Furthermore, if you already have a plot of land, you should take a close look at the development plan of the associated area, which contains the regulations such as:
Floor area ratio / plot ratio - How large the floor area or the total floor space may be in relation to the plot. ... 400m² plot floor area ratio 0.2 = 80 m² floor area of the house, meaning exterior dimensions of the house = 8 x 10 m...
Number of full floors... what constitutes a full floor depends on the federal state in which you want to build... can be found in the state building code, usually 0.75 of the ground floor area, or 2/3 (always exterior dimensions or in the attic cut edge at the 2.30 m height line with exterior edge of the roof covering... see the section for this...
as well as roof shape, colors, design regulations and so on. Everything that is not anchored in the development plan in writing or drawing is regulated by the land use ordinance... available from dtv Verlag.
In the case of a building with a [Einliegerwohnung], you should also obtain the parking space regulations of the respective city... they regulate how many parking spaces you should provide on your property... otherwise there can be an unpleasant surprise.
Regarding wall thicknesses... assume 11.5 cm wall thickness on the upper floor, usually also on the ground floor, except for the partition wall from the living room to the stairs, which should be 17.5 cm... this could also affect the wall to the guest WC and utility room... on the [Einliegerwohnung] side it is also the living room partition wall, which runs obliquely around the corner... also 17.5 cm... because they actually carry the ceilings... here, however, I would significantly simplify, leave out the slanted stuff, let the wall between bathroom and bedroom run straight up and thus give the bedroom more space. The rest would otherwise be wasted space.
Oh, and the utility room ([HAR]/[Hauswirtschaftsraum]) should have a floor area of about 8 m², preferably more, because of the heating and house connections... and it should not be crooked either...