The Festival Hall project was led by London County Council's then chief architect, Robert Matthew, who gathered around him a young team of talented designers including Leslie Martin, who was eventually to lead the project with Edwin Williams and Peter Moro, along with the furniture designer Robin Day and his wife, the textile designer Lucienne Day. The acoustical consultant was Hope Bagenal, working with members of the Building Research Station; Henry Humphreys, Peter Parkin and William Allen. Martin was 39 at the time, and very taken with the Nordic activities of Alvar Aalto and Gunnar Asplund. The figure who really drove the project forward was Herbert Morrison, a Labour Party politician. It was he who had insisted that Matthew had Martin as his deputy architect, treating the Festival Hall as a special project.
A 1948 sketch by Martin shows the design of the concert hall as the egg in a box. But the strength of the design was the arrangement of interior space: the central staircase has a ceremonial feel and moves elegantly through the different levels of light and air. They were concerned that whilst the scale of the project demanded a monumental building, it should not ape the triumphal classicism of many earlier public buildings. The wide open foyers, with bars and restaurants, were intended to be meeting places for all: there were to be no separate bars for different classes of patron. Because these public spaces were built around the auditorium, they also had the effect of insulating the Hall from the noise of the adjacent railway bridge. To quote Leslie Martin, "The suspended auditorium provides the building with its major attributes: the great sense of space that is opened out within the building, the flowing circulation from the symmetrically placed staircases and galleries that became known as the 'egg in the box'." The hall they built used modernism's favourite material, reinforced concrete, alongside more luxurious elements including beautiful woods and Derbyshire fossilised limestone. The exterior of the building was bright white, intended to contrast with the blackened city surrounding it. Large areas of glass on its façade meant that light coursed freely throughout the interior, and at night, the glass let the light from inside flood out onto the river, in contrast to the darkness which befell the rest of London after dusk. The hall originally seated 2,901. The cantilevered boxes are often described as looking like drawers pulled out in a hurried burglary, but none has a compromised sightline. The ceiling was wilfully sculptural, a conceit at the very edge of building technology and, as it turns out, way beyond the contemporary understanding of acoustics. Robin Day, who designed the furniture for the auditorium, used a clearly articulated structure in his designs of bent plywood and steel. The original building had lushly planted roof terraces; the Level Two foyer café had been able to spill out onto the terraces looking out on the river, and original entrances were positioned on the sides of the building, enabling visitors to arrive directly at the stairs leading to the auditorium. The foundation stone was laid in 1949 by Prime Minister Clement Attlee on the site of the former Lion Brewery, built in 1837. The building was constructed by Holland, Hannen & Cubitts at a cost of £2 million and officially opened on 3 May 1951 with a gala concert attended by King George VI and Queen Elizabeth, conducted by Sir Malcolm Sargent and Sir Adrian Boult. The first general manager was T. E. Bean, who had previously managed the Hallé Orchestra. "I was overwhelmed by a shock of breathless delight at the originality and beauty of the interior. It felt as if I had been instantly transported far into the future and that I was on another planet," said journalist Bernard Levin of his first impressions of the building.
The 7,866 pipe organ was built during 1950–1954 by Harrison & Harrison in Durham, to the specification of the London County Council's consultant, Ralph Downes, who also supervised the tonal finishing. It was designed as a well-balanced classical instrument embracing a number of rich and varied ensembles which alone or in combination could equal the dynamic scale of any orchestra or choral grouping, in addition to coping with the entire solo repertoire. The design principles enshrined in its construction gave rise to a whole new school of organ building, known as the English Organ Reform Movement, influencing in the UK alone the cathedral organs of Coventry and Blackburn and the concert hall organs of the Fairfield Halls, Croydon, and the Bridgewater Hall, Manchester: there are also innumerable organs in other countries which have been influenced by it. However, the design of the organ in its housing made maintenance difficult, and by 2000 it had become unusable. It was consequently completely removed before restoration of the Hall itself began in 2005, and after restoration and updating by Harrison & Harrison, a third of the organ was reinstalled. The remainder was reinstalled between 2012 and 2013, and voicing completed in 2014.
The Festival Hall was one of the first concert halls in the world to be built using the application of scientific principles, both theoretical and experimental. Hope Bagenal and his colleagues from the Building Research Station formed an integral part of the design team. The acoustic behaviour of the seats was measured and tested in a laboratory to enable more exacting design. Careful consideration of external noise problems was undertaken. Following the opening of the hall, there was some criticism of certain aspects of the acoustics. This was partially attributable to the fact that some of the original specifications for room surfaces determined by the acoustic consultants were ignored in the building process. A specific problem for performers was the difficulty of hearing each other on the platform. Both the angled 'blast' side walls and the plywood reflectors projected sound away from the stage. The general consensus was that the hall was 'too dry', not reverberant enough, particularly at low frequencies, and that the bass tone was weak. The definition was 'excellent' for chamber and modern music, but the hall was not as effective for music of the late Classical or Romantic period. Sir John Barbirolli commented, "Everything is sharp and clear and there is no impact, no fullness on the climaxes."