Michael Pickwoad, production designer on the series, revealed in an interview with Radio Free Skaro that he expects to start working on new episodes in February or March.
”[Work on series 10] won’t start until well into the New Year,” he said. “So it will be probably be February [or] March before we start getting onto the next one
He added that shooting would then commence “a couple of months” later – implying an April / May start date.
Source and more: Digital Spy