The report, published alongside yesterday’s Autumn Budget, calls for more stringent planning rules on the mix of housing types, size and style, design and tenure mix within a new category of large sites, which could initially have a minimum threshold of 1,500 homes.
Letwin advises that the new committee could arbitrate in cases where developers argue that the new requirements would make their schemes unviable.
"I recommend, in particular, that the expertise of the committee should be sought in situations where that right of appeal has been exercised as a result of a disagreement between the applicant and the local planning authority about whether the diversity proposed as part of the site masterplan will facilitate the maximum rate of build-out consistent with the viability, beauty and liveability of the development," his report says.
Another part of the report suggests that the Planning Inspectorate should be required to consult the expert committee upon receipt of an appeal "where an applicant and local planning authority disagree on the extent of the diversity proposed for an application".
The committee could also offer informal advice to developers and councils during the consideration of an application on a large site. Letwin proposes.
His report says: "I recommend that the housing secretary should guide local planning authorities to consult the national expert committee before approving any such large site application in an area of high housing demand." This route, he says, could "minimise recourse to appeal or litigation".
The committee, the report suggests, could be based on the quality review panel established by the London Legacy Development Corporation to assess new development in the 2012 Olympic Park.
It proposes that members could be drawn from bodies including the Royal Institute of British Architects, the Royal Institution of Chartered Surveyors, the Royal Town Planning Institute, the Chartered Institute of Housing, the National Housing Federation and the British Property Federation (BPF).
Other sources, it adds, could include large and small housebuilders, estate agents, mortgage lenders, institutional investors and representatives of custom-build, self-build and specialised housing provision, as well as representatives of local government.
The report says the committee would consider whether submitted plans would lead to "suitably varied" housing types, whether the homes proposed would address different local housing demands and whether the plan would make the rate of build-out "as great as possible, consistent with the viability, beauty and liveability of the development".
In January, Letwin was commissioned to "explain the gap between the number of planning permissions being granted against those built in areas of high demand".
Letwin’s interim findings, published in June, argued that providing a greater variety of homes on large sites could boost home-building rates. Letwin also said he is "inclined to believe" that landbanking by landowners is a "serious issue for the planning system", but had found no evidence that the practice is carried out by major housebuilders.
Responding to yesterday's final report on Twitter, Rob Krzyszowski, spatial planning manager at the London Borough of Brent, asked: "Does anyone else think the #LetwinReview recommendations are over-engineered? Principle of mix good, but a national committee arbitrating? Just strengthen primacy of local plan requirements in appeals."