Houseguest, Lodger... or Inconvenience?
In the landlord's mind, the "lodger" is often considered to be little more than sofa surfing, even when paying a considerable amount of rent, while the "landlord" is often failing to fulfill his or her legal responsibilities to the lodger and other parties. The "lodger" is often caught in a no man's land (see "roommate or guest?") between being a lodger (who "officially" pays a rent) and a friend who has outstayed their welcome as a guest, and normally isn't treated like a guest (as I've touched on above, the nature of the friendship can change) i.e. usually isn't included when other friends and family visit, doesn't normally eat with the family or take part in other activities, and is often subject to the goalposts being moved on what they thought was the understanding (there would almost never be a written Agreement) much more than an "official" lodger.
If you have a friend living with you, however casually, for most situations, assuming the guest or lodger buys their own groceries, reasonable expenses would be around £100 a month, but £200 a month would be the absolute maximum for things such as extra electricity, extra gas, extra water (but only if home is on a meter), extra phone/broadband (assuming you don't have an unlimited contract already and/or they use your landline for outgoing phone calls) and 25% council tax if you previously got a single person's discount (assuming your home is in a high council tax band). If you're taking more than £100 (for most homes, where guest pays for their own groceries) to £200 a month (for a large, expensive property or having the person there is putting you out a lot - short of harrassment or damage), unless of course you're supporting them on top of that, that person is paying you rent for a home, not simply somewhere to stay. This therefore constitutes a legally binding contract, even if it's only verbal. As a landlord (which this makes you) you have obligations, some of which you are bound by law to fulfil - please see Unofficial Lets.
If you're thinking of helping out a friend by asking them to stay with you for more than just a visit, but you don't want to go down the road of becoming a live in landlord or you want to keep the place as your home only, try to set a time limit for their stay (before they've committed themselves to moving in!), and don't charge them anything near a normal rent - just their share of utility bills, etc, and anything else you provide plus a little extra - that's it!
However, if they have the use of separate bathroom and cooking facilities (e.g. a "granny flat"), you need to be especially careful as any money over £250 a year could legally count as rent, and you will therefore have created a tenancy and will need a court order if your friend refuses to move out!
Explain tactfully that they are a guest, not a lodger, and that while you're happy to help them out short term, you don't want a lodger! However, unless it's just for a short while with an end date, instead of helping your friend, you may well end up in a situation where you both come to resent each other - the host feeling the guest is taking advantage of them, and living off them for cheap, while the guest may resent the lack of freedom in the friend's house and the power they (however unwittingly or unwillingly) have over them. --The take home message here is that unless you're very close to your friend, and your desire to see them safely housed far outweighs any concern of having them in your home indefinitely and being taken advantage of, you should only do this if there's a definite end date!
You can't have your cake and eat it too. Your friend is either staying as a guest, and you're not making money from the situation, but your home remains your home exclusively or you are making some money, but you accept that you are now sharing your home with someone else, on a proper business footing. As such, that person has the right, within reason, to treat the place as their home too - i.e. they are living there, not simply staying there.
Both parties need to remember that they have an agreement - which even if only verbal is a legal contract - informing their living arrangement, which must be above petty jealousies, manipulations and squabbles. If a dispute with your friend/lodger is developing, this needs to be negotiated (unless of course it's a real deal breaker) in a friendly but business like manner - please see Resolving Disputes.
House Guest V Lodger - An Overview of the Usual Legal Position
In most legal systems, lodgers and house guests are both excluded occupiers - that is, they don't normally have the same rights as tenants. However, lodgers are licencees as a minimum (under some legal systems, such as Scottish Law, they are a bit more) but house guests are almost always going to be what is known as a "bare licensee" (although, again, under a jurisdiction such as California even they can claim rights of occupation!). Everyone is a bare licensee at some point, sometimes, several times a day - for example, visiting another person's home (even if they're your tenants) or visiting a shop. Normally, however, unless a bare licensee can demonstrate a significant interest in the property (such as having paid a very large sum - not simply rent - toward purchasing or improving it) or they have children with the legal owner or tenant, they have absolutely no rights to occupy, even temporarily. However, in a situation where there's a contract, as between an owner or tenant and a lodger, which will exist if the householder accepts rent (beyond actual expenses) or services (such as several hours a week cleaning or DIY), the lodger is normally entitled to "reasonable notice" to move out - see Notice Periods.