Spain - planning permission garage to living space?

[MrsUltracrepidarian]

Hi - we have found a lovely house in a great location, but when I asked to see the nota simple, the extension was not described - looks as it has been developed part of the original very large garage space.
I have aske to seen planning permission for it, but am unfamiliar with the Spanish system and don't want to proceed if it has been built without permission.
Does anyone have any experience of getting retrospective planning permission (since I assume it is necessary if converting a garage?) and how long it would take (thinking of asking owner to apply for it and making our offer subject to if he gets it within a reasonable time)