[4] Douglas was born in the Cheshire village of Sandiway and was articled to the Lancaster architect E. G. Paley, later becoming his chief assistant.
[6] As his office was in Chester, most of his work on houses was in Cheshire and North Wales, although some was further afield, in Lancashire, Merseyside, Greater Manchester, Warwickshire, Herefordshire, Worcestershire, Derbyshire, Surrey, and Scotland.
[7] From an early stage in his career, Douglas attracted commissions from wealthy and powerful patrons, the first of which came from Hugh Cholmondeley, 2nd Baron Delamere.
Douglas designed a large number and variety of buildings in the family's Eaton Hall estate and the surrounding villages.
Work carried out on grand houses included additions to Vale Royal Abbey and Hawarden Castle.
Some unlisted buildings have been included because they are of interest, or they demonstrate that Douglas worked in areas at a distance away from his office in Chester.