One of the first decisions taken by Southwark’s new Liberal Democrat-Green Joint Administration has been to back the legal challenge against the Mayor of London and MHCLG’s decision to reduce affordable housing requirements from 35% to 20%.

For me, as Southwark’s new Deputy Leader responsible for Strategic Planning, this was a straightforward decision.

Housing has always been one of the defining Liberal Democrat issues in our borough. For years we have challenged Labour’s failure to build enough genuinely affordable homes, called for stronger affordable housing requirements, and argued that local people deserve to be able to afford to live in the communities they grew up in.

Now that we have the opportunity to put those values into practice, we intend to do exactly that.

Southwark has 23,000 households on its housing waiting list and more than 4,300 households living in temporary accommodation, including more than 5,000 children. The shortage of genuinely affordable housing is the defining challenge facing our borough.

Affordable housing requirements are the benchmark against which councils work with and negotiate with developers. Lowering that benchmark makes it harder to secure genuinely affordable homes through the planning system and risks setting a weaker standard across London for years to come.

That is why Tower Hamlets, alongside Lewisham and Hackney, has launched a legal challenge. Southwark is proud to support it.

Some have suggested that councils should simply accept the Government and Mayor’s decision and get on with negotiating the best deal they can. I disagree. Liberal Democrats have never believed that local government should quietly accept decisions that make life worse for residents. If a decision from Whitehall and City Hall will leave London with fewer affordable homes, it is right to challenge it.

The decision also says something more fundamental about Labour’s approach to housing.

Labour has been in charge at City Hall for a decade. Until this May, it had controlled Southwark Council for sixteen years. It is now in government nationally as well. Yet London’s housing crisis has continued to deepen, with more families priced out of their communities and more children growing up in temporary accommodation.

The response should be to deliver more genuinely affordable homes, not to weaken the expectations placed on developers.

We have to build the city we want to live in. There are generations of Londoners growing up in our city who simply cannot afford to live in the neighbourhoods they call home. The answer to London’s housing crisis is absolutely to build more homes. But “build, baby, build” cannot simply mean asking the same handful of volume housebuilders to do more of what they already do.

The uncomfortable truth is that London’s housing delivery has stalled. High construction costs, interest rates, labour shortages, expensive finance and years of economic uncertainty (all of the above worsened by Brexit) mean the current market is unable to produce the volume and mix of homes we need. Twenty-five per cent of nothing is still nothing.

The problem is not that developers have no role to play. Far from it. But London has become too reliant on a small number of large volume housebuilders. When that part of the market slows, housing delivery across the capital slows with it.

If we want to build at the scale this country needs, we must diversify who is doing the building. That means backing housing associations, community-led housing, smaller and medium-sized builders, development corporations and, crucially, local councils. A more diverse housing market is a more resilient one, capable of continuing to deliver homes even when one part of the sector faces economic headwinds.

History shows that when councils were able to borrow, acquire land and build at scale, they played a central role in meeting London’s housing needs. They can do so again but only if government gives local authorities the powers, freedoms, long-term funding and borrowing powers to build once more. So when ministers say “build, baby, build”, I agree. But empower councils build the genuinely affordable homes our communities desperately need.

When residents voted for change in Southwark, they expected a council that would stand up for local people, including when decisions taken elsewhere make that harder. Supporting this legal challenge is part of delivering on that promise.

I also hope it says something about how Liberal Democrats should approach local government more broadly. Winning power is only worthwhile if we are prepared to use it. That means working constructively with developers while also standing firm on the public interest, standing up to central government, and, when required, standing up to our own Mayor.

Housing is the biggest challenge facing Southwark and London. Liberal Democrats now have the chance to show that there is a different way of tackling it. That means building more homes, insisting on more affordable homes, and giving councils the tools to become major housebuilders once again.

* Victor Chamberlain is the Liberal Democrat Deputy Leader of Southwark Council, Vice Chair of Federal Council and Liberal Democrat Vice Chair of the Neighbourhoods Committee at the Local Government Association