Articled to George Washington Browne, then of Peddie & Washington Browne, from 1892 to 1896, moving thereafter to Robert Rowand Anderson’s office as assistant. During that period he studied at the Edinburgh School of Applied Art and won its travelling scholarship in 1897, spending his time in France. In 1899 he went to London as assistant to E.W. Mountford where he remained until 1901 when he won the Grissell Medal and commenced practice in Birmingham with an untraced architect named Charles. In 1905 he moved to London to form a successful partnership with John Duncan Tate who had been his assistant in Birmingham in 1901-02. They had a significant domestic practice, being responsible for numerous houses in the Home Counties, and in 1907 Forbes designed a house for himself, ‘The Sheiling’ at Chalfont St Giles. Admitted LRIBA on 4 July 1910, his proposers being James MacLaren Ross, Herbert Launcelot Fedden and Edward Gabriel of Edmeston and Gabriel, to whom Tate had been articled. He was admitted FRIBA in early 1916, his proposers being Alfred Cox, Herbert Winkler Wills and Howard Chatfield Clarke.