Forty years ago my parents moved into a smart new-build estate of four-bed family homes. With a "car port" and a fitted Hygena QA kitchen, it felt like Beverley Hills after the east London terrace we'd left behind, although really it was just a cul-de-sac in Hastings. One family even had a swimming pool, but we didn't stop to ask how they felt about the seven Collinson kids, and the numerous other children on the estate, diving in.

But now the splashing has died away. The pool is covered over and the sound of children has gone. The houses are bigger – extensions abound (some slightly less hideous than others) – but where once 20 or even 30 children roamed, now there's just one, sometimes two. Virtually every home is now occupied by retired couples in their 70s and 80s.

One of the unspoken truths of Britain's housing market is that today's families have been locked out of the family home market by their own mothers and fathers. A report from recently launched campaign group the Intergenerational Foundation exposes the stark reality of Britain's housing crisis. There are now 25m unoccupied bedrooms in British homes, and this number is rising at an alarming rate.

"Older people are living longer and staying in the family home rather than downsizing to more appropriate accommodation," says the report's co-author Matthew Griffiths.

The rise in longevity, which has been particularly impressive among men born in the 1920-1940 period, is something we should celebrate. It's not just that people are living longer, they are living healthier for longer, too. In parts of the south-east life expectancy has risen to 83.6 years for males.

But the unintended consequence of longer lives is that older people now have a stranglehold on the family home market. These houses used to be released back on to the market when their occupants passed away in their 70s, and sold to families in their 20s and early 30s. Now, thankfully, many of our parents are living into their 90s, and we do as much as possible to keep people in their own homes rather than putting them into care. But it is having a devastating impact on the release of family homes into the hands of people who really need them.

A family in their early 30s on a standard middle-income job, with one partner not earning but looking after the children, no longer stands a hope in hell of affording a four-bed new-build home in the south-east. It is no surprise that homeownership among the under-35s is falling fast, yet it is still rising for the over-65s.

So do we kick old people out of their homes? Should I be turfing my mother and father out of the house they have so lovingly cared for over all those years?

Of course not. But this is an area where a bit of "nudge economics" could help. According to Intergeneration Foundation, Americans are twice as likely to downsize when their children leave home than British people. It blames the UK's tax system for encouraging older people to stay put, and suggests reform such as an exemption from stamp duty for the over-60s when they move to a smaller property.

It is one of those simple reforms that wouldn't cost thechancellor much, but could have a greater impact than the money wasted, for example, on the affordable homes and shared-ownership initiatives which simply add fuel to the property price bubble.

Other than that, let's pave Britain over. More and more estates like the ones my family moved into on the perimeters of every town in the south-east. Or a council housebuilding programme to rival that of the 1950s and 1960s. Neither sound madly attractive, and would probably be most vocally opposed by the older generation that has done so nicely, thank you very much, from the property boom.