The traditional household of a married couple with children is giving way to cohabiting couples and people living on their own, research reveals.

The overall number of families in the UK rose by 8% between 2008 and 2018, from 17.7 million to 19.1 million, accompanied by a marked change in the ways people are choosing to live.

With Boris Johnson and his partner, Carrie Symonds, living together as the first unmarried couple in No 10 Downing Street, data from the Office for National Statistics (ONS) shows that the number of cohabiting couple families is growing faster than married couple families, up 25.8% over the decade.

“The number of families and households in the UK has continued to rise in line with the growth of the UK population over the past decade. However, the ways that people live have been changing,” said Sophie Sanders, of the population statistics division at the ONS.

“While married couple families remain the most common, cohabiting couples are the fastest-growing family type as people increasingly choose to live together before or without getting married.”

The research also found that the number of same-sex couple families had grown by more than 50% since 2015, and the number of same-sex married couple families had risen more than fourfold.

The number of same-sex married couple families has doubled to 68,000 since 2017, representing 29.4% of all same-sex couple families in 2018, compared with 8.9% in 2015. Same-sex marriage was introduced in March 2014.

“The trends for opposite-sex and same-sex couple families are going in opposite directions,” said Saunders. “The share of opposite-sex married couple families is decreasing, while opposite-sex cohabiting couple families are increasing, although at a much slower rate of change than for same-sex couple families.”

More than two-thirds of families are married with children, but their proportion has declined from 69.1% of all families in 2008 to 67.1% in 2018.

The proportion of cohabiting couple families, the second largest family type, has increased from 15.3% to 17.9%, equating to 3.4 million. The number of lone parent families stands at 2.9 million (15% of the total), making them the third-largest group.

The data also reveals a significant increase in the number of people living alone, exceeding 8 million for the first time. The data suggests this increase has been driven by rises in women aged 45 to 64 and men aged 65 to 74 years living on their own. The number aged 65 and over and living alone increased by 500,000 or 14.8% between 2008 and 2018, to 3.9 million.

“Reasons for increases in these age and sex groups include an increasing population aged 45 to 64 years, rises in the proportions who are divorced or never married and increasing male life expectancy catching up with female life expectancy,” Sanders said.

More men under 65 and more women over 65 live alone because higher proportions of men than women never marry, men tend to marry older than women – and marry women younger than themselves – while partnership dissolution often leads to men living alone without their children. Women’s higher life expectancy is also a factor.

However, lower numbers of young adults are living alone and an increasing number of them live with their parents. In 2018, one in four young adults aged 20 to 34 years (3.4 million) were estimated to live with their parents. That was a 24% rise on the figure 10 years earlier and 1 million higher than the figure for 2003.

Likely causes include high rents and house prices, plus a housing shortage, are probable causes. In 2018, 31.4% of men aged 20 to 34 years lived with their parents, compared with 19.9% of women the same age. The ONS suggested this was because women tended to marry at younger ages than men are possibly more likely to cohabit at younger ages than men.

Experts warned that many of the one in eight people aged 16 and over in England and Wales who live together without being married or in a civil partnership - a proportion that has risen for the past 15 years in a row – have “severely skewed” expectations of what will happen if one of them dies.

“Love just isn’t enough. Cohabiting couples stand to lose it all if the worst happens. You could be left with nothing,” said Dan Garrett, the chief executive of Farewill, a specialist in wills. According to its research, 24% of British people wrongly believe that if they co-own a property with their partner but are not married, ownership of that property would pass to them on their partner’s death.

Another one in five wrongly believe that if they have been cohabiting for more than five years, they will inherit the entirety of their partner’s estate or assume executor status.

Neil Russell, a family partner at Seddons Solicitors, said both beliefs were myths. “There is no such thing as a ‘common law spouse’. You are married or you are not,” he said.

“Government legislation continues to lag behind by not providing legal protection to cohabitees on the breakdown of their relationship. Parliament needs to legislate, but in the meantime people need to be informed of the potential dangers of not securing their position in the event of a relationship coming to an end on the breakup or death of a party.”