Two year tenancy request

Tenancy Types and Management

I am about to let a stable that has been converted into a one bed detached bungalow in my garden, to a lady who has been lodging in my home for a few months. She has requested a two year Tenancy ( I have suggested with a six month break clause)with a rent review option after one year. Firstly :
Is a two year tenancy advisable and legal?
Secondly:
Because the detached building is not registered as an independent dwelling but is ancillary to my home and is intended for letting - my domestic insurers will not insure the building. Do you have any advice please so I may get the building insured?
Thank you.