However, the information tribunal judge agreed that the authority – and development partner Lend Lease – can withhold some details of the viability study on the regeneration of Elephant & Castle’s Heygate Estate.
Affordable housing campaigners have been fighting for two years to see a full version of the document, which was submitted alongside the outline planning application for the scheme, as it is was used to justify an affordable-housing level of 25 per cent.
Southwark Council’s borough-wide target is 35 per cent.
Although Southwark Council published a redacted version of the document to campaigners, the Information Commissioner’s Office (ICO) last year told the authority to publish a full version, arguing public interest outweighed concerns over commercial confidentiality.
The council appealed the decision to the tribunal, which has just published its decision.
Under the ruling, Southwark Council can withhold details of the scheme’s development model, including Lend Lease’s detailed financial calculations and projections on commercial sales and rentals involving other businesses, but everything else must be disclosed.
The judgement said it accepted that Lend Lease’s development model was a "trade secret", adding that preventing disclosure "might encourage other developers to maintain an open book approach to their local authority partners".
It said the publication of some of Lend Lease’ projections for the scheme would disadvantage its commercial negotiations with other businesses, but countered that projections related to private sales and deals with providers of social-rented housing would be less problematic.
It concluded: "When it comes to the rest of the information, in our judgment the balance is different and the importance, in this particular project, of local people having access to information to allow them to participate in the planning process outweighs the public interest in maintaining the remaining rights of Lend Lease and those subcontractors who contributed to the document."
Campaign group 35 Percent said the viability assessment had enabled Lend Lease to build fewer affordable homes than Southwark Council’s planning policy required and that very few affordable properties would be on the Heygate site.
"The public should now be able to see whether Southwark’s decision to waive policy requirements was justified," it said.
Southwark Council said in a statement that it was "broadly pleased" with the decision.
A spokeswoman said: "Without some commercially sensitive information remaining private, developers could simply refuse to work with councils, leaving boroughs without the housing and regeneration we all need."
The tribunal gave Southwark and Lend Lease 28 days to agree on the information the judgement required it to disclose, and to pass that information on to the ICO.
The judgement can be read in full here.