He should simply double-panel the partition, in my opinion that's sufficient for everything. Not that there are still misunderstandings with all the measurements. Oh, I have a tip for the drain. It should not be too high. I don't know if that is also specified in the aforementioned DIN, but with our drain with siphon I can't get any slope anymore with push to open valves because the push to open valves come down too far. With the ones supplied by Ikea it works...