If I were you, I wouldn't build anything at all for now. You are still so young and have only been together for a year. Your boyfriend is still in training, who knows where he will want to work later, maybe he won't like his current company anymore in the future. The plots are very cheap where you are, so you can always build later. Do you really want to stay with your parents, on a farm? It somehow sounds a bit like work to me. Also, you have to consider that you must be able to afford the financing with only one salary if children should come. So why not just wait a few years and see what happens.
Best regards
Sabine