Payment plan for shell construction - market-oriented?

Hello,

as our construction project is entering the final decision phase, I wanted to ask whether the payment schedule submitted by the shell builder complies with market requirements or should be renegotiated.

The following payment schedule was presented to us:

10% Contract signing
15% Completion of basement floor slab
20% Completion of basement ceiling
30% Completion of ground floor ceiling +L-Stones (Tiefhof) and sewer pipes
20% Completion of upper floor ceiling
5% Final invoice

I wonder if 45% of the total amount just for the basement is not considered too high? What are the opinions on this?

Someone is desperately in need of liquidity. I would have expected at most a partial payment for the shell construction.

I would never accept that in my life. You are massively overpaying.

Thank you for the answers. What would be a "reasonable" payment term?

In principle, I have nothing against partial payments, even if the effort is initially increased. However, I was surprised by the staggered schedule.
For your information, the offer is very interesting price-wise, also because I am building together with my neighbor (border construction, different building concepts so no classic terraced house). The shell construction will proceed in stages due to different basement depths. My basement is about 50cm deeper and therefore has to be completed first.

Huh? The shell builder is finished with the completion of the ceiling on the upper floor, right? Why shouldn't he then receive 100% of his payment?

Normally a payment - after completion.