Herne and Broomfield Parish Council and the Campaign to Protect Rural England (CPRE) commissioned Richard Harwood QC to judge whether Canterbury City Council's local plan was legally compliant.
He concluded that the council should have re-assessed the impact on habitats and the environment and whether the proposed distribution of housing remained appropriate and sustainable.
The council increased the housing target in the local plan from 10,200 new homes by 2026 to 15,600 new homes by 2031, according to CPRE Kent.
"A multi-stage process is permissible but it needs to address the proposals and alternatives in the plan which is finally produced, to have an Environmental Report in accordance with the Strategic Environmental Assessment Directive and have been subject to public consultation at the relevant stages," said Harwood.
CPRE Kent has sent the legal opinion to secretary of state Eric Pickles, asking him to intervene.
Canterbury City Council assistant director, planning and regeneration, Ian Brown said: "We received a copy of this opinion on Friday 10 October and of course we'll be looking at what it says.
"We have taken our own advice, which is that we've carried out the relevant studies that we're required to do, but all opinion is helpful when you're trying to deliver something as complex as a plan for the future of the district until 2031. So we will certainly have this opinion assessed."
Canterbury City Council is due to submit the local plan for public examination by a planning Inspector before the end of the year.