A RAPID increase in lone person households will result in 1.7 million more Australians living by themselves in 20 years, the Australian Bureau of Statistics has projected.
And couples without children are set to overtake the nuclear family as the most common family household within three years.
As the number of households reaches 11.8 million nationally in 2031, an increase of 4 million from 2006, lone person households will surge by 91 per cent.
Couples with children are the dominant family household now but by 2031, if trends are maintained, they will drop to 2.5 million as the number of couples living without children rises to 3.8 million.
''Mum, dad and the kids are down to one household in five. Over 50 years the shift has been quite profound,'' said KPMG demographer Bernard Salt.
The rise in single-person households was being driven by an ageing population, said Mr Salt.
''These are not young, sexy singles … but sad, lonely old baby boomers. The widowed, separated and divorced.''
Mr Salt said he doubts whether Australian cities are ''ready for an army of single old people living in suburbia disconnected from the community''.
Social isolation and depression issues would need to be tackled, as would Australia's focus on the quarter-acre block.
However, Ms Qu said the forecast might not be borne out. She said the fertility rate in Australia had recently leapt again and now stood at 1.9 per cent, which was relatively high.
A steady decline in marriage, an increase in cohabitation and ''spectacular'' increases in divorce rates since the 1960s had created a much larger pool of single people with more fluid living arrangements. Living alone was often short term, as people transitioned between relationships or left marriages, she said.
''Financially, people are more able to live alone than in the past. Community attitudes have changed,'' said Ms Qu.
Rapidly changing social expectations are starkly seen in the living arrangements of 25 to 29-year-olds. In 1991, a third (34 per cent) of this age group lived as a couple with children, but this is set to fall to 7 per cent by 2031. And the bureau predicts almost a third of 25 to 29-year-olds will still be living with their parents by 2031.
The decline in women under 30 having children contributed to this age group staying longer in the parental home, said Ms Qu.
''Life has been postponed,'' she said. ''Leaving home is not just a one-off any more. If their job doesn't work out, or relationship doesn't work out, they go home again.''
The ABS projections assume that the rate of change the bureau observed in household types between 1991 and 2006 continues.
However, Ms Qu said the forecast might not be borne out. She said the fertility rate in Australia had recently leapt again and now stood at 1.9 per cent, which was relatively high.
Her own research had also shown more young Australians were interested in marriage again.
''We shouldn't say it is the end of the family. In terms of people's attitudes, they still see marriage as the way to go,'' she said.