Islington’s housing chief and his City Hall equivalent are lobbying surveyors to stop developers wriggling out of their duty to build affordable homes.

Cllr Diarmaid Ward and Sadiq Khan’s deputy mayor James Murray – who used to have Cllr Ward’s job at Islington – have written an open letter to the Royal Institution of Chartered Surveyors (RICS) asking bosses to update their official guidance.

It references Islington’s “game-changing” High Court victory this year regarding financial viability assessments.

Viability assessments determine the cost of a potential project for developers, who often use them to argue they cannot meet a council’s required number of affordable homes. In Islington, that’s half of any major project.

They will often say they have overpaid for the land – coughing up what it could potentially be worth if they get pie-in-the-sky planning permission to build huge numbers of private flats – and therefore can’t afford to build the social homes Islington needs.

Islington changed its own guidance surrounding viability and the valuing of land in 2016 to stop this happening by refusing to recognise anything but the genuine value of a site when making concessions about cheaper homes.

But by the time a project gets to the council’s planning committee it is effectively too late, because the land has already been bought based on what surveyors – who use RICS’ national guidance for valuing – recommend.

Islington can still reject an application for not following its guidance, but then a lengthy appeal process usually ensues that slows down the arrival of the new homes and could see their decision overturned by the Planning Inspectorate anyway.

Now, though, the council is armed with the High Court ruling regarding a former Territorial Army site in Parkhurst Road in which judge David Holgate made the highly unusual move of recommending RICS revisits its own guidance.

Cllr Ward said: “There is a shortage of good quality, genuinely affordable housing in London and Islington is feeling this deeply.

“The council is doing everything it can to help tackle the problem – we have built more new council homes in the last four years than we have in the previous 30.

“But there are still too many families desperately in need of secure, genuinely affordable homes and developers need to do their part.

“Developers should respect local planning requirements and take this into account when purchasing new sites, rather than overpaying for land and then trying to bypass affordable housing requirements.”

Mr Murray added: “The mayor wants to see more social rented and other genuinely affordable homes built in London – so alongside councils building more homes themselves, that means developers doing their bit too. We need clear planning guidance without any loopholes.”