Construction costs are currently skyrocketing

There is a difference between push and pull in data protection. Pull means that authority X requests data from authority Y. This is rather undesired because it can easily be abused. It’s different when one authority passes information to another authority that it already receives anyway, often via the detour of the citizen filling out a form. This is not only massively inefficient but also leads to the databases diverging from each other over time.

A real negative experience in this regard are the 4 birth certificates that one receives for a child and then submits to other authorities. Data protection would be the same if this were automatically forwarded to the offices responsible for child benefits, parental benefits, etc., and at the same time one could tick a box for the child’s ID (mandatory also in the EU abroad).

Exactly, if the data ends up at X anyway because you need something from X, then Y can just as well obtain it directly. The only important thing is transparency and control.

then the previous owner will probably still get mail.

At least that is how it is for us right now.

On 1/1 we were still the owners of our house and also the owners of the current property.

Since 3/15 we have only been the owners of the current property.

But now I have received mail for both regarding the submission of the property tax declaration.

It was similar for me. I also received mail about the property tax for a property I had already sold in 2021. Strangely, they didn't know that yet. But it was stated right away that if I am no longer the owner, I can provide the new owner, and that was it.

if I am no longer the owner, I can provide the new owner, and that's it.

Which, strictly speaking, wouldn't actually comply with data protection regulations, right? So actually, you would have to get the buyer to sign that their data may be forwarded to all authorities that request it. Data protection is important, but I believe the average consumer is regularly overwhelmed by it or simply unknowingly violates it.

I believe that in tax matters, legitimate interest applies and one is therefore entitled to provide the owner and address, as far as known

I suspect the land registry office will not handle it differently either

Which strictly speaking wouldn’t be data protection compliant either, right? So actually, you’d have to get the buyer to sign that you are allowed to pass their data on to all authorities that demand it.
Data protection is important, but I think the average consumer is regularly overwhelmed by it or simply violates it unconsciously.

I don’t think that’s wrong because the new owner is listed in the land register and the tax office would also get the information from there.