James Augustus Louis Hay was born at Akaroa, Banks Peninsula, on 14 January 1881, the eldest child of James Hay, a surveyor, and his wife, Frances Ann Gilchrist Greig. The family lived first at Lincoln, near Christchurch, then in 1895 moved to Napier where Louis attended Napier Boys' High School. The following year he joined the architectural firm of C. T. Natusch as an articled pupil. He later moved to the practice of Walter Finch, and around 1904 was employed by the Department of Lands and Survey in Invercargill. Hay had returned to Napier by 1906 and within three years had set up his own architectural practice. Except for a brief period of employment in Sydney during 1908, he was to remain there for the rest of his life.
From 1909 Hay's work built up steadily. Most of his early briefs were domestic, often for wealthy Hawke's Bay landowners who wanted grand villa-style houses. Although he was strongly influenced by Natusch, Hay's designs reflected his personal enthusiasm for English Arts and Crafts interior fittings and leadlight windows which alluded to Scottish and German art nouveau. The impact of the California bungalow and Hay's fascination with the work of Frank Lloyd Wright were evident in his 1915 design for Otatara at Taradale, and for Waiohika, near Gisborne, designed in 1920.
On 27 September 1918, while in training at Trentham Military Camp, Hay married Margaret Ross McPherson at Wellington. Although due to go overseas on active service, Hay remained at Trentham as a member of the camp orchestra, thereby avoiding an influenza outbreak that killed many on board the troop-ship on which he was to have sailed.
After the war Hay returned to architectural practice in Napier. The quality of his work soon brought him wide recognition. From 1918 to 1920 his most eminent articled pupil was Basil Ward, later of Connell, Ward and Lucas, a practice that by the early 1930s was in the forefront of English modernism in architecture. At the time of the 1931 Hawke's Bay earthquake, in which his wife Margaret was seriously injured, Hay was working on St Paul's Presbyterian Church, designed in partnership with Walter Finch. Like many of Napier's unsupported masonry buildings, the church was destroyed. In July 1931 Hay became a member of the Napier Reconstruction Committee. He also helped to establish an association of local architects to cope with the volume of work, and prevent out-of-town architects monopolising the rebuilding design work.
Hay designed a significant number of reinforced concrete structures for the new town in line with principles of earthquake-resistant construction. His designs are distinguished from contemporary commercial buildings mainly by their exterior stylistic allusions. Characteristically, Hay's AMP building (1933), National Tobacco Company building (1933) and his Hawke's Bay Art Gallery and Museum (1935) derived their ornament not from fashionable art deco motifs but from the work of American architects Louis Sullivan and Frank Lloyd Wright. Sullivan and Wright also provided inspiration for Hay's 1933–34 plans for a Marine Parade entertainment centre and Albion Hotel, which were never built, as well as for his prize-winning municipal theatre design, which was eventually rejected as too expensive.
Although the 1930s were a very busy time in Louis Hay's Herschell Street office, he was regarded as a sympathetic employer who engendered great loyalty in his staff. He held firm views and expected employees to follow his design directives scrupulously, unlike other busy architects who were only too happy to leave detailing to juniors. A meticulous supervisor, Hay was always angered by slipshod work and thought nothing of asking builders to demolish it and begin again. Hay's supervisory role, however, meant that he no longer had time for the personal design work he so enjoyed.
A sociable man, Louis Hay was well known in Napier not only as an architect but as a boat builder, oarsman, waterskier and actor. He was also a noted flautist who at one time seriously considered a musical career but rejected it as too precarious. For many years he was a member of the orchestras of the Napier Frivolity Minstrels and Napier Savage Club. He cut a dashing figure walking in the streets: in summer he wore white suits, bow-ties and a high-crowned panama hat, and was often accompanied by his fox terrier, Spark. Later he drove a large Minerva automobile.
Despite his success Hay was somewhat contemptuous of small-town life; he resented the fact that lack of money and, later, ill health, meant that he was never able to travel overseas. He had to rely for inspiration on the many foreign architectural periodicals he received, which makes the breadth of his architectural references all the more extraordinary. He was not a particularly adept businessman, however, showing a marked reluctance to send out accounts. Despite elaborate plans for improvements, including a library and solarium, his own small Victorian house remained largely unaltered. However, substantial garden walls designed by Hay still stand.
Unfortunately, as Hay's professional opportunities expanded in the 1930s his health deteriorated. After 1940, because of the onset of asthma and the demands of caring for a handicapped son, he did very little work. Louis Hay died at Napier on 4 February 1948, his heart weakened by adrenalin inhalers on which he depended for relief. He was survived by Margaret, a daughter and a son.