The huge shared living space, the lavender walls and the college-dorm chic; most of us can picture Monica and Chandler’s apartment in Friends, the 1990s TV series no doubt being re-run on some channel right now. I preferred This Life, the flatmates drama which defined the 1990s in a rather more British way. But you couldn’t make either of them today. Or, for that matter, The Liver Birds, Men Behaving Badly or even Not Going Out.

Why? Lodgers can no longer lounge around together on some dog-eared sofa because living rooms simply no longer exist in many rented properties. When a landlord sees a living room or a dining room, what they see is enhanced rental yield. Why have shared spaces and shared experiences, when the room can be turned into another bedroom? The tenants can pop a chicken korma ready-meal into a microwave and eat it in their bedrooms. It doesn’t make for good TV drama – but it’s certainly very profitable.

Take, for example, a two-bed flat I know in south London. It was rented until recently to a relative of mine, in her 20s. The lounge space is now a bedroom, rented at £600 a month. The three flatmates (total rental yield on the flat: £1,800 pcm) can’t all be in the kitchen at the same time, there’s so little space. There are two stools in the corner of the kitchen. It may be a flatshare but there’s no sharing. Although she has found somewhere better, her flat hunting was a depressing round of apartments carved up for the maximum rental yield.

Technology is sadly pushing this along. In my late 1980s flatshare, we had one TV in the living room. One phone, in the hallway. And not even a microwave. Today it’s so much easier to peel off into your bedroom with your ready-meal in front of your iPad while watching Net-flix. Social interaction, such a part of the shared living experience, is fast disappearing, replaced by the atomised world of social media.

But some landlords might be in for a shock. The Mortgage Works (TMW), part of Nationwide building society, has told valuers that when a landlord is remortgaging, it’s not acceptable that there is no shared social space in a rented property with four or more tenants. The criteria has been around for some time, but valuers tell me it is now being more rigorously enforced, particularly on three-bed flats that have been turned into four bedrooms plus a kitchen. Once that happens they become by default an HMO (house of multiple occupation) and new rules apply.

One valuer said he now routinely rejects lots of TMW remortgages because they are breaking the rules. He pointed to a three-bed flat in London where the landlord seeking a remortgage was renting it at £2,800 a month. He guessed that the level of rent was almost certainly indicative of a conversion into four bedrooms. He visited the flat, and sure enough it had been converted, prompting the valuer to warn TMW it was in breach of its criteria.

TMW does not insist upon a living room in properties with four or more bedrooms – but it does insist that there is enough social space, such as a decent-sized kitchen-diner. A kitchen with a fold-down table shoved in the corner doesn’t pass muster.

The loss of living rooms hasn’t just been socially damaging – it has also played its part in driving up house prices, making them ever more unaffordable to young adults. If a three-bed flat can command a rent of £2,800 a month once the lounge is converted, then it means a prospective landlord can afford to bid up the purchase price of the property, elbowing aside conventional buyers. If they can only obtain a rent of £2,100, then they will only be able to obtain a smaller mortgage, and pay a lower price for the property.

Our rental market is absurdly loosely regulated. The rules shouldn’t be TMW’s – they should be from the government – and apply to all apartments, not just HMOs.

p.collinson@theguardian.com