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August 3, 2026
Seattle & Eastside ADU Cost Calculator (2026)
Wondering what an accessory dwelling unit will cost on your property? Use our free estimator to get a planning-stage cost range for an attached ADU, detached ADU (DADU), or garage conversion in Seattle and across the Eastside. Adjust the size and finish level to see how the numbers change.
Eastside ADU Cost Estimator
A planning-stage estimate for accessory dwelling units in Seattle & the Eastside, based on Piper Cole Architects' project data.
Estimate only, not a quote. Ranges reflect design, permitting, and construction for architect-led ADUs on the Seattle Eastside and vary with site conditions, slope, utilities, and finishes. Figures align with our published ADU cost guide. For a fixed scope and fee, contact Piper Cole Architects.
How the estimate works
The ranges above are drawn from Piper Cole Architects’ own completed ADU projects across Kirkland, Bellevue, Redmond, Mercer Island, and the wider Seattle Eastside. They reflect architect-led design, permitting, and construction — not construction alone — so they account for the drawings and approvals your project actually needs. For the detailed figures behind the tool, see our 2026 ADU cost guide.
What drives ADU cost the most
Type:
garage conversions reuse an existing structure and cost the least; detached DADUs are new standalone buildings and cost the most. Our garage conversion guide explains the trade-offs.
Site conditions:
slope, soils, and how far utilities must run can swing the budget significantly.
Finish level:
standard, upgraded, and high-end interiors carry meaningfully different costs.
Regulations:
setbacks, height, and lot coverage shape what you can build — start with Seattle DADU rules and Washington’s 2026 ADU laws.
Get an exact number
A calculator gives you a range; a licensed architect gives you a plan. When you’re ready to move from estimate to fixed scope and fee, contact Piper Cole Architects for a consultation.
This tool provides planning estimates only and is not a quote or a guarantee of cost.
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change — always confirm current requirements with the source:
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