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Your Complete Guide to Building a DADU

  • Writer: National Design
    National Design
  • Oct 21
  • 2 min read

Updated: Oct 27

| Make Your DADU Idea a Reality! |




An ADU (Accessory Dwelling Unit) is a secondary living space added to an existing single-family property. It can be attached to the main house (Attached ADU) or detached as a separate small home (Detached ADU / DADU).

Each jurisdiction in Washington State — such as King County, Seattle, and Snohomish County — has its own specific zoning and building regulations regarding ADUs. These rules define maximum size, setbacks, height, parking, and owner-occupancy requirements. In general, ADUs in these areas can range from 600 sq ft to 1,200 sq ft, depending on the property’s zoning and lot size.


ADU regulations:




The 4 Stages: From Concept to Construction


  Step 1  — Feasibility Study


We begin by verifying what type and size of ADU can be built on your property according to local zoning and building codes. Our team contacts the city or county planning department to confirm all the specific requirements — including setbacks, lot coverage, and allowable structure height.


Typical timeline: up to 7 days.



Step 2 — Design and Documentation


This phase starts with developing the floor plan, elevations, and exterior design — a close collaboration with the homeowner to achieve the best layout and aesthetics. Once approved, we prepare the full architectural set and coordinate with the structural engineer.


Depending on site conditions and jurisdiction, additional documents may be required — such as:

  • Civil Engineering (Drainage Report)

  • Geotechnical (Soil) Report

  • Arborist Report

  • Topographic Survey

  • Septic System Design

After all documentation is complete, the project is reviewed internally and prepared for submittal to the city or county for permit review.


Typical duration: approximately 1.5 to 2 months, depending on project complexity.







Step 3 — Permit Process


While homeowners can submit the permit package themselves, we strongly recommend letting our office handle the submittal and communication with the city.


Permit coordination fee: $1,500 (includes the following):

  • Digital preparation of all submittal files (PDF sets, forms, and applications)

  • Uploading through the online permit portal

  • Responding to city plan reviewers’ comments

  • Coordination with structural, civil, and other consultants

  • Tracking review progress until permit approval


City/county fees not included.


Review timeline: typically 2 to 6 months, depending on the jurisdiction and complexity.



Step 4 — Construction Phase


Once the building permit is issued, the construction of your DADU begins — starting with foundation work and continuing through framing, utilities, finishes, and inspections. Each stage of construction must pass official city or county inspections to ensure full compliance with code.


After the final inspection, your new DADU is officially ready for occupancy.



Смотрите также ВИДЕО о нормах ADU для штата Вашингтон



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