In his final report on his independent review of build-out rates, Sir Oliver Letwin recommends new planning rules for sites of more than 1,500 homes in high demand areas, which would require a diversity of housing product.
According to Letwin, the key way of raising build-out rates is to improve the diversity of housing types on large sites.
In the report, the former Tory Cabinet minister said: "I have concluded ... that increasing diversity (and hence improving build out rates) on large sites in areas of high housing demand will require a new planning framework for such sites (which can apply both to the further development of large sites already under construction and to new large sites that have yet to be allocated or permitted)."
The report said the proposals could be initially delivered through a written ministerial statement but would, in the long-term, require amendments to primary legislation, new secondary legislation and a new planning policy document that could be annexed to the National Planning Policy Framework (NPPF).
Letwin concluded that revising the current requirements in the NPPF for residential developments to have a mix of tenures, types and sizes "does not, in my judgement, offer the prospect of significant increases in the rapidity of build out on such sites".
Instead, he said primary legislation should be amended to define the size and boundaries of the new category of large sites that could be allocated in local development plans.
New secondary legislation should amend the Town and Country Planning (Development Management Procedure) (England) Order 2015 to include type, size and tenure mix.
This would make "housing diversification" a reserved matter on all outline planning applications and require applications on large sites to be accompanied by a "diversification strategy".
The new planning policy document recommended by Letwin should "set out the diversification principles that are to apply to all planning decisions relating to such large sites in areas of high housing demand in future," he said.
Housing diversity could cover housing of differing type, size and style, design and tenure mix, according to the report.
If the applicant argues that it is not viable or desirable to take market risk in relation to diversity, "the applicant should set out the methods by which the relevant parts of the site will be sold to other parties more able to take such market risk.", the report said.
Disputes over the mix of housing types should be settled by a new "national expert committee", Letwin said.
However, the plans prompted concerns among some planning experts.
Writing on Twitter, Matthew Spry, senior director at planning consultancy Lichfields said the proposals amounted to another parallel planning system for large sites.
"This poses existential question for role of local plans, and how the two processes interact," he said.
Spry added that it was unclear under the mooted model, who would be responsible for the upfront work to promote the large sites and how it would relate to the process of public plan-making, consultation, assessment of alternatives and the avoidance of conflicts of interest.
He also voiced concerns that introducing further planning reform could risk holding up sites being brought forward under the current system, which he said "is now quietly very effective at bringing forward such proposals".
He concluded: "Overall, I wonder whether recommendations carry risk of unintended adverse consequences and requires legal and policy change that outweigh the possible benefits in a system that's not actually failing."
Also responding on Twitter, Richard Harwood QC of 39 Essex Chambers, said: "My initial thought is that the large sites proposals can simply be addressed in planning policy - through local plans and development briefs."
Communities secretary James Brokenshire MP, said: "Sir Oliver has found that it takes 15 years to complete building on some of the largest sites, which is far too long.
"It is clear action is needed so developers work with us as partners to deliver 300,000 properties a year by the mid 2020s.
"We will consider the recommendations in Sir Oliver’s report to determine next steps to ensure we build the homes our country needs."