Floor plan for building a semi-detached house - ideas

and to be individual, [...] if you hardly know yourself at all. Is it actually sensible to have a construction plan drawn up by the draftsman (without an architect,

Or neither: a draftsman is a kind of architect’s assistant, if you hardly know yourself at all, hardly anything (useful) more individual will come out than a catalog plan could offer you. Apart from the dimension- and house-shape-related handicap, the selection of proposals isn’t narrow either, semi-detached houses are currently very trendy, so every provider has something to offer. I would probably go to a joint solid construction contractor with the semi-detached partner in such a building area. One who is also a developer should already have suitable initial draft plans in the program.

Am I allowed to build a maximum of 2 full floors each with 90m² floor area?

That is correct. GF = floor area, here as an absolute value and not related to the plot area by a floor area ratio = floor area ratio, unusual but possible.

The building envelope for this plot is apparently absolutely limited in width (7.50 m) and only indirectly in depth via the floor plan (absolutely 90 sqm).

The large excerpt reveals what was barely recognizable in the "small snippet": The building envelope is 7.5m * 12m (more precisely 11.99m) in size, which corresponds to the maximum floor plan. With full utilization, it will be very tight with garage and driveway (max. 45m²).

since a roof terrace is also included.

Why? If no further "small snippets" are missing, the floor area is only determined for all full floors. §20 Land Use Ordinance (3) The floor area shall be determined according to the external dimensions of the buildings on all full floors... (4)... ancillary facilities within the meaning of § 14, balconies, loggias, terraces ... shall be disregarded. It will rather become tight in terms of height. At 8.25 m, only 2.75 m remain per floor. You have to be economical with the ceiling construction.

Approximately we are at 20 degrees at 12 meters house / 6 meters middle at 120 cm height.

Tangent Alpha = opposite side/adjacent side = h/s h = s * tan a = 6m * tan 20° = 2.18m In combination with a knee wall, which the other floors leave from 6.20m wall height, that results in a nice attic studio (keeping in mind the full floor boundaries).

Just a stupid question: For a rather narrow building of 7.50m, I still consider a room height of 2.60m to be appropriate. Could I then calculate it like this: Basement ceiling at 0 Room height 2.60m Ceiling approx. 0.30m Upper floor 2.60m Ceiling 0.30m = 5.80m 8.25 ridge height - 5.80m = 2.45 room height in the attic. (Minus 15cm roof insulation + 15cm allowed tolerance = +/- 0) Would an attic with 2.45 room height at the highest point be possible. 40cm knee wall possible since 6.20m (-5.80m) wall height is allowed. What does the tangent and the expert say about this? (I don’t have a calculator here)

Since a third "floor" is desired, namely a parent area, according to @Escroda's statement a staggered floor with bedroom, dressing room, and small shower bath can be accommodated. To comply with the height, one compromises a bit... I don't see a problem with that now. However, I believe that children don't need a playroom, not even in a house. What are the children's rooms for themselves? That a toddler only sleeps there? Well then... I just see this as an expansion of a house, moreover there is still a basement, which will be equipped with an office but also offers space otherwise. I would rather plan a utility room on the children's floor and possibly move a hobby room for everyone to the basement. Anyway: now two parties have to agree on the house shape first. I can bet that after the house is built you will have a few friends less ;)

My thought experiment for a staggered floor assumes exploiting the eaves height (which should provide less than half a meter of knee wall) and installing a shed roof.

Height tolerances here - as far as I have read correctly - refer to the position of the reference point "top edge of finished ground floor" relative to an absolute height meter value for each property. But, for example, not for the ridge height above ground floor level.