VAT reduction during house construction

I think it will fail because of the listing in the contract, if I have understood correctly:

The service contract must specify that a separate remuneration is agreed for parts of the overall performance. Partial services are only possible if individual items such as service descriptions, quantities, and prices are included. If only a fixed price for the entire work is agreed, partial services are excluded from the outset.

And why not? 5% of X is a pretty concrete amount, even if a net sum Y was agreed upon.

Nope. Just because you install a more expensive staircase compared to the standard, that doesn't mean the earthworks become more expensive. On the other hand, 15% of the final amount does. As I said, as a contractor I wouldn't take the risk of having to pay thousands of euros in additional taxes during the next audit in 4 years if I have the customer committed anyway. Most likely, the customer only builds once in their lifetime.

How do you think the general contractor would behave? Issue an invoice with 19%? Or with 16%?

In my opinion, it would be correct for interim invoices as follows:

2nd half of 2020
Interim payment 1+2: net + 16%

1st half of 2021
Interim payments 3-7: net + 19%
Final invoice total net + 19% minus gross interim payments

The question is whether I base interim payments 1+2 on the gross amount (as agreed with private customers) and deduct the 16%, or base it on the net amount and then have a larger additional payment of the 3% VAT difference from interim payments 1 and 2 at the final invoice.
Option 2 could catch the client completely off guard.

oh dear!
This also concerns us. The house (turnkey) is expected to be completed by 09/2020. In the developer purchase contract, a total amount x including 19% VAT, as well as proportional installments corresponding to construction progress are listed (30% at ..., 28% at... etc.). There are no special inspections in between. The only option is to inspect the construction progress.
About 20% are still outstanding.

Which option is correct?:
a) from 01.07.2020 the remaining amount will be recalculated (remaining amount / 119 * 116)
b) upon acceptance of the completed house the total amount will be recalculated (total amount x / 119 * 116)

Builder purchase contract or contractor's work contract?

For us, it is exactly the same situation as with , only with acceptance 11/20. How would it be with us ( : For us GU)?

oh dear!
We are affected by this as well. The house (turnkey) is expected to be completed in 09/2020. The developer purchase contract includes a total sum x including 19% VAT, as well as percentage installments corresponding to the construction progress (30% at ..., 28% at ... etc.). There are no special acceptances in between. There is only the option to inspect the construction progress.
About 20% is still outstanding.

Which variant is correct?:
a) from 01.07.2020 the remaining sum will be recalculated (remaining sum / 119 * 116)
b) upon acceptance of the completed house the total sum will be recalculated (total sum x / 119 * 116)

Variant a)

16% tax is added to the net price and the installment payments made are fully deducted from this amount. The resulting amount is the remaining sum to be paid.