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Phase 2 · Design and engineering · Step 2.2

Make the lift-vs-retrofit decision

With the foundation assessment in hand, you and the designer make the call: build on the existing foundation (with retrofits), lift the house and build a new foundation underneath, or lift and add a new ground floor too. This decision shapes everything downstream.

Who
Homeowner, Designer, Structural eng.
How long
1-2 weeks
Cost
Free (decision) — drives $20K-$150K+ downstream
You end up with
Documented strategy decision in the project file

If you skip this: The lift-vs-retrofit decision determines schedule, foundation cost, and whether the family can shelter in place during the build. Make it explicitly with the engineer and designer in the room — implicit decisions get re-litigated mid-project at huge cost.

Three options on the table

Option A — Build on the existing foundation, with retrofits. The structural engineer designs perimeter steel, additional interior footings, or a new foundation wall to carry the new load. The existing house stays in place. Most common path on post-1940 houses with adequate footings.

  • Cost adder: $15,000–$60,000 for retrofit work, included in the GC bid line for foundation.
  • Schedule: no schedule penalty.

Option B — Lift the house and build a new foundation underneath. A house mover (a small handful of specialty firms serve Seattle — your structural engineer or GC will know the current short list) lifts the house on temporary cribbing and beams. The old foundation comes out. A new full-height foundation goes in. The house drops back onto the new foundation. The first floor stays unchanged.

  • Cost adder: $20,000–$100,000 for the lift + foundation rebuild, on top of the second-story addition cost.
  • Schedule: adds 4–8 weeks.

Option C — Lift the house and add a new ground floor (or basement) underneath. Same as B but the new foundation is full-basement height, giving you a brand-new ground floor or finished basement under the existing house, then the second story addition on top of that.

  • Cost adder: $80,000–$200,000+ on top of base addition cost.
  • Schedule: adds 8–16 weeks.
  • Outcome: you end up with effectively a three-story house and the most square footage of any option.

How to make the call

Three inputs:

  1. The foundation assessment from step 1.3. Tells you whether the existing foundation is rescuable.
  2. Square footage you want. If you only need ~600 sq ft of new space upstairs, Option A is usually fine. If you want 1,000+ sq ft of new space and a usable basement, Option C suddenly looks good.
  3. Where the family lives during the build. Option A is the only path that even theoretically lets the family stay in place (and even then, most don't). Options B and C absolutely require moving out.

What to document

A short decision memo signed by you, the designer, and the structural engineer. Two paragraphs is enough. This becomes the anchor for every downstream cost and schedule estimate.

Where this information came from

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