Dharavi in Mumbai is usually considered to be Asia’s largest slum, with a million people living and working in just under a square mile. Photo: Bethany Clarke/Getty Images

Access to safe and affordable housing is vital in both the north and south. But how much will it cost and do politicians care enough about the urban poor?

The official figure for the number of people who live in slums is 863 million people, but that number is more likely to be 1.3 billion and by 2050 the world’s slum population will reach well over two billion on current trends. So the target in sustainable development goals to “ensure access for all to adequate, safe and affordable housing and basic services, and upgrade slums by 2030” seems overambitious at best, and simply delusional at worst.

To know whether a target to “upgrade” slums can be achieved it has to be measurable, but for many slums there are only rough population counts without detailed enumeration. Experience in India shows that when a proper, community-based census does take place, the real numbers turn out to be much higher than the estimates.

Dharavi in Mumbai, possibly Asia’s biggest slum, has one million people crammed into one square mile; Mumbai itself has more than nine million slum dwellers, up from six million a decade ago. Yet in 2009, India launched just the sort of plan that the UN goals would encourage, aiming for a slum-free India within five years. Since then the slums have grown unabated. This suggests there is a range of problems in delivering the UN target.

First, the world is moving inexorably to city dwelling, with more than half of humanity now living in cities. Ninety percent of new urban growth is taking place in developing countries and one third of their city population already lives in slums. While 95% of slum dwellers are in developing countries, even metropolises like London have a growing problem of “beds in sheds”.

Second, the physical and legal challenges of slum settlements can be overwhelming. In a city like Caracas, where the poor want to live as close as possible to jobs in the city centre, most slums occupy precarious sites that are susceptible to landslides. Elsewhere, slums might sit on the slopes of an active volcano, bestride an earthquake fault or in the flood-prone basin of a former lake – all sites eschewed by mainstream housing developers. Almost all slums have complex issues about land rights, and the lack of land ownership creates a vicious cycle – not only public authorities, but banks and developers are unwilling to invest without clear land titles.

Third, replacing bad housing – or even repairing it – is costly and complicated – estimated costs vary considerably and global figures are scarce. One estimate from 2005 for the previous Millennium Declaration to improve 100 million slums by 2020 was that the 15-year target would cost an enormous $300bn. And even this assumes 80% of the costs will be met by the slum dwellers themselves.

The cost of upgrading slums doesn’t mean it can’t be done. Sao Paulo won an award for its slum upgrading programme, implemented over two decades. But compared with other cities, its slum population (800,000) is modest. The task facing Mexico City, which hosts the Neza-Chalco-Itza slum area with its four million inhabitants, is more typical of the developing world’s big urban centres and far exceeds the budgets of municipal authorities.

As politicians try to court the growing middle classes the world over, housing policies shift and the benchmark for affordability is raised. Meeting poorer people’s housing needs requires bigger subsidies just at a time when most countries are cutting back on public spending – or even pulling out of subsidised housing altogether. If this is happening in rich countries such as Britain and the US, it is even more likely in poorer countries with tighter budgets and limited tax bases.

The above recognises that policy and price are two important determinants of whether the SDG target can ever be achieved. But a third determinant is possibly the most important: participation. Programmes to improve slums or rehouse their occupants often don’t engage properly with the people themselves. This means that they are likely to be resisted and – if they do go ahead – more likely to fail.

Putting “the urban poor at the centre of strategies for urban development” is the very reason the organisation Shack/Slum Dwellers International was set up. Its president, Jockin Arputham, may have been recognised with a Nobel Prize nomination, but their work is far from over. The urban poor are still unseen and unheard and until that changes, the UN’s ambition to curb slum dwelling has no chance of being achieved, whatever resources are devoted to it.

That said, just because the target is hard doesn’t mean it shouldn’t be attempted. Slums may be poor, but they are economic powerhouses, with their mostly young populations finding innovative ways to generate income in an informal economy.

Most of the resources to upgrade slums will inevitably come from these people, whether through the community savings schemes run by Interação in Brazil, women’s group Mahila Milan and their self-build housing in Mumbai, or the Centre for Community Initiatives’ pit latrine programme in the slums of Dar es Salaam. Such projects often need seed funding and an injection of expertise to get going. The development community can provide this. But if more governments were to change their approach and begin to trust, promote and invest in slum dwellers, then they might start to keep pace with the problem rather than falling ever further behind.

Original post and comments: Guardian Global Development Network