Is your land classified as rustic (rural) or urban?
If rustic “usually” you are allowed to expand to up to 2% occupancy of the plot size ie the footprint of the building to be no more than 2% of the plot size (with a possible total construction of 4% ie to include additional storeys.)
However there are categories of rustic land for which different rules apply and so you need to clarify what your is. Check at your town hall if you aren’t sure.
It also depends on the number of sq metres of your plot. Depending on local rules and regs, the minimum size usually required to build anything new (even an extension to an existing building) would be 10000m2. In some regions it’s even more.
As Terry says, for something as important as this, you really should be talking to your architect, if not the town hall directly, and not rely on forum advice, otherwise you could get into a lot of hot water!!