Early career
Holden left Leeson's practice in 1896 and worked for Jonathan Simpson in Bolton in 1896 and 1897, working on house designs there and at Port Sunlight, before moving to London to work for Arts and Crafts designer Charles Robert Ashbee. His time with Ashbee was short and, in October 1899, he became chief assistant in H. Percy Adams' practice, where he remained for the rest of his career.
A number of Holden's early designs were for hospitals, which Adams' practice specialised in. At this early stage in his career, he produced designs in a variety of architectural styles as circumstances required, reflecting the influences of a number of architects. Holden soon took charge of most of the practice's design work. From 1900 to 1903, Holden studied architecture in the evenings at the Royal Academy School. He also continued to produce designs in his spare time for his brother-in-law and Jonathan Simpson.
His red brick arts and craft façades for the Belgrave Hospital for Children in Kennington, south London (1900–03), were influenced by Philip Webb and Henry Wilson and feature steeply pitched roofs, corner towers and stone window surrounds. The building, now converted to apartments, is Grade II* listed.
In 1902, Holden won the architectural competition to design the Bristol Central Library. His Tudor Revival façades in bath stone incorporate modernist elements complementing the adjacent Abbey Gate of Bristol Cathedral. The front façade features oriel windows and sculpture groups with Chaucer, Bede and Alfred the Great by Charles Pibworth. Internally, the design is classical, with the furniture designed by Holden and the stone carving mostly by William Aumonier. It was described by architectural historian Sir Nikolaus Pevsner as "free Neo-Tudor" and "extremely pretty" and by Andor Gomme as "one of the great masterpieces of the early Modern Movement". It has been compared with Charles Rennie Mackintosh's Glasgow School of Art and it is sometimes suggested that Mackintosh's designs for the later part of the school were inspired by Holden's, although Pevsner noted that Mackintosh's designs were in circulation earlier. The building is Grade I listed.
Near Midhurst, West Sussex (now officially in Easebourne parish), Holden designed Tudor-style façades for the Sir Ernest Cassel-funded King Edward VII Sanatorium (1903–06). The building features long wings of south-facing rooms to maximise patients' exposure to sunlight and fresh air. The design is in keeping with the building's rural setting, with façades in the local tile-hung style. Pevsner called this "certainly one of the best buildings of its date in the country" and "a model of how to build very large institutions". He designed the sanstorium's V-shaped open-air chapel so that it could be used for both outdoor and indoor worship. Both buildings are Grade II* listed. Other hospitals he designed in this period include the British Seamen's Hospital in Istanbul (1903–04) and the Women's Hospital in Soho, central London (1908).
For The Law Society he designed (1902–04) a simplified neoclassical extension to the existing Lewis Vulliamy-designed building in Chancery Lane with external sculptures by Charles Pibworth and a panelled arts and crafts interior with carving by William Aumonier and friezes by Conrad Dressler. Pevsner considered the façades to be Mannerist: "The fashionable term Mannerism can here be used legitimately; for Holden indeed froze up and invalidated current classical motifs, which is what Mannerist architects did in the Cinquecento."
In 1906, Holden won the architectural competition to design a new headquarters for the British Medical Association on the corner of The Strand and Agar Street (now Zimbabwe House). The six-storey L-shaped building replaced a collection of buildings on the site already occupied by the Association and provided it with accommodation for a council chamber, library and offices on the upper floors above space for shops on the ground floor and in the basement. Described by Powers as "classicism reduced to geometric shapes", the first three storeys are clad in grey Cornish granite with Portland stone above. Located at second floor level was a controversial series of 7-foot (2.1 m) tall sculptures representing the development of science and the ages of man by Jacob Epstein. The building is Grade II* listed. Alastair Service considered it "perhaps his best London building".
In 1909, Holden won the design competition for an extension to the Bristol Royal Infirmary. Subsequently, dedicated to the memory of King Edward VII (died 1910), the extension (1911–12) was built on steeply sloping ground for which Holden designed a linked pair of Portland stone-faced blocks around a courtyard. The plain, abstract blocks have towers on the corners and two-storey loggias, and are a further simplification of Holden's style.
The practice became Adams & Holden in 1907 when Holden became a partner and Adams, Holden & Pearson when Lionel Pearson became a partner in 1913. In 1913, Holden was awarded the RIBA's Godwin medal and £65 to study architecture abroad. He travelled to America in April 1913 and studied the organisation of household and social science departments at American universities in preparation for his design of the Wren-influenced Kings College for Women, Kensington. Other buildings by Holden before the First World War include modernist office buildings in Holborn and Oxford Street, an extension in red brick of Alfred Waterhouse's Shire Hall in Bedford, and Arts and Crafts Sutton Valence School, Kent.
Holden also worked with Epstein on the tomb of Oscar Wilde at Père Lachaise cemetery in Paris (1911–12). In 1915, he was a founding member of the Design and Industries Association and he was a member of the Art Workers' Guild from 1917. Unsuccessful competition entries for which he produced designs include Strathclyde Royal Infirmary (1901), Manchester Royal Infirmary (1904), County Hall (1907), the National Library of Wales (1909), Coventry Town Hall (1911) and the Board of Trade building (1915).