Semi-detached house with or without underpinning - assembled

Hello everyone, This question has been asked many times and wonderfully answered by you. We have again the old problem but with a small change. We are building without a basement and the neighbor has a basement. We have agreed that the neighbor with the basement wall on our side will start first and then the underpinning will be built on the wall. We share the costs. Now the topic came up whether, if we build with a 2-week interval and not 1-2 years interval, we even need the underpinning at all. The neighbor is increasing the basement concrete wall thickness from 17 to 26 cm to get better support. We put earth in and compact it until it is sufficient for the floor slab. Here are different opinions. We decide to get advice from the structural engineer. Can you also please help us with advice. Many thanks in advance, HarryBo

I do not understand the question.

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

Do not understand the question either. It should be rewritten. The house must be taken into account in the structural analysis regardless. Either with underpinning or appropriate basement walls. How much time passes between the constructions does not matter.

Is it the case that you need a deeper foundation and the neighbor wants or should help build it? Or does he specifically want to save this foundation through a wall reinforcement?

The ground under your house is loosened by the construction method. A wider wall is not effective here. Different settlements occur because the first builder has already imposed loads.

Or should the wall be designed so that a massive compaction of the workspace fill to be built in is sufficient for your foundation?

I would wait for the engineer's recommendation and then negotiate the costs with the neighbor. Because he precisely saves himself the very elaborate underpinning.

Hello everyone, I will try to explain better. My neighbor and I want to start immediately because the development has been delayed by 6 months. He wants to finish building his basement including the basement ceiling with waterproofing in 2-3 weeks and the structural engineer said I don’t need underpinning for my floor slab. But he will check it thoroughly again when the plans are available. Neighbor has increased the basement wall from 17.5 to 24 cm. My concern is to wait 2-3 weeks but if it is delayed then I can hold my shell builder until the basement is finished. I am not sure if 2-3 weeks are enough for a passive house basement. Neighbor is basically nice, he thinks that if I build underpinning or supporting wall beforehand and then he builds the basement—it would be complicated and unnecessary cost. I was willing to cover 50%. Could you please give advice on what would be reasonable and if there are alternatives. Thanks in advance, HarryBo

If your neighbor's structural analysis includes your house, you do not need underpinning. If not, you need one.