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ADU & Addition Planning Guides

ADU & Home Addition Insight for Seattle-Area Planning

ADU and home addition planning guides for homeowners comparing added living space, garage conversions, detached units, bedroom additions, permits, site constraints, costs, and construction sequencing.

Plan added space around site conditions, code, and daily use

ADUs and additions require early planning because site access, setbacks, utilities, structure, drainage, parking, privacy, and local code can shape the project before design details are finalized.

These guides help homeowners compare added-space options, understand common permitting questions, and prepare for construction decisions that affect schedule, cost, and long-term use.

Before planning an ADU or addition, confirm these constraints

  • Review lot size, setbacks, access, utilities, and drainage before design advances.
  • Clarify whether the space will be for family, guests, rental use, or flexible living.
  • Identify structural tie-ins, roof connections, and utility upgrades early.
  • Plan permit timing before selecting final finishes.

ADU and addition planning topics covered here

Attached and detached ADUs Garage and basement conversions Bedroom and living-space additions Setbacks and site access Utilities and structure Addition costs and permits

Planning library

ADU and addition planning guides

Planning questions

Common questions before building added living space

What should be checked before planning an ADU?

Review zoning, setbacks, lot access, utilities, parking, drainage, privacy, structural needs, and local permit requirements before committing to a design direction.

How is a home addition different from an interior remodel?

Additions often involve foundation work, roof tie-ins, exterior walls, structural framing, site access, utility extensions, and more permit coordination than a typical interior remodel.

Can an existing garage or basement become added living space?

Sometimes, but feasibility depends on ceiling height, access, egress, utilities, fire separation, insulation, parking, local rules, and the condition of the existing structure.