Seattle DADU Cost 2026: $220K–$470K for a Detached Unit
A DADU — Detached Accessory Dwelling Unit, Seattle's own term for a standalone backyard cottage — typically costs $220,000–$470,000, with a common 750 sq ft build landing at $265,000–$380,000. That's $40,000–$120,000 more than an attached AADU of the same size, because a DADU needs its own foundation, roof, and mechanical systems.
Last updated: •Reviewed by the aduglossary editorial team
By the BVLLC Editorial Team · Zoning figures verified against SDCI's Ordinance 127376 (Jan 2026) and aduzoning.org; cost ranges from published Seattle-area builder pricing. Last updated July 2026.
Quick Answer
Seattle ADUs: $220K–$470K
Permits: $3.5K–$15K · Updated April 2026
500 sq ft DADU
$200K–$290K
Studio-sized detached build
750 sq ft DADU
$265K–$380K
Most common size
1,000 sq ft DADU (2BR)
$330K–$470K
Code max for 2 bedrooms
Max size, 3+ bedrooms
1,200 sq ft
Per Ordinance 127376, effective Jan 2026
DADU vs. AADU premium
+$40K–$120K
Own foundation, roof & mechanicals
Rear/side setback
5 ft (0 ft on alley)
Verify by zone with SDCI
What Does DADU Mean in Seattle?
DADU is Seattle's own code term for a Detached Accessory Dwelling Unit — a fully separate structure on the same lot as a single-family home, distinct from an AADU (Attached Accessory Dwelling Unit), which is built into or onto the existing house. The distinction isn't just vocabulary: Seattle Department of Construction and Inspections (SDCI) applies different Green Factor landscaping requirements, fire-separation rules, and utility-connection standards to detached versus attached units, and the cost gap between the two is the single biggest lever in a Seattle ADU budget.
Detached units cost more because they cannot borrow anything from the existing house. A basement conversion (an AADU) reuses an existing foundation, roof, and often an existing electrical panel or water line. A DADU needs all of that built new: a full foundation engineered for Seattle's Seismic Design Category D, a complete roof structure, a dedicated electrical service (often a separate meter), and its own heating and water systems compliant with the Washington State Energy Code.
DADU vs. AADU: How Much More Does Detached Cost?
The table below compares a detached DADU to an equivalent-size attached AADU (basement or garage conversion) at the same square footage, standard mid-finish construction, full seismic engineering and WSEC energy compliance.
| Size | Detached DADU | Attached AADU | Detached premium |
|---|---|---|---|
| 500 sq ft | $200K–$290K | $80K–$140K | +$100K–$150K |
| 750 sq ft | $265K–$380K | $105K–$175K | +$110K–$205K |
| 1,000 sq ft | $330K–$470K | $135K–$220K | +$125K–$250K |
AADU figures reflect basement/garage conversions reusing existing foundation and structure. Sloped lots or custom architect-designed DADUs push detached costs toward and past the upper bound. See the full Seattle ADU cost guide for garage-conversion and above-garage pricing. Last updated July 2026.
Seattle DADU Zoning Rules That Drive Cost
Seattle's DADU-specific zoning changed materially between 2025 and 2026, and the current rules directly affect what you can build and what it costs. Under Ordinance 127376, effective January 2026, DADUs may reach 1,000 sq ft for units with up to 2 bedrooms, or 1,200 sq ft for 3+ bedroom units — larger than the flat 1,000 sq ft cap that applied through 2025. Height increased from the historical 18–22 ft limit to 32 ft in most Neighborhood Residential zones, with up to 42 ft allowed under tree-protection, setback, or Green Factor conditions.
| Rule | Current standard |
|---|---|
| Max size | 1,000 sq ft (≤2BR) / 1,200 sq ft (3+BR) — Ordinance 127376, effective Jan 2026 |
| Height | 32 ft standard; up to 42 ft with tree-protection/setback/Green Factor conditions |
| Rear setback | 5 ft (0 ft where lot abuts an alley) |
| Side setback | 5 ft |
| ADUs per lot | 2 max (1 DADU + 1 AADU, or 2 AADUs — not 2 DADUs) |
| Owner-occupancy | Not required (eliminated 2019; reinforced by WA HB 1337 in 2023) |
| Off-street parking | Not required (eliminated 2019) |
Two rules push cost specifically for detached units. First, DADUs must meet a Green Factor landscaping score, an additional planting/permeable-surface requirement that AADUs are typically exempt from — budget $1,000–$3,000 for landscape design to satisfy it. Second, DADUs within 5 ft of a property line generally require 1-hour-rated fire walls, an assembly cost that attached conversions rarely trigger since they sit inside the existing building envelope.
Zoning figures verified against SDCI-sourced summaries current to January 2026 (Ordinance 127376) and cross-checked against aduzoning.org. Zoning is lot- and overlay-specific — confirm your exact allowance at seattle.gov/sdci before finalizing plans.
Full Seattle ADU permit requirements & feesFinancing a Seattle DADU
Because a DADU costs $40,000–$120,000 more than an equivalent attached unit, financing decisions matter more for detached builds. Seattle homeowners with equity built up before 2020 commonly cover a full DADU through a HELOC or cash-out refinance without disturbing a low-rate primary mortgage; larger custom DADUs above roughly $350,000 more often use a construction-to-permanent loan. See our full Seattle ADU financing guide for lender comparisons, rates, and qualification requirements.
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Seattle DADU Frequently Asked Questions
How much does a DADU cost in Seattle?
A detached DADU in Seattle costs $220,000–$470,000 for a 500–1,000 sq ft unit at standard finishes, with a typical 750 sq ft build landing at $265,000–$380,000. High-end or architect-designed units in central Seattle neighborhoods can reach $400,000–$600,000+. Seismic Design Category D engineering, Washington's energy code, and sloped-lot grading are the three Seattle-specific costs a national cost calculator won't show you.
What does DADU actually stand for, and is it different from an ADU?
DADU stands for Detached Accessory Dwelling Unit — Seattle's own zoning-code term for a separate backyard structure. "ADU" is the umbrella term covering both DADUs and AADUs (Attached Accessory Dwelling Units, like a basement or garage conversion built into or onto the main house). Every DADU is an ADU, but not every ADU is a DADU — the distinction matters because Seattle applies different Green Factor landscaping and fire-separation rules to detached units.
How much more does a detached DADU cost than an attached AADU in Seattle?
A detached DADU typically costs $40,000–$120,000 more than a comparable attached AADU (basement or garage conversion) of the same square footage, mainly because a DADU needs its own foundation, roof, and full mechanical systems (HVAC, water heater, electrical panel) that an AADU can often share or extend from the main house. A 750 sq ft basement conversion might run $105,000–$175,000, while an equivalent detached DADU runs $265,000–$380,000.
What is the maximum size for a DADU in Seattle?
Under Seattle's Ordinance 127376 (effective January 2026), DADUs may be built up to 1,000 sq ft for a unit with up to 2 bedrooms, or 1,200 sq ft for 3 or more bedrooms. This is a citywide increase from the 1,000 sq ft flat cap that applied through 2025 — verify the exact figure for your lot and zone with SDCI, since some overlays and lot-coverage limits can still constrain the buildable footprint below the code maximum.
What are the setback and height rules for a Seattle DADU?
Standard DADU setbacks are 5 ft from the rear property line (0 ft allowed where the lot abuts an alley) and 5 ft from side property lines. Height is capped at 32 ft in most Neighborhood Residential zones, up from 18–22 ft before the 2025 reforms, with up to 42 ft allowed under certain tree-protection, setback, or Green Factor conditions. Confirm your lot's specific zone (NR1, NR2, NR3, etc.) at SDCI before finalizing a design — corner lots and lots on slopes have additional rules.
Can I rent out both my house and my DADU in Seattle?
Yes. Seattle has had no owner-occupancy requirement since 2019, and Washington's statewide HB 1337 (RCW 36.70A.696) reinforced this in 2023 by barring cities from imposing one. You can rent the primary home and the DADU simultaneously as long-term rentals. Short-term rentals under 30 days require a Seattle STR Operator License and are restricted on non-primary-residence units.
Can I build a DADU and an AADU on the same Seattle lot?
Yes. Since 2019, and confirmed again under state HB 1337, Seattle allows up to two ADUs per single-family lot — one attached and one detached, or two attached units. You cannot build two detached DADUs on one lot.
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