A million more young adults in the UK are living with their parents than were two decades ago, research suggests.
A quarter of 20 to 34-year-olds do so, the study, by think tank Civitas, says.
Since 1998, this has risen by 41% in London, where housing is most expensive, but by much less in cheaper areas like north-east England (14%) and Yorkshire and the Humber (17%).
And for 23-year-olds across the UK, the proportion living with parents has risen from 37% in 1998 to 49% in 2017.
Civitas editorial director Daniel Bentley said: "As owner-occupation and social housing have each become more difficult to enter, hundreds of thousands of young adults have taken one look at the high rents in the private rented sector and decided to stay with their parents a bit longer instead."
He added that it was essential the government took this into account when forecasting future housing need.
Saving up to buy
Primary school teacher Lauren, 29, from Chesterfield, moved back home after university.
She and her partner have been trying to save up to buy a house but she told BBC 5Live: "It just seems to be impossible".
"We're hopefully looking [to buy] around this time next year… it just seems to be getting put back and put back.
"It is frustrating because we don't want to be at home well into our thirties because it's not fair on our parents, and equally we want to start our lives together."
She says her mum and dad have never asked for any board, but that she gives "something every month anyway" as she feels it is the right thing to do.
"They're very good, they never hint at 'when you are you going to move out?"', she says.
Aidan, 23, from Durham, is another primary school teacher who moved back home after university.
He says: "I could afford to live in my own house but I love it.
"It means I can save over half my wage for a house deposit and I get to spend time with my family after my mum died whilst I was at university."
The study also suggests youngsters who do move out are much less likely to live on their own than they were in the late 1990s.
Single-person households have dropped to 30% in recent years, it says.
This is in stark contrast to most of northern and western Europe, the report says, where single living has been increasing rapidly.
In France and the Netherlands, 35% of households are single-person. And this rises to more than 40% in Germany and Denmark.
A spokesman for the Ministry of Housing, Communities and Local Government said: "For the last 30 years, governments of all stripes and types have failed to build enough homes, but we're turning that ship around.
"We still need to deliver more, better, faster, but more than 222,000 homes were delivered in 2017-18, the highest level in all but one of the last 31 years.
"We've also set out an ambitious package of measures to help build 300,000 properties a year by the mid-2020s.
"This includes over £44bn investment, rewriting the planning rules and giving local authorities the power to build a new generation of council houses.
"We are also supporting investment in build-to-rent homes to improve supply and affordability in the private rented sector."