I believe that most contracts contain a clause regarding the adjustment of the offer price to the current value-added tax rate. As a rule, the tax has always been increased so far, so the construction company would otherwise have had to bear the cost.
But in my opinion, it is also important to consider how the contract is structured. Since usually no completed partial services are agreed upon, but lump-sum deductions, it is quite possible that the deductions are billed at 16%. And then, if the final invoice is perhaps not issued until 2021, 19% is due on everything.
For those who are now concluding contracts and want to start building this year, a visit to the tax advisor for contract design could be worthwhile. Then you could possibly put the shell construction and interior construction into separate contracts. If the shell construction is then completed this year, 16% is due, and 19% on the interior construction the following year.