Douglas's output included new churches, restoring and renovating existing churches, church furnishings, new houses and alterations to existing houses, and a variety of other buildings, including shops, banks, offices, schools, memorials and public buildings. His architectural styles were eclectic. Douglas worked during the period of the Gothic Revival, and many of his works incorporate elements of the English Gothic style. He was also influenced by architectural styles from the mainland of Europe and included elements of French, German and Dutch architecture. However he is probably best remembered for his incorporation of vernacular elements in his buildings, in particular half-timbering, influenced by the black-and-white revival in Chester. Other vernacular elements he incorporated include tile-hanging, pargeting, and the use of decorative brick in diapering and the design of tall chimney stacks. Of particular importance is Douglas's use of joinery and highly detailed wood carving.