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The joint Government inspectors examining the draft Local Plan for Tonbridge and Malling have thrown a spanner in the works.
Inspectors Louise Crosby and Luke Fleming say the authority has failed to comply with its duty to co-operate with neighbouring authorities in drawing up the plan.
The accusation is the other side of the coin, after Sevenoaks District Council had its Local Plan rejected by its Government Inspector Karen Baker, after coming up with a plan that fell far short of the target housing figure the Government wanted it to achieve.
Ms Baker said that Sevenoaks had failed to inquire with the neighbouring boroughs of Tunbridge Wells and Tonbridge and Malling to see if they could take any of its 3,392-homes shortfall.
Sevenoaks challenged the decision in the High Court, but the judge found against them in November.
Now the Tonbridge and Malling plan inspectors have written to the borough's planning policy manager, Ian Bailey, to say the authority had not provided evidence of sufficient engagement with Sevenoaks in discussing whether it could help meet the shortfall.
In its defence, Tonbridge and Malling had said it was not clear until Sevenoaks published its final plan in December 2018 what the size of its housing shortfall would be, and the borough had been obliged to publish its own plan in January, barely a month later.
Tonbridge and Malling told the inspectors that if they had delayed the submission of their plan to try to accommodate some of the unmet need from Sevenoaks, they would have had to effectively start all over again.
But the inspectors rejected the defence, saying: "It appears from the evidence before us, that the council knew for a number of years, prior to the submission of their plan for examination, that it was highly unlikely that Sevenoaks would be able to meet its housing requirement in full.
"Despite this, there is no evidence that the council engaged in any meaningful discussions with Sevenoaks to consider how the strategic matter of unmet need could be resolved.
"Instead the council has relied on the fact that Sevenoaks did not formally ask them for help."
Although both Tonbridge and Malling and Sevenoaks claimed they were constrained in the amount of new housing they could take by the existence of Green Belt land in their boroughs (93% in Sevenoaks and 70% in Tonbridge and Malling), neither had undertaken any joint evaluation of the relative values of both region's Green Belt areas.
The inspector said they will not make a final ruling until they have received a response from Tonbridge and Malling but warned: "We consider it a very strong likelihood that there will be no other option other than to invite you to withdraw the plan from examination, or, failing that, for us to issue a final report recommending that the plan is not adopted."
The Leader of Tonbridge and Malling Borough Council, Cllr Nicolas Heslop (Con) said: "I am disappointed.
"The inspectors' letter contains what we believe are factual inaccuracies and we will be challenging these.
"The Government has acknowledged that the Duty to Co-operate process is simply not fit for purpose, and they have announced that they will be withdrawing it."
Cllr Heslop said: "Given that we submitted a plan that met our own housing needs in full, we find it bizarre that the implications are that we need to draw up a new plan with the unwelcome consequence of having to provide for even more housing than we have allocated in the existing draft local plan."
If the council is sent back to the drawing board, the borough may be forced to plan for significantly increased housing levels - not only to accommodate some of Sevenoaks' shortfall, but also because it rushed the submission of its plan in January last year to avoid having to comply with new Government regulations that came into effect shortly after that would have increased its housing targets still further.
As it is, the council has submitted a plan to deliver 6,834 new homes by 2031 - that's a rate of 696 a year.
Read more: All the latest news from Malling