As chief government architect he was responsible for many of the federal buildings constructed in this period.

Drawings for public buildings designed by Horwood and his staff during his tenure as Chief Architect of the Department of Public Works are held at the Library and Archives Canada in Ottawa.

As Dominion Architect, his most important building was the Dominion Astrophysical Observatory and residence, Victoria, British Columbia, West Saanich Road (1915–16), which is a National Historic Site of Canada.

He also designed buildings for an Experimental Farm in Brandon, Manitoba.

Other designs include an addition to the Royal Canadian Mint refinery on Sussex Drive (1916) and a number of post offices across Canada: Summerside, Prince Edward Island (1915); Hampton, New Brunswick (1914–15); Chester, Nova Scotia (1915); Almonte, Ontario (1914–15); Burford, Ontario, King Street (1914–15); Fort Frances, Ontario (1916); Milverton, Ontario (1914–15); Palmerston, Ontario, William Street at Bell Street, (1915); Shawville, Quebec (1915–16)