House-builders will adjust and if necessary reduce build rates in order to maintain profitably. Should house-builders choose to deliver fewer houses than required by the 5 year trajectory, the LPA would be held to account by the Government and the Planning Inspectorate. In which eventuality, developers would as now seek to overturn at appeal any refusal by the LPA of applications to build on greenfield sites, not allocated in the District Plan and Neighbourhood Plans.
This is also confirmed by the report of the House of Lords Select Committee on Economic Affairs ‘Building more homes’ (15 July 2016)