I also have to check what the surcharge would be for increasing the knee wall from 1.00 m to 1.25 m or 1.4 m. Also, what the slightly enlarged roof overhangs would cost, since they are required in the development plan and I find them quite nice as well. I don't think I would change the ceiling height on the ground floor. With a room size of about 38 sqm for the living-dining room and kitchen, the room isn't yet so huge that it feels too low. I almost dare not say it, but I don't really like the high rooms in old buildings. Maybe that's because size-wise, I'm more of a Hobbit.
Changing the ceiling height is, of course, just a luxury point – although if you place value on the resale option of the property, it could be an argument. Also worth considering – if you stick with the idea of Danwood & the Point – leaving the roof open to the ridge on the upper floor, with visible roof beams. That's also luxury, but I find it a significant improvement of the spatial impression – the net cost difference should be around 3 to 5k €.
Out of all three options, I would first adjust the knee wall (we increased ours to 150 cm) – in terms of price/performance ratio, this offers the most benefit (upstairs). And I'm fairly sure that this also opens up some options in the bathroom for you to accommodate your desired larger shower without having to build over the stairs. For example, if you can shift the bathtub another 20-30 cm towards the outer wall (which should be quite possible since the installation doesn't require the thick casing), you could also shift the bathroom window towards the north and possibly execute it as a dormer or slanted window and use the gained space at the bottom of the plan for the shower. Such a window style may fit very well into your region.
Also consider a double casement window or stairwell window – otherwise, it tends to be rather dark there. Additional costs for a double casement window (triple glazed with electric shutters) are about 2.2k €.
I have my first consultation appointment at Danwood on Thursday. Maybe something nice will be found in the "Family" series. Although the selection is somewhat more limited, it might still suit my budget. I will do the walls and floors myself, possibly also all the tiles. So basically ready for technical installation.
If you want to bring EL yourself, the Family series is of course a very good option – especially since the Family 104 has a very similar floor plan. However, you have probably read the “rules” of the Family program: no changes to statically relevant components (e.g., your idea of building over the stairs generally wouldn't be allowed), window positions cannot be changed (only height and type of opening), and the selection of bathroom fixtures & tiles is strictly limited (you can also remove them entirely, although the credits then do not at all compensate for a sanitary installation by a German company). However, you can mirror the floor plan.
It is obviously much cheaper – even if you calculate the EL out in the Today series, the price difference is still at least 5k € and up to 10k € depending on the final configuration. Some options are significantly cheaper in the Family series (e.g., increasing the knee wall to 1.5 m costs only 1.5k €, but is only possible if the roof pitch is adjusted). Air-to-water heat pump or ground-source heat pump technology, KfW40, etc. are also possible in the Family series.
Depending on the sales department and their relationship with Danwood, you can occasionally have something changed outside the “rules” as a special request – for example, the roof overhang should be possible in your case since it is prescribed. Otherwise, to my knowledge, it would not be allowed in Family.