Ahead of the election on 5th May, the various London mayoral candidates have been keen to emphasise their commitment to protecting the welfare of Londoners, present and future. However, one major issue missing from the mayoral debates so far is a recognition that this cannot be achieved by treating London as an island within the M25 – as both Ken Livingstone and Boris Johnson have done in their time in office. This isolationism is no longer sustainable given the threat posed by chronic housing shortages, as well as the size of the opportunities that an integrated regional approach to public investment would offer to both Londoners and their neighbours.

The incoming Mayor, taking over a strongly established GLA, has the chance to make their mark by recognising the crucial linkages between their turf and the Wider South East. Practically, this means taking serious account of how ‘cross-border’ economic, housing and labour market links will affect policy decisions. But it also means working more actively with representatives from the wider region to resolve shared problems and make the best of the joint potential of this southern heartland.

These themes emerge strongly in the final set of reports from the current Mayor’s Outer London Commission (OLC), but they have barely figured in the electoral manifestos of the leading mayoral candidates. Caroline Pidgeon has promised a specific dialogue with the rest of the South East about accommodating London’s household growth when brownfield sites in London ran out. But neither Zac Goldsmith nor Sadiq Khan have shown any more inclination than their predecessors to look beyond the borders of their electorate.

In part, this is because these are not electorally appealing issues, especially given the sensitivities around the green belt. But with the housing crisis set to dominate the political agenda in the capital for years to come, the question of how London engages with its neighbours has become important to address.

This will not succeed if the Mayor is simply perceived as asking neighbouring regions to house those who they’ve failed to accommodate within the capital. Leaders from across the wider South East, who’ve held two preliminary summits with the current Mayor, are expecting London to do its bit in releasing extra land for development, but they have a wider agenda and are responding to the fact that the housing crisis is region-wide. Its common cause is the overall tightness of land supply across the South East, exacerbated by greenfield development quotas which have bitten most directly outside London.

The issue is shared because neither mayors nor planners elsewhere can ‘control their domestic borders’. Housing completions need to double to meet the estimates of housing need, but despite all the efforts currently proposed by the OLC and others, this is still unlikely. In that case, some of London’s projected population growth will simply get diverted into neighbouring sub-regions. But since capacity there is also limited, the likely knock-on effect will be for more local residents to look further afield for affordable housing – spreading London’s housing footprint even deeper into semi-rural East Anglia, Wales and the South West (if not to cities further north).

This would be a perverse and environmentally unsustainable outcome of ‘compact city’ planning policies. Instead, a much better idea would be to take a city-region approach – channelling growth into well-connected strategic locations closer to London, which would enhance both economic and environmental sustainability.

The OLC reports offer a series of recommendations for how the new Mayor can boost housing output from brownfield sites within London. However, this alone won’t close the supply gap, requiring new initiatives to bring other land into residential development. To address this in a sustainable way, the OLC recommends that the new Mayor should take a lead in ensuring strategic reviews of green belt are undertaken on a co-ordinated basis, both inside London and beyond. Another more specific proposal recommends a focus on the development of five Growth Corridors along major transport axes in and out of London, with an integrated combination of housing, employment and enhanced transport links.

As the Centre for London’s recent Manifesto for London also recognises, opening-up more land for development must be pursued in a controlled fashion that can command broad support. This may be best achieved through identifying specific well-bounded areas, with potential for dense development, to be excluded from the green belt – removing the fear of continual incursion into other areas. This should also be buttressed by ‘deals’ to enhance environmental quality across other nearby green belt areas, and to upgrade communications links.

Of course, these ideas are speculative – the point is that willing partners and public confidence will be required in order to find solutions to the housing crisis across the South East. As the most powerful political actor in the extended region, the Mayor of London could play a crucial leadership role in this process, by helping to negotiate deals with the government and to build habits of co-operation among regional partners.

But most importantly, the new Mayor must recognise that London simply isn’t a free-standing city-state, and that it can’t ‘consume its own smoke’ in accommodating its projected population growth. Securing a decent quality of life, both for Londoners and their South East neighbours, will require region-wide efforts to re-model a much-valued – but outdated – green belt for the 21st century.

Ian Gordon is Emeritus Professor in Human Geography at the London School of Economics and a member of the Outer London Commission