Go Far®The short version
A detached ADU (DADU) typically takes 10–15 months from the first design meeting to move-in. An attached ADU or interior conversion — basement or garage — usually runs 5–9 months, because the shell already exists and site work is minimal. Almost all of the difference between a fast project and a slow one happens before construction starts.
Phase 1: Feasibility and design (1–3 months)
Zoning check, site survey, utility scoping, and design development. This is where good projects are won: confirming setbacks and lot coverage under Washington’s ADU law, finding where sewer and power will route, and settling the floor plan before drawings go to engineering. Changing your mind on paper costs days; changing it during construction costs weeks.
Phase 2: Engineering and permitting (2–4 months)
Structural engineering, energy calcs, and the permit application. Review times vary by city — a complete, well-prepared submittal in the Snohomish County area commonly clears in 8–16 weeks, and correction cycles are where padded timelines come from: every round of reviewer comments adds weeks. Some cities, including Seattle, publish pre-approved DADU plans that can cut permit review dramatically if one fits your lot; our Seattle ADU rules guide covers how that works.
Phase 3: Construction (3–10 months)
Conversions run 3–5 months. Detached new builds run 6–10 months: excavation and foundation, framing and roof, utility trenching, then the long interior stretch — rough-ins, inspections, insulation, drywall, finishes. Western Washington weather deserves respect but not fear: crews here know how to pour and frame through the wet season, and a project that permits in fall builds through winter and finishes in spring.
What actually causes delays
- Incomplete permit submittals — the top cause, and entirely preventable.
- Utility surprises — undersized electrical service or a sewer line that needs upsizing, found late instead of scoped early.
- Mid-project design changes — every change during construction ripples through trades and inspections.
- Long-lead items ordered late — windows, electrical panels, and appliances should be ordered the day permits are filed, not when the slab cures.
Plan the money alongside the schedule
A year-long project also has a payment timeline — deposits, draws, and the financing that funds them. Our ADU cost guide and ADU financing guide pair with this one so the budget and calendar are planned together.
Start the clock with a real plan
Go Far Construction builds ADUs and DADUs across Lynnwood, Seattle, and Snohomish & King County. A free feasibility conversation maps your lot, your utilities, and a realistic month-by-month schedule before you commit to anything.