Just spoke with the heating engineer. He said that the installation is specified by the heating manufacturer. However, he will check it again and get back to me.
Parkett is being installed as the flooring here. The other gap is the transition between the living room and the hallway. Possibly a door will be added here (I am still undecided).
There are no cats here.
I had imagined the loophole to be a bit bigger, but it is what it is and it doesn't really bother me. That is between the countertop and the wall cabinets of the kitchen.