On 1 April 1919, with the war over, Symes stayed in the army and reverted to his permanent rank of lieutenant. He was promoted to captain again on 16 November 1923, with his date of rank backdated to 1 January 1923. Serving mainly on his regimental duties through the first half of the interwar period, Symes was selected to attend the Staff College, Camberley, on 20 January 1931, and was promoted to brevet major on 1 January 1932. His fellow students there included men such as John Nichols, William Gott, Miles Dempsey, George Hopkinson and Maurice Chilton. On completing the course he was posted to the Royal School of Artillery as a General Staff Officer (GSO) on 15 January 1933. He then became a brigade major with the 14th Infantry Brigade, then commanded by Brigadier Henry Maitland-Makgill-Crichton, on 1 November 1934, The brigade, part of the 5th Infantry Division, was then stationed in North Yorkshire before being posted to Palestine on internal security duties during the 1936–1939 Arab revolt. Symes held this post until being sent to India as a GSO (Grade 2) on 6 September 1937. He was finally promoted to the substantive rank of major on 1 August 1938, nearly twenty years after he had first held the rank, and then to brevet lieutenant colonel on 1 January 1939. On 11 December 1939, three months after the Second World War began, he married an Australian woman, Katherine Bellairs Lucas, in a ceremony at St John's Anglican Church in Colaba, Bombay, India.