LONDON (Thomson Reuters Foundation) - Architects and planners in rapidly growing cities must work with slum dwellers to build skywards and create high-rise, self-built housing blocks, a leading Pakistani architect says.

As millions more people pour into Pakistan's largest city, Karachi, local architects should work in city slums to allow residents to develop their homes into multi-storey buildings that can accommodate more urban migrants, said Arif Hasan.

The academic, architect and planner who has worked with slum communities in Karachi for over four decades, said rapid urbanisation means cities must try new approaches to private developments of apartment blocks.

"I don't think you need to redevelop these settlements," Hasan, 73, told the Thomson Reuters Foundation.

"I think you need to upgrade them, and make it possible for the people, if they want to build upwards, that they build upwards," he said, speaking at a conference organized in London on Tuesday by INTBAU, an international architecture and heritage organization.

Of Karachi's total population of around 15 million, activists estimate that about 60 percent now live in self-built slum homes known locally as "katchi abadis".

Across the city, these vary from improvised wooden shacks to sizeable brick homes which have been developed over decades to link into electrical and sewage systems.

Slums near the city center have "densified" massively in recent years to house a growing urban population, said Hasan. Most houses are shared by more than 10 family members and men are often forced to sleep in nearby parks, he added.

The Orangi Pilot Project (OPP), a humanitarian architecture organization Hasan chairs, has begun advising residents on the design and engineering skills to build high rise.

OPP has worked with slum dwellers to develop low-lying homes in Orangi Town - the 8,000-acre (3,200 hectare) Karachi neighborhood widely cited as Asia's largest slum - since 1982. But architects now offer solutions to its challenges of building higher, such as structural, plumbing and ventilation issues.

"For single and double storey squatter settlements, we have done this in a big way. But it is no longer enough. High rise informal settlements are a different kettle of fish," he said.

HOW TO 'KILL' LAND SPECULATION?

While building up can create more capacity in katchi abadis, authorities must also take aim at the most profligate users of land, Hasan said.

Private gated housing estates for wealthy Karachiites accommodate as few as 80 people per hectare, said Hasan, and contribute to the city's outward sprawl and commuters' hours-long journeys through overburdened transport networks.

"We need to bring that up to 450 persons per hectare, so that we can preserve land" by setting a minimum housing density by law, said Hasan.

Karachi's population has grown massively since Pakistan gained independence in 1947, when it numbered just 450,000. While newcomers are forced to the periphery, unused land still exists in the city center, where investors hold land to sell on as prices rise, Hasan said.

"Killing this kind of speculative development is very important," said Hasan, adding that a heavy tax on undeveloped land and property was the "only way".

In addition, a "land ceiling act" should be introduced to prevent any person from owning more than 500 square meters of urban land, he said.

"That, again, would bring down prices, that would make land available for use, it would make it possible to have a more 'pro-people' land use than what we have," he said.

Such laws have existed in cities worldwide - including in Europe and North America - and could provide answers beyond Karachi to the housing crises many global cities now face.

But Hasan said he was not holding his breath.

"There is a lot of discussion of this but it's going to be a very long battle, I believe, because those who make laws and those who own property are one in the same."

(Reporting by Matthew Ponsford, editing by Jo Griffin.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, traficking, property rights and climate change. Visit news.trust.org)