Build a terraced end house with an additional unit (GÜ) on your own

Put the access point on the ceiling or better_ at 2.20 meters height on the hallway wall. Make sure to lay it in the empty conduit. If one thing is certain, it is that higher data rates will be required in the future rather than constant ones. And with multiple access points, you are on the safe side and can still change the medium at any time.

Is an access point necessary on the ceiling in every hallway or wouldn't a mesh system also suffice?

I'm not a network expert, but: shouldn't one access point be more than enough for about 6*10 m without a basement? My FritzBox is in the basement, and I have network coverage throughout the house without mesh and repeaters. For more WF.

similarly sized RH, with basement, white tank and reinforced concrete ceilings:

FritzBox in the living room: ground floor great, upper floor bad, attic no chance.
-->LAN rulez!

Normally, an access point in the hallway on the ground floor should be sufficient. Still, just run a cable to the hallway upstairs as well, it really doesn't matter anymore.

So the ground floor and the upper floor are clear - then I could save the access point in the attic?

I had overlooked DG. It depends a lot on the ceilings. A wooden ceiling or concrete hollow block will probably only need one access point for the whole house. With concrete containing a lot of steel inside, you will likely need an access point on every floor for decent reception. If you take 3 Ubiquiti AC Lite, you’re looking at ~240€ and have a clean solution, which is probably the best approach.