Reduced value-added tax until the end of 2020. Procedure for invoices?

VAT is due on delivery and/or performance. The delivery must have taken place, the service must have been rendered. So, service rendered in June, invoice issued in July, 19%. Service rendered or goods delivered in December, invoice same month 16%, invoice in January also 16%.

How does it work with larger purchases like a kitchen or new house? Bought kitchen/house in July at 16% and still get it delivered/built next year, is it still 16% or a price adjustment?

What does "bought a house" mean? Really bought the finished piece of house? Then possibly 16%, provided completion in 2020. If you mean by that "signed contract with general contractor," house construction takes place in 2021, then 19%. If parts are completed and invoiced in 2020, then 16% on those parts, provided the tax office agrees (the special case is discussed in the other thread).

Another example, the kitchen. Offer and signature in July 2020, delivery May 2021. The offer will say 16%, but you will pay 19% in May 2020.

Privately, you always buy the net merchandise value plus the VAT valid at the time of delivery/service!