 Dunch was the only son of Hungerford Dunch MP of Little Wittenham and Down Ampney and his wife Catherine Oxton, daughter of William Oxton of Hertfordshire.He was born in Little Jermyn Street, London, 14 December 1657, and baptised 1 January 1658.The freedom of the borough Wallingford was conferred on him on 17 October 1695, and he was at one time proposed as its high steward, but was defeated by Lord Abingdon, who polled fifteen votes to his six.