City of York Council's local plan was submitted to the Planning Inspectorate in May 2018.
In a letter to the council following examination hearings on several key aspects of the plan, including the duty to cooperate, housing numbers and green belt issues, inspectors Simon Berkeley and Andrew McCormack say that the authority's approach to delineating proposed green belt boundaries is "far from straightforward".
"That is not necessarily a problem or a criticism in itself. However, it does seem to us that a simpler methodology could have avoided some of the concerns we have," the inspectors said.
Highlighting an example of their concerns, the inspectors said they are "struggling to see why issues relating to air quality and flood risk should have any bearing on the delineation of green belt boundaries". Their letter says they see "no obvious connection between these factors and the purposes of including land in the green belt".
The letter says the inspectors consider that there are elements of the approach taken to delineating green belt boundaries that are not adequately robust. "Indeed, in our opinion, there are intrinsic flaws embedded in the methodology."
Consequently, the inspectors said they have "serious concerns about the justification for the precise green belt boundaries proposed in the local plan, particularly in terms of their consistency with the National Planning Policy Framework".
The inspectors set out three "potentially realistic options" to remedy the situation. One way out, they say, would be for the council to "convincingly explain to us how we have misunderstood the methodology and that it adequately justifies the proposed green belt boundaries".
Alternatively, they suggest, the council could seek to demonstrate that, "notwithstanding the methodological flaws, the green belt boundaries proposed are justified and reasonable". Otherwise, they add, the authority could withdraw the local plan from examination.
In a response to the inspectors, the council acknowledged their concerns about the evidence underpinning the approach taken to defining green belt boundaries. The authority said it would "need some time to consider fully and to respond to your possible options for the way forward".
However, it added that its provisional view is that there is "a strong prospect" of the council being able to demonstrate that the green belt boundaries are justified and explain any misunderstandings over the methodology.
In June last year, City of York Council consulted on a raft of proposed amendments to the submitted local plan, including a nine per cent cut in its annual housing target from 867 homes to 790.
In May, an objector failed in a bid to persuade the High Court that the authority should have regarded a site on the edge of the city as being within the green belt when it considered development proposals for the location.