As the building is part of the house (even if it isn't attached to the house, outbuildings are classed as part of the home) you would have to insure both properties under your mothers name (as you don't own either property). A basic home office is usually easy enough to cover (A lot of policies have cover as standard). but if you have people visiting the office and are running a business from there, that is a different story and you might need a specialist policy to cover both. Take some advice on this before proceeding as if your mothers insurance company think a business is being run from the premises and they haven't agreed to it, it would invalidate the existing policy.