Addition foundation, tie-in, and shell

Sequence

1. Excavate the addition footprint. Remove existing landscape, dig to footing depth.
2. Form, rebar, pour the addition footing. Renton CED footing inspection before pour.
3. Form, rebar, pour the foundation walls. Cure 5–7 days.
4. Strip forms, waterproof, backfill.
5. Tie-in detail. Structural connection between existing and new foundation per the engineer's design.
6. Frame the addition. Floor joists, walls, roof framing, sheathing, WRB, roof underlayment.
7. Window and door rough-in in the addition.
8. Roof tie-in to existing roof.

Total: 4–8 weeks depending on weather and addition complexity.

Coal Mine Hazard Area: inspection implications

If the lot is in a Coal Mine Hazard Area, special foundation requirements from the geotech report apply per RMC 4-3-050. Confirm with Renton CED at permit issuance whether independent geotech review of foundation conditions is required before pour.

The tie-in is the critical detail

Two failure modes at the foundation tie-in:

• Differential settlement. New foundation settles into virgin soil; existing has already settled into position. The engineer's joint detail has to accommodate the difference.
• Water intrusion. The joint between existing and new exterior wall is a known leak path if not flashed correctly. WRB has to be continuous across the joint.

Both are solved by detail — the structural drawings should show the tie-in explicitly.

Inspections

• Footing inspection before pour.
• Foundation wall inspection before backfill.
• Framing inspection after sheathing, WRB, and roof underlayment.