Douglas designed some 500 buildings. He built at least 40 new churches or chapels, restored, altered or made additions to many other churches, and designed fittings and furniture for the interiors of churches. He designed new houses, altered or made additions to others, and built various structures associated with those houses. Douglas's works also included farms, shops, offices, hotels, a hospital, drinking fountains, clocks, schools, public baths, a library, a bridge, an obelisk, cheese factories, and public conveniences. As his office was in Chester, most of his works were in Cheshire and North Wales, although some were further afield, in Lancashire, Staffordshire, Warwickshire and Scotland.