained lots shift the number up. These are planning estimates from current bids and recent projects — verify current city fees against the city's published schedule before committing a budget.
Are attached ADUs cheaper than detached ones in Seattle?
Often yes. An attached ADU (AADU) on a typical 5–7k Seattle lot plans at $180k–$240k for construction, vs $240k–$320k for new detached construction. Attached projects share foundation, roof, and one utility tap with the primary house — that's where the savings come from. The trade is that conversions hide more risk in existing conditions.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
What's included in the Seattle construction line?
$240k–$320k for a typical 5–7k lot. That covers foundation through final finishes — framing, MEP rough-in and trim, insulation, drywall, cabinetry, fixtures, flooring, exterior siding, and roofing. Site work, design, and permits are tracked as separate lines above. Numbers are planning estimates, not city-published fees.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
How much do design and permits add in Seattle?
Seattle design typically plans at $12,000–$28,000, with permit and review fees of $9,200–$20,800 on a 5–7k lot. Both scale with project size and review complexity. City-fee ranges shown here are planning estimates — confirm current numbers against the city's fee schedule linked below.
Go deeper: Read the Seattle Design Review for ADUs: when it applies (2026) guide
Why are Seattle ADU costs higher than the national average?
Three reasons: Seattle labor and materials run higher than national averages on Puget Sound; older housing stock means more utility, side-sewer, and panel work; and SDCI's review and stormwater standards are detailed. None of those are negotiable, but all are predictable with a feasibility review before design.
Go deeper: Read the Seattle ADU financing: 7 loan products compared (2026) guide
Are SDCI permit fees included in the construction estimate?
No. The construction line in our table covers what the general contractor builds. Permit, side-sewer, City Light, and stormwater fees are separate and are billed by SDCI and the relevant utility — verify current numbers against the SDCI fee subtitle.
Do I have to upgrade my electrical service for a Seattle ADU?
Often yes. A 100A primary-house service typically can't support an additional dwelling unit's loads. The upgrade is real cost but predictable — Seattle City Light coordinates the service-side work; your electrician handles panel and feeder work on the building-permit side.
Go deeper: Read the Seattle electrical panel upgrade for ADUs: 100A to 400A (2026) guide
Does Seattle require a side-sewer permit for an ADU?
If the ADU connects to a new or upsized sewer line, yes. SDCI issues side-sewer permits separately. The cost is project-specific and is not part of the building-permit fee.
Go deeper: Read the King County side sewer permits for ADUs: cost & timeline (2026) guide
What can a homeowner do to lower Seattle ADU cost before design?
Three things: get a feasibility walk before drawings, request a side-sewer CCTV inspection on older lots, and confirm your electrical service capacity. Those three together prevent the most expensive surprises we see.
Seattle cost & permit sources we used
Every claim on this page about Seattle permit fees, ADU code, submittal portals, or utility billing traces to an official city or zoning-code source. Construction numbers in the cost table come from current Golden State ADU project data and are presented as planning estimates — not city-published fees.
- Seattle Department of Construction & Inspections (SDCI) · city pageAccessory Dwelling Unit — SDCI
Defines AADU vs DADU in Seattle and the required permit pipeline that drives design and review fees.
Accessed 2026-06-03
- SDCI · fee scheduleHow Much Will Your Permit Cost?
Authoritative source for current SDCI permit application, review, and issuance fees in Seattle.
Accessed 2026-06-03
- SDCI · fee schedule2026 Fee Subtitle (SMC 22.900)
Published Seattle fee subtitle listing per-trade plan-review, inspection, and surcharge rates.
Accessed 2026-06-03
- City of Seattle · portalSeattle Services Portal
Official submittal portal where Seattle ADU permits are filed and fees are paid.
Accessed 2026-06-03
- SDCI · city pageSide Sewer Permits
Side-sewer/storm-side connection permits are issued separately and add to ADU project cost in Seattle.
Accessed 2026-06-03
- Seattle City Light · utilityNew Electric Service for ADUs — Seattle City Light
Electric service upgrades or new ADU services are coordinated separately through Seattle City Light.
Accessed 2026-06-03