ᐅ Semi-detached house built with or without underpinning

Hello everyone,
This question has been asked many times and you have always provided great answers.
We have the old problem again but with a small change.
We are building without a basement, and the neighbor has a basement.
We agreed that the neighbor will start first with their basement wall on our side, and then the underpinning will be built on that wall. The costs will be shared.
Now the question came up: if we build with a two-week gap instead of a 1–2 year gap, do we still need the underpinning at all?
The neighbor is increasing the thickness of the basement concrete wall from 17cm (7 inches) to 26cm (10 inches) to get better support.
We will fill in soil and compact it until it is sufficient to support the foundation slab.
There are different opinions on this. We will consult the structural engineer for advice.
Could you please also help us with your advice?
Thank you in advance,
HarryBo

I don’t understand the question.

In my opinion, underpinning is only necessary if you build first.

I don’t understand the question either. It should be rewritten.

In any case, the house must be accounted for in the structural engineering, either by underpinning or appropriate basement walls.
The amount of time between construction phases does not matter in this regard.

Is it the case that you need a deeper foundation and the neighbor wants or should help build it? Or is the goal to save this foundation by reinforcing a wall?

The soil under your house will be loosened by the construction method. In this case, a wider wall is not effective. This leads to uneven settlement because the first builder has already applied loads.

Or should the wall be designed so that a thorough compaction of the backfill in the working space to be built is sufficient for your foundation?

I would wait for the structural engineer’s recommendation and then negotiate the costs with the neighbor. After all, they save themselves the very complex underpinning work.

Hello everyone,
I will try to explain better.
My neighbor and I want to start immediately because the utility connection has been delayed by 6 months.
He wants to finish building his basement including the basement ceiling with waterproofing within 2-3 weeks, and the structural engineer said I don’t need underpinning for my slab. However, he will check everything carefully again once the plans are ready. The neighbor has increased his basement wall thickness from 17.5 to 24 cm (7 to 9.5 inches).
My concern is whether waiting 2-3 weeks is possible, but if it is delayed, I can keep my shell builder on hold until the basement is finished.
I am not sure if 2-3 weeks is enough time for a passive house basement.
The neighbor is basically nice and says that if I build underpinning or a supporting wall first, and then he builds the basement, it would be complicated and cause unnecessary extra costs. I was willing to cover 50% of the costs.
Could you please advise what would be sensible and if there are any alternatives?
Thanks in advance,
HarryBo

If your neighbor's structural engineering includes your house, you do not need underpinning. If not, you will need it.