AST or Contractual Licence?

Tenancy Types and Management

I live in a house with a granny annex. I want to rent out the annex. It has its own front door, but it also has an internal connecting door to my house (which I will lock if there is a tenant in). Please can you advise me whether I should use an AST or some other form of agreement, because although I will be a resident landlord, it is not like having a lodger in a bedroom who shares the kitchen etc. The granny annex has its own bathroom and kitchen and is completely self-contained. Thank you