250 sqm is probably the maximum for a semi-detached house, 150 sqm for an end-terrace house. For under 400 sqm, in my opinion, there is no space left for a detached single-family house.
Our plot was actually not in high demand because it was pretty overgrown. But my husband has now spent quite a few hours every evening after work cleaning it completely and now it looks really great. I think it shouldn't be a problem to sell it well now.
Maybe you will find a buyer who takes over the old plot with your house project. If it is a two-generation house, it might also be interesting for someone who wants to live in one apartment themselves and rent out the other for financing. Sounds not entirely uninteresting. Is quiet commercial activity also possible?