The following is pretty obvious but I have never seen it set down.
The three reasons being:
- Shortfall in stock of housing sites in Local Plan: When you allocate a site in a local plan the stock of potential housing sites increases. As the plan ages and isn’t updated the stock shrinks.
- Shortfall in the flow of Allocated Housing Sites to Permission: Once it is the stock it needs to flow from an allocation towards delivery. The flow also includes windfall sites.
- Shortfall in the flow of Permitted Housing Sites to Permission: Now it is down to housebuilders to get building.
- Shortfall in the flow of housing sites coming into the Local Plan: This can be represented by a formula representing the flow of numbers per year that need to be added to a local plan to replenish its stock to target, based on the length of the plan and frequency of review. So if a plan 15 years has a 6,000 target it needs to replenish 2,000 new units every 5 years. Any delay the flow increases.
I have mentioned this before but never fully set it out. It intrigued the ministry. What is interesting is its potential to get rid of diversionary bullshit (of which the LGA are the experts) about what the real source of the problem is. If you have sufficient site level data (which at the moment you only have in London) you can set out the contribution of each component and set out the rate at which each needs to increase to reach target. This would be far better than intervention measures such as the Delivery Test which only monitors one. What is more it makes it possible to use stock/flow simulation models (using ODE equations in a computer model) to forecast future supply.