Does the existing site have unrestricted public access? To achieve a ACV registration you would need to establish a body which meets the necessary criteria (does not have to be a neighbourhood forum, but the constitutional status of some groups which have e.g attempted to register has been challenged successfully by landowners.
And if the Council itself is now seeing the site as a potential development opportunity, are they likely to be supportive of an ACV application.
Thirdly, will an ACV registration help greatly? It buys time, but if the local community has no resources to buy the site then this route may not help.
Our neighbourhood forum has been through a somewhat similar situation, in respect of an undeveloped backland in North Kensington (London) which had been used as an adjunct to a commercial nursery garden since the 1960s. When we started preparing a neighbourhood plan, the landowners put it on the market for residential development, and the Council (RBKC) took the view such development was appropriate. With a lot of local support, we have succeeded at examination of our Draft NP in this and two further backlands being included in the final Draft Plan as Local Green Space and meeting the NPPF criteria for such a designation.
The referendum on the NP (St Quintin and Woodland) is next month. The landowner and developer involved have applied for judicial review of the Council’s decision to accept the Examiner’s recommendation and progress the Draft plan to referendum – so this will be a further case of a NP being challenged by the development industry. Most such challenges have failed in the courts.
Local Green Space designation is the strongest form of planning protection for a site that has been an open space, or has become used as such over decades. If achieved it brings down the value of the land to a level where, potentially, a local community can raise funds to create some form of communal open space. A Local Green Space designation can be made by a local authority (including parish and town councils) as well as in a neighbourhood plan.
So step one might be to approach the council and ask if they would be willing to make such a designation, and if not why not. They will doubtless point to their housing targets, but then you will at least know where things stand. Neighbourhood plan preparation is a lot of work, and the criteria for Local Green Space designation (at para 77 of the NPPF) set a high bar.
Best of luck