How Long Does It Take to Build a DADU in King County?

Construction Timeline: 4 to 8 Months

Once your permit is approved and you break ground, a DADU in King County typically takes 4 to 8 months to complete construction. Add that to the 3–8 month permitting phase, and the total project timeline from design to move-in runs 10–18 months. Understanding what drives the construction timeline helps you plan realistically and manage expectations.

Phase-by-Phase Construction Timeline

Site Preparation and Foundation: Weeks 1–4

Site clearing and grading: 3–5 days
Excavation: 2–5 days depending on foundation type
Foundation forms and rebar: 3–7 days
Concrete pour: 1 day
Curing time: 7–14 days before framing can begin
Utility rough-in under slab (if applicable): 2–3 days before pour

Foundation type significantly affects this phase. Slab-on-grade is fastest (2–3 weeks total). Crawl space takes 3–4 weeks. Full basement takes 4–6 weeks.

Framing: Weeks 3–7

Floor framing (if raised foundation): 2–3 days
Wall framing: 3–5 days
Roof framing: 3–5 days
Sheathing (walls and roof): 2–3 days
Window and exterior door installation: 1–2 days
Framing inspection: 1–3 days to schedule and pass

Two-story DADUs add 3–5 days to framing. Complex roof designs add 2–3 days versus simple gable or shed roofs.

Mechanical Rough-In: Weeks 6–10

Plumbing rough-in: 3–5 days
Electrical rough-in: 3–5 days
HVAC installation: 2–4 days
Insulation: 2–3 days
Mechanical inspections: 1–3 days each (plumbing, electrical, HVAC, insulation)

This phase involves multiple trades working in sequence. Good scheduling prevents gaps between trades — experienced builders minimize idle days.

Exterior Work: Weeks 7–12

Exterior work often overlaps with mechanical rough-in:

Weather barrier (housewrap): 1–2 days
Siding installation: 5–10 days depending on material
Roofing: 2–4 days
Exterior paint/stain: 3–5 days
Gutters and downspouts: 1–2 days

Pacific Northwest weather is a major factor here. Rain delays are common October through April. Smart builders schedule siding and roofing during drier windows when possible.

Interior Finishes: Weeks 10–16

Drywall (hang, tape, mud, sand): 7–10 days total
Interior paint: 3–5 days (primer + two coats)
Flooring: 3–5 days
Cabinetry and countertops: 3–5 days for install (plus 4–8 weeks lead time for fabrication)
Tile work (bathroom, backsplash): 3–7 days
Trim and doors: 3–5 days
Fixture installation (plumbing, electrical, hardware): 2–4 days
Appliance installation: 1 day

Site Work and Final: Weeks 14–18

Utility connections finalized: 2–5 days
Driveway/pathway paving: 2–3 days
Landscaping restoration: 3–7 days
Final grading and drainage: 1–3 days
Cleanup: 1–2 days

Inspections and Occupancy: Weeks 17–20

Final inspections (building, electrical, plumbing, mechanical): 1–2 weeks to schedule and pass all
Punch list corrections: 1–2 weeks for any minor items
Certificate of occupancy: Issued once all inspections pass

What Speeds Up Construction

Pre-ordered materials: Windows, cabinets, and specialty items ordered during permitting arrive on time for construction. This alone can save 4–6 weeks.
Experienced subcontractor network: Builders with reliable subs avoid scheduling gaps between trades.
Simple design: Rectangular footprint, standard roof, mid-range finishes = faster construction.
Summer/fall start: Starting construction in June–September avoids the worst of rain season for foundation and framing.
Finish selections made early: Choosing all finishes before construction starts prevents delays waiting for decisions.

What Slows Down Construction

Weather: Pacific Northwest rain can delay foundation, framing, siding, and painting. Budget extra time for winter projects.
Material delays: Supply chain issues can affect windows, specialty items, and appliances. Order early.
Inspection delays: Failed inspections require corrections and re-inspection. Working with code-knowledgeable builders minimizes failures.
Change orders: Mid-construction design changes disrupt scheduling and add cost. Finalize the design before breaking ground.
Subcontractor availability: King County’s active construction market means popular subs book out weeks in advance.
Complex sites: Slopes, poor access, and difficult soil conditions add time at every phase.

Your Role During Construction

As a homeowner, you can help keep things on schedule:

Make finish selections before construction starts
Respond quickly to any questions or decisions your builder brings to you
Maintain access to the construction area (move vehicles, secure pets)
Communicate proactively if something concerns you — don’t wait

APEX DADU Delivers On Time

We manage construction schedules tightly — pre-ordering materials during permitting, maintaining reliable subcontractor relationships, and building in weather contingency for Pacific Northwest conditions. APEX DADU clients know their timeline from day one, and we communicate progress every step of the way.

Contact APEX DADU to discuss your project timeline