Retrofit warm and cold water meters?

Yes, the practice is used by us, we deduct the costs as a flat rate. Currently, we are satisfied with the solution.

OT:
Is this coordinated with your tax advisor? If not, definitely get in touch again.
A few years ago, there were court rulings regarding mixed-use properties where the tax office insisted on payment via separate accounts. Secondly, the practice part can also automatically become business assets.

OT: Is this coordinated with your tax advisor?

Yes, it works the same way in the current house since we don’t have any meters. I will ask him again to avoid any nasty surprises, thanks for the hint

Secondly, the practice part can also automatically become business assets.

It is, regardless of how the operating costs, water, electricity are charged. Unfortunately, because if we want to sell one day, there could be a considerable tax payment (on the difference between purchase/sale price), since real estate and land prices are constantly increasing!