Temporary VAT reduction by 3% until 31.12.2020

From the document:

"The new VAT rates of 16 percent and 5 percent are to be applied to deliveries, other services, and intra-community acquisitions made between July 1, 2020, and December 31, 2020 (see paragraphs 2 and 3). The decisive factor for the application of these VAT rates is always the time at which the respective transaction is carried out on page 5. The time of the contractual agreement is as irrelevant as the time of payment receipt or invoice issuance (see section 12.1 paragraph 3 UStAE). The same applies to partial services (see paragraph 2), for which paragraphs 19 to 25 contain special transitional rules."

What does this mean, for example, for our kitchen or our house acceptance?
The kitchen will be installed in September and the house accepted. All previous advance payments were made with 19%.
The kitchen deposit was also previously calculated with 19%.
The contract with the construction company contains a clause that secures a VAT increase. ("If VAT is increased, the construction price will be adjusted accordingly"). In my layman's opinion, it would legally be an overreach of one party to the contract if, in case of a VAT reduction, this were not correspondingly applied to the construction price, right?

Very briefly: Probably yes. However, that does not mean that the construction company sees it the same way. You might have to sue. I would ask very gently beforehand if the BU does not mention it on their own.

The problem is that you have to read the entire document. The paragraph you copied actually only describes that for performance fulfillment and acceptance within the specified period, billing must be done with 16% VAT.

But that doesn’t help you, because the question is whether the entrepreneur has to pass on the reduction to you. As has been discussed here several times (I won’t copy the link again where I summarized it in layman’s terms, it’s one or two pages earlier) what is decisive is what is stated in the rest of the document. Contract before 01.03.20 = entrepreneur must pass on the reduction. Contract between 01.03. and 30.06. = can but does not have to.
All clauses etc. are then civil law issues.
But be careful, this is merely the prevailing opinion on the status quo of the draft.

In consumer law in general, there is a rule that the consumer must not be placed at a disadvantage compared to the entrepreneur. That means if the entrepreneur protects themselves against an increase but, on the other hand, does not allow the consumer to benefit from a reduction, this constitutes a disadvantage for the consumer (e.g. termination rights in subscriptions; the consumer's termination period must never be longer than that of the company). Here, it is not a completely clear case insofar as it is almost a reversal of the speculative case, but I see (as a layperson!) the case quite clearly. Your deadline issue probably rather concerns the case where there is no clause in the contract at all.

Document decisive. Contract before 01.03.20 = Entrepreneur must pass on reduction. Contract between 01.03. and 30.06. = Can but does not have to. All clauses etc. are then civil law issues.

...which I have already written...

There is a general rule in consumer law that the consumer must not be placed at a disadvantage compared to the entrepreneur. That means if the entrepreneur protects himself against an increase but on the other hand does not allow the consumer to benefit from a decrease, this constitutes a disadvantage to the consumer (e.g. cancellation rights for subscriptions, the consumer’s cancellation period must never be longer than that of the company).
This is not a completely clear case here, since it is basically about a reversal of the speculative case, but I see (as a layman!) the case quite clearly.
Your deadline story probably revolves around the case where there is no clause at all in the contract.

That’s correct, face meant it that way too.
But: In case of doubt (if the company simply does not pass on the reduction), one will have to sue. And of course, the question arises how sensible that is. I still hope that companies will rather try to avoid trouble, but who knows...