I doubt you would need planning permission for letting an existing granny flat which is part of your own house @Rachel1621 , based on Section 2:5 of https://www.gov.uk/government/publications/letting-rooms-in-your-home-a-guide-for-resident-landlords...
My nasty neighbours complained to the Council about me running an illegal B&B business.... The man from the council ambled round & said "well, it's only Airbnb, that's just like having lodgers, really...?"
So it begs the question, Will you be operating an Airbnb, or a 'proper' B&B? Maybe that depends on whether you're serving food? - I don't know.