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. SDCI inspection of rebar before pour.
3. Form, rebar, pour the foundation walls or stem walls. Cure 5–7 days.
4. Strip forms, waterproof, backfill.
5. Tie-in detail. The structural connection between existing and new foundation — continuous footing rebar across the joint, stepped joint with isolation, or above-grade structural tie at the wall.
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. New shingles or roofing keyed into the existing.

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

What the family experiences

If you're sheltering in place, this is the easiest phase. The work is happening outside and the existing house's interior is mostly untouched. Some noise during framing; not much dust.

The tie-in is the critical detail

Two failure modes are common at the foundation tie-in:

• Differential settlement. New foundation settles into virgin soil; existing foundation has already settled into its position. If the engineer's joint detail doesn't accommodate the difference, cracks appear at the tie-in within the first year.
• Water intrusion. The joint between existing exterior wall and new exterior wall is a known leak path if not flashed correctly. The WRB has to be continuous across the joint, and the flashing has to lap correctly.

Both are solved by detail, not by execution alone — the structural drawings should show the tie-in detail explicitly, and the GC's superintendent should walk the joint with the engineer before backfill.

Inspections

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

When this is done

When the addition is framed, dried in, and inspected. The existing house's interior demo can start.