Quirky flat layout with tiny bedsit independent of flat- legal issues re. renting larger flat out and keeping bedsit?

[PotentialLandord]

Hi- I'm looking at potentially buying a flat to let out, with a view to possibly selling up and retiring to the area when older. I have various connections in this area to do with a hobby. I've viewed one recently that has an odd layout that could work brilliantly now and in the future- but don't know if it's possible to rent it out legally? It's a 3 bed flat in a large old building, where it has a separate room that has shower/ toilet across the hall- totally separate from the main flat. It would work now to rent out the flat and then use the room with shower/ toilet as a tiny bedsit when I am in the area with my hobby- currently 1-4 nights a month, a bit more when I have leave. Then when I retire I could live in the flat and potentially rent the room as a tiny AirB&B- or have hobby friends stay etc...

How would I do it though? They are all very clearly part of the same flat on deeds etc.. would it be possible to split bills this way and if so how? Currently the room is being used basically for storage by the current owners- but would be perfectly feasibly as a teeny tiny bedsit. So I could have the rent from the 3 bed flat, and then also save on B&Bs when I'm there for my hobby. I wondered about doing it as a lodger, but I don't think it works as the flat has to be my main home which it wouldn't be.

Any ideas about this and the legalities of renting the flat- when I'd be technically witholding 1 room for my own personal use to stay in a few nights a month? Not possible to put water and utilities on separate bills, and also issue with council tax. But if it is technically my storage room- that I just happened to sleep in now and then- how would that work?

I have a very similar set up as in I have a totally separate annexe to my flat. I am looking into a very similar arrangement in future by retaining my annexe for my use. The only way to legally do this that I can see is to rent the flat as rooms in your house rather than as an entire separate dwelling. So it could be rented to a family but the adults would have a tenancy agreement for exclusive use of the rooms but not the entire property. Or it could be an HMO if sharers and they would have tenancy agreements for exclusive use of one bedroom per tenant. The bills would likely need to be included in the rent and I keep them in my name and pay them. It's not ideal if I'm honest!

You would need to check if the lease for the flat permits you to separate it into two separate self contained flats, likely you would need landlord consent for which they will charge. You would also need planning permission. This would result in two separate properties for council tax. That would then allow you to rent it out, but how much do you really want to be a landlord with all that entails?

Alternatively you would need to do it as though you had a lodger sharing your space. Again you would need to check the lease for the flat. This is the easier route.

Personally I would either buy a smaller property and with the excess, invest it more generally rather than in property or go the lodger route.