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.
Douglas constructed around 500 structures, with a focus on building at least 40 new churches or chapels. He also undertook renovations, alterations, and additions to numerous existing churches, and designed the interiors of these buildings' spaces.