Payment plan for shell construction - market-oriented?

So 4 - 6 advance payments for a construction period of 6 months are completely normal. That only one invoice is issued is not normal. You just have to be careful not to overpay during the advance payments. The architect is usually responsible for that. A security retention should also be withheld. I do not think a prepayment before the start of construction is good. But it is okay that something is billed immediately after the construction site setup.

First of all, thank you very much for all the information. However, I must admit that I am now even more confused.

I have no problem paying in installments after the work has been completed, I just think the percentage division with 45% until the basement ceiling is completed is too much. Am I wrong here?

I would now also consider the number of payments to be normal. Whether the basement corresponds to 45% of the total project? Presumably, you are right with your feeling that this is not the case. Of course, it depends on the project - but just negotiate down another 10-15 percent so that it feels right. I find 10% at contract signing too high - what is there really to plan for that you should go into advance payment? It’s not a general contractor contract after all...

I would now also consider the number of payments to be normal. Does the basement correspond to 45% of the total project? You are probably right with your feeling that this is not the case. Of course, it depends on the project – but just go down another 10-15 percent so that it feels right. I find 10% at contract signing too high – what big plans could there be for which you should make an advance payment? This is not a general contractor contract...

He argues that he is advancing payments upon contract award and has the basement walls and ceiling prefabricated. In the past few days, I have visited several of his construction sites (shell construction completed). The construction site itself makes a good, neat impression, and the machines used there were also in a well-maintained condition.
However, I am not an expert and can therefore only judge from a "gut feeling" perspective. I will try to negotiate further with him.
Thanks for the many tips

that he goes into advance payment with the order placement and has the basement walls and ceiling prefabricated.

Exactly as it should be – he goes into advance payment. That’s exactly how it should be on the ground floor as well. He builds the walls, puts a ceiling on, and when it’s finished he gets the money for this construction phase. Whether the ceiling and walls are cast on site, come prefabricated from the factory, or are conjured up by someone else – that should not be your problem :) He actually shouldn’t cancel the order because of something like that – I would ask critically. In case of doubt, if the contractor bails out before the start of construction, you have nothing – except losing 10% of the contract sum.
Whether this advance payment is rather common in some places: no idea, here actually not.

We have seen such payment plans in offers from various trades and immediately rejected them. They were mostly young companies (max. 3 years old). We now pay an installment after the completion of the foundation slab, ground floor walls, ceiling/walls of the upper floor. We do not make advance payments and would currently advise against it. It reads as if the company is not liquid enough to make advance payments.