Friends, as the song goes, will be there for you. Flatmates, on the other hand, are there for you whether you like it or not. They are there for you when you get out of bed in the morning. They are there for you when you get home in the evening. (They are even there for you when you call home to check your messages.) In fact, they are your live-in confidante, career adviser, therapist and friend - if you're lucky.

Blame rising property prices or the fact that Irish people are getting married later in life, but more people are sharing with non-family members well into their late 20s and early 30s. Contrary to popular belief, "dorm life" is not the sole preserve, or luxury, of free-wheeling students. After all, flatmates give you all of the emotional support but none of the complications that accompany familial relationships or romantic commitments.

Flatmates, not friends, are the new family. College life used to be one of the only times when friends and, on occasion, nemeses lived under the same roof (or at least in the same apartment block). Not any more. "Post-college neo-dorm" life is also a time of midnight chats and impromptu parties, but with lots of spare cash. It's a chance for one last carefree hurrah for before settling down with a mortgage, partner - or both.

In short, it's a 24/7 slumber party with wine, beer, sparkling water, oodles of snacks and a fridge full of your favourite food.

"Non-family households with unrelated persons", as the Central Statistics Office puts it, had risen by almost 30 per cent by the last census in 1996. And although many of those included here are couples, the number of flatmates has also undoubtedly soared over the five years since that census. And, for some people, this new domestic arrangement might actually be the first opportunity to experience the dynamic of a non-dysfunctional family unit.

Trevor White, publisher of the Dubliner magazine, spent the last two years living in New York with two gay male flatmates - an Irish astrophysicist and an American film producer - before moving home. At first, he was apprehensive about the logistics of sharing with two gay men, but their different lifestyles made the experience all the more colourful. (Plus, he could hardly go wrong with a loft apartment on Fifth Avenue.) But, crucially, his flatmates were hugely supportive of his professional aspirations.

"I was feverishly planning a magazine, writing a screenplay and working for a publisher and, as they were slightly older, they were a huge influence on me," White says. "I now live alone in Dublin. It would be very difficult to find a replacement for them! They introduced me to a side of New York that I had never expected. They were also great arbiters of style, coming out with lines like, 'You cannot go out in that shirt!'"

Not surprisingly, a whole television industry has been built up around this extended adolescence. It started over 10 years ago with MTV's The Real World. Since then, it has run and run, with This Life (angst-ridden London lawyers), Friends (angst-ridden New York friends), Will & Grace (angst-ridden gay man/straight woman), Big Brother (angst-ridden social experiment) and Melrose Place (angst-ridden soap where anything goes).

The last-named did more than all the others to make having flatmates the lifestyle choice. Melrose Place was set in a Spanish-style apartment complex in Los Angeles which was occupied not by grungy students, but by glamorous thirtysomethings with real jobs. They were stuck in the transition between college and adulthood for seven long years. But, boy, did they enjoy it.

While the apartment complex in Melrose Place provided the ideal space for close physical (and sexual) contact, Will & Grace depicts the squeaky clean, relatively uncomplicated flatmate experience where a gay man and straight woman share without sexual tension or the hormonal competitiveness of same-sex flatmates.

The gay man/straight woman dynamic can also make it easier if one flatmate introduces a partner. Psychologist and psychoanalyst Domhnall Casey says this is tantamount to "putting the cat amongst the pigeons, which is why the gay man/straight woman dynamic can work so well. There's less likely to be jealousy of the other's partner, so they're free to advise each other on romantic problems."

While flatmates are more likely to confide their darkest secrets to each other rather than risk the ire or judgment calls of parents, partners or siblings, they do need to be careful not to take any previous baggage with them into their new home, Casey adds. "Flatmates need to be careful not to project the insecurities of their previous family situation on to their new 'family' unit."

For example, some people take on the role of mother, father or, worse, child. "If flatmates don't remain aware of the pitfalls, they can easily fall into the habits of sibling rivalries they have already experienced," Casey says.

"If there's more than two flatmates, people could fall in and out of favour. But it's a great learning experience." Like the characters in Friends, a close bond can develop between flatmates in a relatively short time.

Trevor White agrees: "Any flatmate invariably finds out more about you than you would hope to reveal. It's the Big Brother syndrome. You spend so much time with someone who discovers the most obscure and exotic aspects of your personality. I was lucky. My flatmates were very worldly people, with a great generosity of spirit."

Of course, it can also all go horribly wrong. Personality clashes, lifestyle differences and, ultimately, the desire for personal space can turn a formerly happy flatemateship into an acrimonious "divorce".

That includes the dreaded task of splitting up the CD collection. But Casey says a successful flatemateship has its personal and emotional rewards. So the next time you find yourself lying in a bubble bath while your flatmate stands next to you shaving their face in the mirror or putting on make-up (or both), remember that modern, platonic domestic arrangements don't get more blissful or hassle-free than this. Forget pre-marriage courses. They're grossly out of date. Get yourself a flatmate.