Mid Sussex District Council has launched a legal challenge after being advised that its District Plan could be set for failure.

The Plan aims to guide growth and development within the district up to 2039, laying out when and where things will take place, and protecting the area from speculative development.

Following an examination of the Plan in November, the council waited five months to hear from the planning inspector overseeing the process. The inspector’s announcement that the council had failed in what is known as a Duty to Cooperate, left leaders shocked.

The Duty to Cooperate is a legal requirement that says planning authorities – such as the council – must ‘engage constructively’ with their neighbours on planning issues that cross administrative boundaries. This includes taking on some of the homes that areas such as Crawley do not have the space to build – something Mid Sussex has been doing for years.

In her letter, the inspector said it looked like the needs of Crawley ‘were, in effect, ignored in the absence of ongoing and constructive engagement’. And she said there was no ‘meaningful evidence’ of what the council could do to help Horsham, which has been ‘extensively affected’ by the Water Neutrality Zone, so can’t meet its own needs.

Council leaders said neighbours would benefit from 1,000 new homes and were unhappy with the inspector’s finding that it had failed in its Duty to Cooperate simply because it didn’t specify which homes would go where.

Leader Robert Eggleston said: “Mid Sussex has drawn up its Plan following the advice of very senior planning lawyers and advisors, so we are understandably amazed at the inspector’s rationale.

“It is extraordinary that the inspector has ignored the evidence we provided. The same evidence that has previously satisfied other inspectors.”

If the council follows the inspector’s findings, it would need to start its Plan making from scratch – a process which would take at least 30 months, cost more than £1m, and leave the district unprotected from speculative development.

As the council is due to be dissolved in 2028 under the government’s Local Government Reorganisation plans, it was felt necessary to legally challenge the inspector’s views.

Conservative leader Gary Marsh said: “I have a lot of experience of planning, and this has left me speechless. Our Plan was well supported by our communities, neighbouring councils, and many leading house builders. This is deeply worrying.

“Plan-led housing growth is the proper way to do things; not like this. The inspector could have modified our Plan at the next stage to remedy the technical issue identified. This approach is irrational and disproportionate.”

Mr Eggleston added: “We are left with no choice. We must protect Mid Sussex from unplanned, speculative development.

“We have worked diligently for many years to undertake the work thoroughly and in accordance with government guidance and the law. The inspector’s ill-founded conclusions are a slap in the face for proper Plan making and the patient, careful work needed to build consensus amongst communities and with our partners.”