Urban planner Ryan Gravel has one question for people campaigning against higher density living: where do you want the extra people to live?

It's a pertinent issue for both his home city of Atlanta, bracing for an extra 2.5 million residents in the next two decades, and Australian cities such as Perth, expecting an extra 1.5 million people by 2050.

Mr Gravel, in Perth to speak at the Urban Development Institute of Australia's national conference, says we need to think seriously about how our cities deal with population growth, including rethinking our reliance on urban sprawl.

"I can't make you live in a high rise or ride on a train if you don't want to, but where do you want those two-and-a-half million people to live?" he asked.

A WA Government report released last year highlighted the challenges facing the Perth and Peel regions, which stretch along a thin coastal strip for 150 kilometres from Two Rocks in the north, to Mandurah's outer suburbs in the south.

"A 'business-as-usual' approach will not adequately provide accommodation, services and infrastructure to support this influx," the Perth and Peel @ 3.5million report said.

Shift in thinking needed on new developments

In tackling the problems of urban sprawl, Mr Gravel said there needed to be a change in thinking from both the private and public sector in how new suburban developments were financed.

"There's this expectation that we're going to follow you down the road with our pipes and school buses and emergency services and roadways," he said.

"And I think we need to stop that. If developers want to build out there, that's fine, do what you want, but we need to make sure if we're going to subsidise something it is something that is in our collective best interest."

To attract people to the inner city, Mr Gravel drove the much-lauded Atlanta BeltLine project, an urban redevelopment program designed around a rundown former railway link that encircled the inner suburbs.

Urban planner Ryan Gravel is behind the lauded Atlanta BeltLine project. ( ABC News: Rebecca Turner )

When completed, the revitalised railway corridors will host cycle paths, light rail and parks — and connect 45 inner-city neighbourhoods.

Tipped for completion in 2030, it has already attracted $3 billion in private sector investment, including retail developments and housing.

Mr Gravel said the overwhelming success of the project was that it changed the culture of Atlanta, making an inner-city lifestyle more appealing after decades of growing urban sprawl.

"The future is being attracted to the city because that's what it wants — walkability, it wants alternatives to driving, it wants social spaces, it wants to connect with people, it wants diversity," he said.

The cultural shift has been so significant that last year Atlanta's government won overwhelming support in a referendum to introduce a sales tax increase to raise more than $2.5 billion to fund public transport projects.

Falling house prices, unemployment can lead to 'suburban ghettos'

He also warned that dark side of sprawl — suburban ghettos — could be exacerbated by falling house prices and a weak jobs market.

"When poor people were stuck in the inner city, they had general proximity to jobs and other things," he said.

"But when you're stuck at the end of a cul-de-sac somewhere, stuck in a traffic jam, you're invisible to the world. People don't even know you're there."

But the suburbs were still an appealing place to live for many people, with smart communities tapping into the changing needs of their residents.

"Suburban communities that are thoughtful right now are thinking about how to protect themselves from that future — transit, jobs, creating walkable places," he said.

"You don't have to be a downtown. There's a lot of people who don't want to live downtown but you have to prepare for that future you want."