Planning permission and "finca rustica"

Hi there, and thank you for reading my first post on this site. I hope that you can help or at least point me in the right direction, please ?! My parents and I bought a finca / olive farm in Valencia five years ago, and have been in discussion with the local government over extending the property. They have agreed to a max of 25% of the current square footage as the property borders a national park. Is this "normal" please ? Also - the architect has mentioned something about there being no restriction on the erection of "non-permanent" buildings (ie wooden cabins) - does anyone know whether this is truly the case as if it is permitted then we may do this instead of extending the property. Many thanks in advance for your help. Julia.