What does the property look like (location relative to the street, development)? Would it possibly be (in case of financial need) still divisible and maybe another small house could be built? My current project is the renovation of a semi-detached house on a 550m2 plot for rental purposes. The architect also had a concept to build a "small" single-family house (6m wide, approx. 100m2 living space) on the remaining land. Unfortunately, it is a listed building area and therefore not possible for the time being.
Unfortunately, the development plan (and thus indirectly the city, which wants to avoid high building density in this settlement) does not allow that, otherwise it would be a good idea!
Wishing you much success with your renovation project!