Convert house to two apts

Rent a room. It's tax free and legal

you don’t have to.

You would absolutely need planning permission

Ive seen it done. Knock a door in the side wall for the "apartment". Partition the house so it has enough room on one side for a bathroom, bedroom and cooking area. Make sure there is a door between your side and their side. If its rent a room tell them they are free to use all of the house but sure they wont need to. Thats enough to tell them without telling them to keep to their side. If they dont get the message say goodbye and get someone else in.

Airbnb it or rent a room. No planning permission needed at all.

Anything would work, as long as someone could walk through it if they wanted. If you were ever in an old georgian building converted into flats you would see these partitions all the time. Something along those lines but obviously you need to think about it with sound in mind.