DADUSeattleWA.com · Free educational tool
Explore an Attached ADU Budget
Change the conversion type, square footage and existing conditions to explore a planning budget.
Illustrative project budget
$250,000 – $410,000
Example: Basement conversion, 600 sq. ft., standard finishes. This is an illustrative starting assumption, not a contractor quote.
Sample assumptions: October 2026. Version 2026.10.1.
How the planning range is calculated
Uses editable sample 2026 rates, minimum project costs, incremental construction allowances, planning/design fees, modeled taxable construction charges and a separate planning reserve. These are educational starting assumptions rather than a verified Seattle bid dataset.
Included in construction: Standard kitchen and first bathroom, normal labor, ordinary building systems, typical overhead/profit and structure appropriate to the selected project type are included in the base rate. Special conditions and owner-paid fees appear separately.
Not necessarily included: Land purchase, financing, major hidden conditions, unforeseen geotechnical work, extraordinary utility infrastructure, future price increases and unknown site-specific agency requirements may not be priced.
Source status: Illustrative seed assumptions, not independently validated market averages. Editorial review: 2026-10-27; pricing version 2026.10.1.
Permit totals are budgeting allowances, not official SDCI fee calculations. Recheck official sources and city requirements before relying on any cost. Seattle permit fee guidance.
Frequently asked questions
Does this calculator confirm my project will be permitted?
No. Seattle zoning, building, fire and utility requirements depend on the specific site and design. Use Seattle SDCI guidance for permit and eligibility information.
Why does the estimated budget change with size?
Construction changes with area but fixed design, review and infrastructure costs do not scale evenly. The model also applies minimum construction amounts to small projects.
Are these current contractor bids?
No. Rates are editable example assumptions retained from an earlier planning model, not independently verified market averages, contractor offers or official city fees.
Learn why costs change
- ADU options and conversionsUnderstand the differences between conversion and addition projects.
- DADU costs and budgetingReview Seattle project-budgeting topics.
- What estimates includeCompare line items and common omissions in construction estimates.
This is an independent educational estimate, not a contractor quote, zoning approval, legal advice or official fee calculation. Unknown subsurface, structural and utility conditions can exceed the displayed range.