West Oxfordshire District Council’s draft local plan, which sets out a vision for the district between 2011 and 2031, was submitted for examination in July.
In his preliminary findings, planning inspector Simon Emerson said the council had fulfilled its duty to cooperate with neighbouring authorities.
However, he said the council had "not sufficiently worked with its neighbours across the [Housing Market Area] in determining its own housing requirement".
He said the council’s proposed housing requirement of 10,500 dwellings is "not justified and has not been derived from a process which complies with the requirements of the National Planning Policy Framework".
He said the proposed housing requirement fell "significantly short of the [Strategic Housing Market Assessment’s] recommendation".
Emerson added: "The council’s evidence to support its housing requirement has been worked-up independently of its partners in the rest of the HMA without due regard for consistency across the HMA and the potential wider implications of its actions."
Cllr Warwick Robinson, cabinet member for strategic planning and housing, said: "In light of the Inspector’s findings we will give further consideration to our local plan and decide on the most appropriate course of action before informing Mr Emerson early in the New Year. He will then make the appropriate arrangements to move forward.
"We will of course keep everyone informed of the local plan progression. There will be public consultation on any main changes and we will continue to work with other Oxfordshire local authorities on our common housing and infrastructure issues."
The council said the inspector had expressed support for elements of the plan, including its overall approach to where new housing developments should be located.
It added that a second hearing, which was provisionally planned for February, has now been postponed to allow the council to carry out further work.