In terms of the law in England and Wales, a brothel is any property more than one person is offering sexual services, whether at the same time or not, and whether or not any money (ie prostitution) is involved.
Quite a lot of places are brothels
They're illegal to run, control, or allow in any property you are responsible for.
In practice, if you don't annoy the neighbours, no-one will care.
Don't do them from any well-known short-term rental site - they will mind if they know (being a US operation, the company takes a staggeringly dim view of prostitution.)
In terms of hotels, if they'd throw you out for working solo, they'd do it for this!