In 1716 he replaced James Gibbs as one of the two surveyors to the Commissioners for the Building of Fifty New Churches – the other being Nicholas Hawksmoor. He designed one church for the commissioners – St George's, Hanover Square – and collaborated with Hawksmoor on the design of two others, St John Horsleydown in Southwark and St Luke Old Street. At St Alfege's Church in Greenwich, he recased the medieval tower and added a steeple in 1730, the rest of the church having been entirely rebuilt by Hawksmoor for the commissioners in 1712–14. James also designed St. Mary's Church, Rotherhithe in 1714–15 and St Lawrence, Whitchurch near Edgware around the same time. He also re-clad the medieval tower of St Margaret's, Westminster 1735–37 in a 'papery gothick manner' (VCH Middlesex Vol.XIII City of Westminster Part 1 P.144).