What are the rules for putting a Static Caravan in your Garden?

Are you doing a favour for Ian?

who is Ian

you will have 2 homes and pay big tax on the main home.

front or back ?.... Neighbours be unimpressed for sure

If you google "static caravan in garden" there are many results that I can't be bothered to read, but the gist is that it depends what you do with it.

Simply ask your local Council Planning Department or check their planning policies on line.  The rules often differ from council to council depending on whether they have specific relevant policies or by-laws or not. Typically a STATIC caravan will be treated in planning terms as an outbuilding, so you will need consent if it is, for example, in front of the building line, too high, too big, too close to a boundary, too close to a listed building, in a conservation area etc.

You need a good concrete base, plus some method of getting soil/waste water away.

Water and electric/gas in should be easy.

why not choose a de luxe summer house ?  -it will look a lot nicer and won't cause friction with the neighbors

You'll need a hoss, a potatoe patch and an accent for the full effect !