Since no one else appears to have answered this........if you have title (titulo) to the land the land belongs to you. Building requires permits (up to 5 separate ones in certain areas). The normal proceedure is for your architects plans for building to be stamped with these permits. It is a VERY good idea to get all necessary permits BEFORE you start building, otherwise you lay yourself open to having your building work stopped, having to take it down or handing over hefty slush funds.
If you have a property in certain town centres, this may be leasehold rather than freehold, so technically you are renting the land from the ayuntamiento yet you own the house on this land.
This is a complicated field........that is why it is a good idea to live here first & get to know all this stuff. Difficult to get your head inside the subtleties from a distance.
Clifton