What does the development plan say, do you have to build immediately? I would only buy the plot of land. In the time the plot is reserved, in my opinion you cannot make a good plan for a house. There are so many details to work out, you will only get annoyed afterwards. An empty plot of land can also be resold without existential threatening losses (unlike most newly built houses).
This gives you the time to approach the house project with the calm it needs.