The London Borough of Southwark said that, under the policy, all schemes of at least 0.5 hectares or more than 10 homes would have to provide 35 per cent affordable housing – or submit a viability assessment saying why this was not possible.
The council said that the move is part of its attempts to increase the transparency of its planning decisions.
Last year, the council was forced to disclose viability information related to the controversial redevelopment of the Heygate Estate in the borough.
Councillor Mark Williams said: "We know the public has had concerns about the way developments are negotiated, and we promised that we would find a way to put all this information in the public domain.
"We know developers don’t want to share this information for commercial reasons, but we want our residents to see the workings and understand that we do all we can to get the best deal for Southwark.
"We think we’ve found a great way forward – if developers don’t want to face this public scrutiny then they will have to commit to 35 per cent affordable housing from the outset. It’s a win-win for the people of Southwark."
The draft development viability supplementary planning document (SPD) has gone out for informal consultation until 5 January, when a formal consultation will begin and run until 16 February.
The council said that the SPD could be adopted from 15 March 2016.