The extension was required in view of the unavailability of actual number of people speaking the language in the 2011 Census, they said.
"We will formally write to the state government seeking another six months time before we can submit the report and recommendation," the committee's secretary and director of arts and culture department M War Nongbri told reporters after a meeting held at the secretariat here.
According to the committee's secretary, one of the stumbling blocks was unavailability of the actual number of Khasi speaking people in the state from the Census department as per the 2011 Census.
Another issue was the delay in completing National Register of Citizens (NRC) in the state, committee member E P Kharbhih said.
Asked on the number of people required to be recognized, another member Dr Sylvanus Lamare said for inclusion in the Schedule, any language must be spoken by at least five lakh people.