The committee’s final decision has been deferred until September for reasons of refusal to be heard.
However, an appeal from developers Persimmon Homes is expected.
Welsh Government will have the final say on whether or not the development will go ahead.
In the letter, Dr David said: “The officer recommendation in the report presented to the committee was to approve the application, subject to it first being referred to you for consideration.
“It was however clear from the meeting of elected members that there is absolute cross-party unity against this development and the recommendation was unanimously rejected.
“The motion to seek reasons for refusal was moved by Labour councillor Elizabeth Aldworth and seconded by Plaid Cymru councillor John Taylor. Members of Caerphilly County Borough Council who know best the areas that they represent are well aware of the both the folly of this application and the scale of unified cross-community opposition to it.”
Dr David also talks about how the site of the proposed development falls outside the settlement boundary of Caerphilly and would go against Welsh Government policy to “prevent inappropriate development in the countryside”.
He also points out that the development would have a detrimental impact on the natural environment and “could lead to a loss of habitat for some protected species”.
The letter goes on to discuss the potential impact on nearby Lansbury Park, which ranks high on the Welsh Index of Multiple Deprivation.
Dr David said: “The removal of this open space and amenity would have a detrimental impact on the health and wellbeing of residents and potentially contravenes the provisions of the Wellbeing of Future Generations (Wales) Act 2015”.
The AM also called for developers to build on brownfield sites in the Caerphilly basin instead.
Dr David told Caerphilly Observer: “The letter is intended to inform the minister about the good decision that was made at the planning committee, outline some of the sound reasons for refusal (without pre-empting the Planning Committee in September) and identify the cross party and cross community unity on this issue.
“It is vital that all political parties and residents continue to work together on this.”