MEP rough-in (electrical, plumbing, HVAC)

What "rough" means

The mechanical, electrical, and plumbing systems go inside walls and ceilings, but not finished or trimmed out. Everything you can't see once drywall goes up.

• Electrical: feeder from the main panel to the cottage subpanel; subpanel installed; wiring runs to each outlet, switch, fixture, and appliance circuit; box installation.
• Plumbing: water supply lines, DWV (drain-waste-vent) stacks, fixture rough-ins, water heater connections.
• HVAC: ductless mini-split refrigerant lines (the most common Seattle DADU choice), HRV/ERV ducts and unit, sometimes heat-pump water heater plumbing.

The order matters

Three trades working in the same walls. Standard sequence:

1. Plumber first (DWV stacks need vertical runs; large pipes need their space).
2. HVAC next (ducts are the next biggest envelopes).
3. Electrician last (small wires fit around what's already there).

A good GC sequences this. A bad one has all three trades fighting over the same chases on the same days.

Inspections

Each trade gets its own rough inspection by SDCI (electrical, mechanical) or King County Public Health (plumbing). All three have to pass before insulation.

Cost

• Electrical: $15K–$35K rough plus finish.
• Plumbing: $15K–$30K plus connection fees.
• HVAC: $9K–$22K complete heat-pump and ventilation package.

These are line items in the GC bid — ask for them broken out.

What can go wrong

• Service upgrade not done yet. If the SCL service application (step 3.2) wasn't filed early, the new 200A panel may not be ready when the electrician needs to land the cottage feeder.
• Side sewer not connected. The plumber's DWV stacks need somewhere to drain. If the side-sewer contractor hasn't finished the connection, plumbing rough is on hold.
• Ductless line-set routing conflicts with windows, beams, or drywall returns. Caught now, fixable; caught at finish, ugly.