The Welwyn Hatfield Strategic Housing Land Availability Assessment (SHLAA): Phase 3 - Determining the potential for housing windfall is an assessment of the levels of windfall development which are likely to take place in the borough between 2014 and 2029.
Windfall development is defined as development which takes place on sites that are not allocated in the Local Plan, or specifically identified in the plan-making process. The National Planning Policy Framework (NPPF) allows local planning authorities to include an allowance for windfall development in their estimates of future housing supply if they have evidence that it is likely to continue to be a reliable source of supply.
SHLAA Phase 3 assesses the potential for future windfall development by analysing past delivery rates for different types of sites, historic windfall trends, the types of sites identified in the SHLAA Phase 1 assessment (urban capacity), and expected future trends. The document identifies a windfall allowance of 290 dwellings for the period 2014 to 2029, which equates to 19 dwellings per year.