Douglas's most important patrons were the Grosvenor family of Eaton Hall, Cheshire. In 1865 he was commissioned to design the entrance lodge and other structures for Grosvenor Park in Chester, and St John's Church in the village of Aldford in the Eaton Hall estate for Richard Grosvenor, 2nd Marquess of Westminster. When the marquess died in 1869 he was succeeded by his son Hugh Grosvenor, 1st Duke of Westminster. Douglas received a large number of commissions from the 1st Duke and from his son, the 2nd Duke, throughout his career. It is estimated that for the 1st Duke alone he designed four churches and chapels, eight parsonages and large houses, about 15 schools, around 50 farms (in whole or in part), about 300 cottages, lodges and smithies, two factories, two inns and about 12 commercial buildings on the Eaton Hall estate alone. He also designed buildings on the duke's Halkyn estate in Flintshire, including another church.