Types of shared accommodation
Flat / house share
This is a property rented out as whole by a group of sharers under a joint tenancy. Joint tenants are "jointly and severally liable", which means that any one (or all tenants) can be held responsible for the rent payments and other obligations of the contract. Things can get tricky particularly when one flatmate wants to leave before the others, as their notice has the effect of ending the tenancy for everyone and a replacement contract (with a replacement flatmate) is not an automatic right. However, generally landlords allow a replacement flatmate to be found and the contract to be re signed. You may find adverts for rooms on the site that have been placed by the flatmate who is leaving. There is an unwritten code that says a leaving flatmate must do all he or she can to find a replacement so as to cause the other flatmates as little inconvenience and expense as possible. You will also find rooms advertised by the existing flatmates, as after all , it is they who will be living with the new flatmate so will be keen to be part of the process.
Rooms to rent (live out landlord)
Though rooms in flat and house shares will also be described as 'room(s) for rent', there is a distinct situation where a landlord rents their property out by the room. This has advantages and disadvantages over flat and house shares. The main advantage is that as it is let out by the room, each flatmate has their own separate agreement with the landlord, which means leaving will not cause so many potential problems. The main disadvantage being that the landlord is responsible for finding a new tenant. Though flatmates may ask that they find a replacement, some landlords who perhaps want to fill the room as quickly as possible, find a new tenant without involving the other flatmates. This means of course that compatibility is not necessarily going to have been taken into consideration!
Rooms to rent (live in landlord)
This is where a person (or family) owns and lives in a property and rents out one or more rooms. Flat/housemates in this situation are often referred to as lodgers. Again, there are pros and cons to this arrangement. Some don't like 'live in' landlords because they feel they can't fully relax as the owner of the property is there or because everyone isn't on an equal footing as with a flatshare. The plus side is you may find a property is better looked after as a result. Read more about your rights as a lodger.
If 2 people rent a 3 bed property under a standard assured shorthold tenancy then rent out the 3rd room, collecting the rent themselves, this is called subletting. This is not permitted under most standard tenancy agreements. It does however go on, and there will be cases where the landlord has agreed (perhaps off-record) for it to happen. It can also suit some people in certain situations where neither party wants any commitment, as there are no contracts involved. Such a casual arrangement obviously has it's risks: as a flatmate you have no rights if they decide one day to change the locks and not let you back in, and in a similar way, those subletting have no rights should the subtenant decide not to pay their rent.
Disclaimer We cannot guarantee the accuracy of this guide. The law surrounding such matters is complex, so in no way should this guide be taken as any kind of legal advice. If in doubt seek legal advice: the Citizen's Advice Bureau should be able to help you.