I don't know exactly how a semi-detached house is defined, but I think it isn't specifically built as one (it can also look completely different and doesn't have to be wall-to-wall). I assume your price quotes include the basement plus ground works, right? And what does the ground work with a slab cost? The difference would then be the cost for the additional basement. To me, it sounds like the space in the basement per m² is significantly cheaper than comparable space in the house. Then wouldn't a technical room, small office, and small workshop be more cheaply accommodated in the basement, right?