Don't let yourself be unsettled. Build your house exactly as you envision it. If at some later point the other semi-detached half is built, then they just have to adjust to you. Ideally, you build a basement. Then later no underpinning from the neighbor will be necessary – if you build with a slab and the later neighbor builds with a basement, it will simply be significantly more expensive for them.
And on the subject of the basement price: it will surely cost you at least 60k – depending on how much material you need to dispose of or not. In our building area (nördl. Ba-Wü) basement builders pay between 40 and 70 € per ton for disposal.