Second villa planning permission

The first thing to do is to check with the planning dept of the town hall to establish if the land is zoned for building and if it is large enough to obtain permission to build.   

The laws change,  so what say was big enough previously might not be now and vice versus

We have (this month) received our "Licencia de primera ocupación" for our new house in Granada city, which we finished building almost exactly two years ago - be advised, it´s not very easy  !
Most important requirement is fluency in Castellano (and/or Catalan where you live?).
We bought the land in Dec 2014 (it already had a ruin on it), of course we had architects, but also vitally important is a lawyer, since one of our next door neighbours didn´t like us from the start, thanks to them we were in Court two days ago with a demand for "conciliación" - this story is not over.
Each locality has it´s own building regulations, our architects did not know several obligatory requirements for new builds e.g. obligatory solar water system and obligatory acoustic tests.
Due to things like this, we have gone over budget by maybe 15%, and we still have an unresolved court action from the neighbours.
I wish you luck !

Oh ! I forgot the most important thing (in my case, at least) -
PHOTOGRAPH EVERYTHING NOW (and continuing).
This has saved my ass, because I have photos taken before we bought the plot, showing the neighbour´s house.
But also, according to my lawyer, showing an old donkey shed which we can possibly develop in the future, because it was pre-existing.

... of course, you did ask about planning  permission !
This is ultimately the architect´s responsibility, they won´t get their project built if they don´t obtain the "Licencia de obras", and likely they won´t get paid either.
Also they will be legally responsible for the site until you get the "Licencia de primera ocupación". Many trips to the local "Urbanismo" (planning dep´t)  will be required to iron-out the many issues that will arise and you will be appointed a named employee of Urbanismo to oversee your project (other officers of the Concil will get involved according to their speciality and your particular issues).
I think you can read from my other answers, the importance of local architects, as they should know more about the local regulations.
Also you have to appoint a technical architect ("aparejador"), who will be the chief of works and liaise with the builders, ensuring the builder complies with the many regulations. The aparejador will probably be on site every day and, likely, visit Urbanismo several times.