Dear Virginia, When I got a flat, one of my friends moved in as a lodger. Everything was fine for a few months – until she got a permanent boyfriend. He's now staying nearly all the time and it's driving me mad. I've tried discussing it with her, but she just says I'm jealous and he's not costing any more money, so what am I complaining about? I know I'm being irrational, and I don't want to cause ill-feeling, but what can I do? Yours sincerely, Edith

Life isn't all about money, for heaven's sake. And you're not being irrational. Your flatmate sounds totally insensitive. Living with another single person is one thing; you are both equal, you both have half of the fridge, roughly the same number of baths or showers, you make the same amount of mess and, on the whole, you halve the chores. You also have each other to have a gossip with at the end of the day and, as single women, you share many of the same problems – you understand if the other is having painful periods, or suddenly gets emotional; you gape at what each other has bought out shopping; and, if you like, you can loll about with no clothes on with no embarrassment.

Enter a bloke and the chemistry changes completely. Suddenly you, Edith, are sitting about like some kind of gooseberry. You want to share the sofa with your friend to watch telly, and lo and behold there's a man sitting in your place. The balance is all wrong. It may be your flat, and you may feel you have, ultimately, more rights than your girlfriend, but when it's two against one, it's very easy to feel dreadfully lonely and left out.

It's not just the irritation of shaving hair in the bathroom basin and the stuffing of your fingers in your ears when you hear them next door, bonking all afternoon. You'd be angry if your flatmate were to move a platonic girlfriend into the flat. You let the room to a single person, and now you've found you've got a couple.

I'd tell this girl that either this man goes or she goes. And if she decides she's going to spend most of her time with her boyfriend, but use your flat as a base, I still think it's an unacceptable arrangement. You don't want to live alone. You don't want to worry at night about burglars breaking in and there being no one to call on for help. You didn't let the room to an empty space. And you didn't let it for the money alone. You let it for the security and the companionship as well.

Presumably this bloke has his own place. Why doesn't your friend move in with him and leave you to find another single girl to share with? And this time, before the next one moves in, explain that although you might not mind men staying over occasionally – if you're given warning – the moment she becomes a couple she's got to go.

It's all too easy, when you own a flat, to forget your rights as you cower under the psychological pressure of living with a couple. I've been emotionally terrorised by lodgers, until friends have dinned in the message: "It's your house!" and the penny has finally dropped.

My technique for lodgers now is to insist that they agree to a list of stringent rules before they move in. No dogs. No use of the garden. Everything to be washed, dried up and put away before they leave the kitchen. No noise after midnight. I'm a right old tyrant. But once you have rules, you can relax them. Exercise a liberal, anything-goes policy to start with, and you can never go back to staking out the boundaries again.

Readers say:

Lay down the law

Edith, you are not being irrational, your friend is taking advantage of you. Say something to her. Keep it neutral, so that she doesn't think you're having a go at her boyfriend. Say you're not happy sharing your living space with an extra person and therefore you'd like it to stop. Introduce new rules: ie, he's allowed to stay two weekends a month. If she doesn't like it, then give her a month's notice. Keeping it neutral gives you more authority – but don't be surprised if she reacts melodramatically.

Having experienced difficult situations with flatmates, the best advice is to confront the problem, however nervous you are. If your friend refuses to acquiesce, you will have to accept that sadly, she is not a true friend.

Lucy Collins, Manchester

Ask her to leave

Sharing with a couple is quite unlike sharing with one other person. You end up feeling like a gooseberry in your own home. It's nothing to do with jealousy – it's about not feeling comfortable.

You are within your rights to ask her to limit the time her partner spends in the flat – to, say, one night a week. However, this will probably cause resentment, so it is probably better to ask her to leave. You have been good flatmates; you are still friends, but you never wanted to share your flat with a couple.

It sounds like your time as flatmates has reached a natural end and you will both feel better for making a change.

Martin Chadderton, London EC1

Make him pay his way

If she is your lodger and paying you rent then she is allowed to have a boyfriend stay over. Then again, I think it sounds more like he's living there. You should ask that maybe if he stays that much he should contribute to bills. She may buy his food, but I bet he watches the TV, uses your shower, etc. Just set down the rules, if she doesn't like it she can always find somewhere else to live.

Sharon Gregory, Blackpool

Start again

This has little to do with jealousy. Your so-called friend is bullying you with this accusation. When you decided to rent a room out, would you have rented it out to twins or friends who wanted to share? Would you have chosen a male lodger? Occupying space in a house and using the facilities is worth money, whether you are increasing the utility bills or not (and does he never boil a kettle?). You're getting 100 per cent more than you signed up to, not to mention the increased noise of two people talking.

You're in charge, and if you're not enjoying things at home, you can change things. I would serve her notice, but do it as amicably as possible. Understand that this was not driven by jealousy. Explain it that is not working out for you, and look for a replacement.

Susie, Oxford

Stand firm

Three phrases spring to mind. "Attack is the best form of defence"; "An Englishman's home is his castle" and "two's company, three's a crowd". I suspect your friend can see your point of view, but is happy for things to carry on. If she admits there is a problem, the status quo would have to change, so rather than concede that you have a point, she attacks, accusing you of jealousy.

Do not let her get away with this. You are not being unreasonable. The friendship is bound to be put to the test whatever you do.

Linda Acaster, Leicester