Rosemary Bennett

First it was grown-up children moving back in to the family home. Now there is a new threat to the peace of the empty nest.

The fastest-growing household is the “multi-family”, in which offspring come home and bring their partner.

There are now 286,000 multi-family households in the UK, up 39 per cent on a decade ago. Student debts, soaring property prices and the difficulty in securing a mortgage all contribute to the new “boomerang-plus-one” trend.

The annual Families and Households survey from the Office for National Statistics (ONS) also found that there had been a sharp decline in the traditional nuclear family of married parents with children.

At a total of 1.9 million, there are half a million more children living with parents who are not married than there were 10 years ago. However, as a proportion of all families with children, they are in a minority, making up only one in seven.

Two thirds of children, 8.4 million, live with married parents, although this is down by 400,000 from a decade ago.

The figures suggest cohabiting is no longer a trial run for marriage, but a permanent state for many couples. Cohabiting parents are more likely to break up than married parents, with 33 per cent splitting before their child’s fifth birthday, four times the rate of married parents.

Lawyers say the legal system has not kept up with the trend, leaving many parents high and dry when they split. “People are happy to have children and not be married in far greater numbers than ever before. However, the law has simply not kept up with what happens if these relationships end,” said Fiona Wood, a partner at Pannone Solicitors.

“A cohabiting mother in what might be thought of as a traditional household, where she either doesn’t work or earns a low income, currently isn’t protected at all. They can claim child maintenance from their former cohabitee until the children grow up, but they cannot make financial claims for themselves.”

Proposals to change the law to give long-term cohabitees similar rights to married couples were rejected by the previous British Government.

There are three million children living in lone-parent families, which is a fall of 36,000 since 2003.

Harry Benson, a director of the Marriage Foundation, said this was a surprise given that the pace of relationship break-up is still high. “I think the high cost of living could be driving lone parents to repartner or remarry to try and shoulder the burden,” he said.

The number of people living alone has also risen sharply to 7.7 million, with widows aged over 65 accounting for 20 per cent of that number.

However, the biggest increase in people living alone has taken place among those aged between 45 and 64. There are 2.4 million in that age group on their own, which is 28 per cent higher than a decade ago.

Statisticians said that the surge is a result of the baby boomers, who are less likely to be married in the first place or more likely to be divorced, reaching this age bracket.

The number of younger people living on their own has fallen by 19 per cent to 1.7 million. The ONS said the proportion of under-35s living with their parents was a major factor.