The courtyard area is best defined by IPC:
Courtyard – The smallest rectangular area that provides a
minimum electrical and mechanical clearance (courtyard
excess) around the combined component body and land
pattern boundaries.
So exactly what @TViT needs. But as this standard defines a fixed courtyard for a given density level there simply is no need to have this configurable dynamically at design time. (Having a well made footprint generator that has a setting for this is sufficient. Which is why i pointed to our generator scripts.)
Of course a more dynamic way of defining this might allow for even more usecases to be captured but i would assume these to be of low priority. (I suggest to make a feature request on launchpad for this. But make a well reasoned usecase description that shows where the standard way of using a static footprint defined courtyard does not suffice.)
For now the best advice is to modify the courtyard of your personal footprints such that they define the courtyard area with the clearances you desire. (But remember if you define the clearance the same for all parts then you only need to have half the clearance designed in every component as courtyard is checked against courtyard of nearby footprints.)
And kicad specifics can be found in both the KLC and the forum FAQ: