The entire house would then be approximately 15m wide and about 7.1m deep...
I wonder how that is supposed to work? I assume these are the exterior dimensions. For a semi-detached house, that leaves 7.5m x 7.1m per housing unit. Not included yet: wall thickness. Assuming timber frame construction, the exterior wall is 36cm thick; I am unsure how the fire protection wall between the two housing units is designed, but let's be generous and count 40cm total for the partition wall, so 20cm per unit. That leaves an interior dimension of (7.5m - 36cm - 20cm) = 6.94m and (7.1m - 2 x 36cm) = 6.38m. So 6.94 x 6.38 = 44.28 sqm of habitable usable area. Since it is to be a semi-detached house, you also have to factor in a staircase and interior walls for both units. Open living is nice and all, but at least a WC and bathroom will probably still need a wall, and that has also proven itself for bedrooms. Another approx. 17cm less per wall. Even if you opt for a space-saving half-turn staircase, at least 2.5 sqm must still be accounted for. So I come to just about 40 sqm of habitable floor space. I roughly calculate further: the ground floor is supposed to contain the entrance area, open living room, and WC. I take 5 sqm for WC and entrance (which is not generous; I just took this number and am not even sure if it will work that small), 2.5 sqm for the stairwell, leaving 32.5 sqm. But the whole thing is rather square in the floor plan (interior dimensions 6.94m x 6.38m, and we haven’t even added plaster yet!) and that doesn’t make it any easier. Same story upstairs: just to fit a double bed in the bedroom, you need at least one wall of 4 m. I deduct that from the 6.94 m, leaving 2.94 m (subtracting the necessary interior wall thickness from the 4 m means no 2 m wide double bed, the residents will have to sleep on a maximum of 1.80 m). And so on and so forth. It gets even tighter if it is to be built solidly. I calculate back and forth, but just the thought of it already gives me claustrophobic feelings… We are currently living on 76 sqm spread over two floors (open living area, bathroom, storage room upstairs, bedroom, small basement, third room downstairs). I know what I’m talking about. Lots of stairs are healthy, but I wouldn’t build like that. And our floor plan here is more rectangular with a longer side; that makes the room planning easier. Does it really have to be a semi-detached house? With two housing units stacked on top of each other you would save a stairwell. Given such tight living space, I think that is worth considering. Do the two units have to be equal in status? Otherwise, a somewhat more generous main unit combined with a granny flat (and only one staircase) could also be an idea. I find it difficult to design two such small, almost square housing units so that there is a sensible layout (a sufficiently large closet also belongs in such a bedroom, which also needs more space, etc.). And I think, precisely for that reason, there will be no off-the-shelf house here. I would sit down with a good planner (whether you choose one independently or one from a home builder) and think about how this very tight space can be optimally used according to your requirements. And, as I said, I would reconsider the whole plan to build such a semi-detached house with these specifications once again.