Masonry and roof construction - Which stone/roof beam?

hello I have a concern. I am in the process of building an extension or an anteroom. Basically like a hallway where shoes and jackets are put away. Width about 4.50m and depth about 2.50m The floor slab is already in place. Now the question is, what should I use to build this anteroom? Ytong, hollow block, ...? What size should the stones be? 17.5 or 24cm? Possibly later a shower should be built on top, so the bathroom expanded, but only about 2m x 2.5m... As I said, the shower might be placed on top. My question is which stone and which roof beams? I would prefer to have a wooden beam ceiling. Thanks in advance.

The foundation slab is already in place. Now the question is, what is the best material to use for building this anteroom? Ytong, hollow block, ...?

Well, if the foundation slab is already in place: exactly as drawn in the building application.

Planned was ytong 24er. Because of the insulation value. The consideration is hollow block stone?

I wanted to say, the question sounds overall very naive and like an illegal construction.

True. For me, the question is whether ytong or hollow block stone is better... Or another alternative? The size is specified as 17.5 cm... A friend said... Better to do 24 cm because of insulation. But it’s only an anteroom, so I don’t know what to choose now?