A static caravan is subject to the same rules as putting up a shed or other outbuilding. Providing the van would fall within the permitted development limits set for structures in your garden then planning permission would not be required. The size and position rules are too many to list here (your planning department will advise) but the most important thing is that it must be used in connection with the main house and not, under any circumstances, be used as an independant unit of accommodation.
Not sure if this is applicable to Scotland so please contact your local planning dept.