The reason being given for releasing land from the greenbelt is that Oldham (and Greater Manchester at large) will run out of building land over the course of the 19-year Spatial Framework. Save Royton's Greenbelt has put this claim under forensic scrutiny and has come to the conclusion that there is no evidence to support it. In fact, we have concluded that the GMSF is engineering a building land shortage by using a mathematical conjuring trick.
To take Oldham's case specifically, the GMSF has determined that there will be a shortage of 4,000 building plots over the course of the plan, which underpins the rationale for releasing land from the greenbelt. This number is specifically the shortfall between the number of plots Oldham needs to meet its target (15,137) and the amount of land logged in the 2018 edition of the Strategic Housing Land Availability Assessment (11,130). This amounts to 4,000 plots, roughly the amount that will be released from the greenbelt (see Greater Manchester’s Plan for Homes, Jobs and the Environment, p. 123).
Moreover, the Strategic Housing Land Availability Assessment (SHLAA) also provides a trajectory for when the land will become available for building (the structure of this register can be viewed in the table below). By plotting the SHLAA trajectory (see Housing topic paper, Appendix A, p. 1) against Oldham's building target we can determine exactly at which point the building land will run out in Oldham. From the graph above, we see that the projected number of builds will exceed the land supply in 2028/2029, and will run into serious trouble in the 2033/2034 period. It is clear from the graph there will be a shortfall of 4,000 building plots by 2037. The GMSF puts forward a compelling argument for their case, but there is a fundamental flaw in their approach.
The SHLAA is not a finite supply of land, it is a rolling supply. As plots drop off the register as land is built on, other land is added as it becomes available. Twenty years is a long time and plenty of land will become available over this period. It is entirely possible for the SHLAA to increase in size as well as decrease. The 2012 edition of Oldham's SHLAA only logged 9,118 plots, while the 2018 edition logs 11,130. Over 2,000 homes have been built across Oldham over that period and land availablity has actually increased. By comparing the trajectory of 2012 SHLAA to the projected household increase we can see from the graph below that building land was projected to run out in the 2022/2023 period. Over a similar 20-year timeframe (2012–2033) the shortfall would be around 4,800 plots. The 2018 edition pushed this "cliff edge" back to 2028/2029 and reduced it to 4,000 plots.
There is a strong parallel between the two SHLAAs here: both editions of the SHLAA register enough land to support building over a 10-year period, but a 20-year period results in a shortfall of around 4,000–5,000 plots. There is an important reason for this: the SHLAA is only designed to supply land over a 10–15 year period. If you have a project that exceeds this timescale (as the GMSF does) then the SHLAA will not initially provide all the land that is required. It is very likely that the next edition of the SHLAA will push back the "cliff edge" back even further, or at the very least greatly reduce the amount of land "shortage".
Clearly there is a fundamental flaw in how the building land shortage is being calculated. The GMSF is planning a 20-year project using a register that operates on a 10–15 year timescale. It has not proven that there will be a land shortage, it has built an "illusion" of a "land shortage" into the design of the GMSF itself.
By setting a timescale that is longer than the operational timespan of the SHLAA any building project can arbitrarily create a land "shortage". For example, the 2018 SHLAA predicts a surplus in 2028, a shortage of around 800 plots in 2033, and 4,000 plots in 2037. If the GMSF had set a timescale of 25 years instead of 19 years then the land "shortage" would be about 9,000 plots.
The GMSF is clearly playing a numbers game to circumvent greenbelt projection laws. By calculating a land shortage in the manner that they have, you can annex as much land from the greenbelt as you want by simply extending the timescale of your plan. It is impossible to envisage that greenbelt protection laws were designed to allow for greenbelt release under such a scenario.