MARK COLVIN: The number of Australians living alone is set to double over the next two decades.

A rapidly ageing population, combined with a rising divorce rate and a trend towards getting married later, are bringing radical changes in the makeup of households.

The trend is revealed in the Australian Bureau of Statistics' latest figures on social change.

And as Andrew Geoghegan reports, the changes will present significant social challenges.

ANDREW GEOGHEGAN: The concept of family is being seriously challenged by the latest official statistics. It shows that 1.8 million people were living alone four years ago.

But cast an eye into the future and the trend in living arrangements has up to 3.7 million Australians living by themselves within 21 years. That's an increase of more than 100 per cent.

Phil Browning is a demographer with the Australian Bureau of Statistics.

PHIL BROWNING: Some of the reasons for that relate to the ageing population, delayed marriage, and also increases in divorce and separation.

ANDREW GEOGHEGAN: And if you're not living by yourself then there's a good chance that you'll be living as a couple without children.

Phil Browning again.

PHIL BROWNING: The couple families with children, that's projected to increase only slowly, or possibly it might even decrease between 2001 and 2026. And this population will decrease if we continue to move away from this type of family at the same rate of change as we've seen from 1986 to 2001.

Whereas couples without children, they're projected to increase quite rapidly, from 1.9 million in 2001 to between 2.9 million and 3.3 million in 2026.

Now this is related to delaying having children, as well as the movement of the baby boom cohort into the older age groups, where children are likely to have left home, what you've probably heard of referred to as the empty nest sort of phenomena.

ANDREW GEOGHEGAN: These smaller households will present significant social challenges.

Randall Pearce is the General Manager of the social research firm, IPSOS Mackay.

RANDALL PEARCE: Living on your own, people see either experience as aloneness or they experience it as freedom and independence, so I mean, you could have a society with a large proportion of people living on their own, I guess, on the one hand perhaps subject to depression, and on the other hand feeling somewhat liberated and independent.

So it's going to be a bit of a mixed result there in terms of overall wellbeing but there are I guess much more tangible implications arising from this trend, including how we build our houses and how we construct neighbourhoods.

I mean, on the one hand, single people living on their own will require more modest accommodation.

ANDREW GEOGHEGAN: But humans are herd animals and Randall Pearce says that will mean singles will look for places where they can find a sense of family.

RANDALL PEARCE: We will seek to reconnect with the herd if we're sort of living away from them. So that means that there will be lots of opportunities for restaurateurs and people who organise clubs and activities to attract these singles as a means of reconnecting with the herd.

MARK COLVIN: Social researcher Randall Pearce ending that story from Andrew Geoghegan.