Numbers and Delivery
Notwithstanding the complex and diverse issues addressed in local plans, housing supply and its achievement through the planning system comes down in large part to a ‘numbers game’ played out between the key protagonists (developers and their agents, consultants, local planners, as well as interest/lobby groups ) through the various stages of planning. A local plan, whatever else it contains, must contain a housing number—the target number of units to be built (for which land must be available) over the planning period, typically now 20 years although often only 10-15 years in the past. There must be a target number for each authority—it may be further broken down between different main development locations.
How are these numbers derived, and how are they tested for adequacy? Over several decades, the most popular and central approach to this task has involved the use of demographic household projections. The strengths and limitations of this approach are discussed further in Chap. 10. In structure planning practice in the 1980s and 1990s, it was possible to see this approach being blended to varying degrees with approaches based on economic development requirements and employment forecasts, and approaches based (at least notionally) on the concept of ‘environmental capacity’, the latter being perhaps in some cases a signal that local communities wanted to see less new housing built. Clearly, in some localities, such as national parks or green belts, there might be very little land that was not covered by a restrictive designation, and therefore the housing number would be supply constrained. Another feature of practice in that period was that, very often, social or public sector housing needs were considered separately to, and possibly additionally to, the need for ‘general’ (i.e. market) housing. Estimates of need for social housing might be based on traditional waiting list data, or (increasingly in the 1990s) local housing needs surveys. In the period since 2000, official guidance and local practice has shifted more towards doing an integrated analysis of the overall housing market and setting social/affordable housing requirements within the broader picture. Methodologies for these ‘Strategic Housing Market Assessments’ and similar systems are discussed further in Chap. 10.
Housing numbers may be partially or largely accommodated on sites which are identified on the plan’s Proposals Map. More critically, however, the local authority must also maintain a Strategic Land Availability
Assessment (SHLAA) which is a database of all relevant known sites for housing development, showing their capacity (units), key information about ownership, etc., and about potential constraints (e.g. infrastructure, viability) affecting availability, as well as their expected timing of start and completion. Authorities must be able to demonstrate at all times that they have at least a five year supply of developable sites, with indicative availability up to a 15 year horizon. Arguably, this document is the most critical link in the chain of implementation through plans to actual housebuilding. Increasingly, local authorities also link these databases to their work on planning the provision of infrastructure, including estimating costs as a basis for levying the Community Infrastructure Levy (CIL).