There has been no more interested spectator to the emergence of the new Pallant House Gallery than Professor Sir Colin St. John Wilson, better known as Sandy, and as the celebrated architect of an even greater structural challenge, the British Library.

It was his wife MJ Long, along with her partner Rolfe Kentish, who were the chosen architects for a project which was greatly inspired by the fact that Sandy Wilson had agreed to give – and loan his unrivalled collection of British art of the second half of the 20th century to a revitalised, refurbished, reborn Pallant House Gallery. It was a perfect coming together of needs and desires: Chichester finally gets a modern cultural icon to balance its Georgian past and for Sandy Wilson “when the building opens I will see my collection for the first time.”

There is very little space in his homes in London and on the sea at Bosham to show many of the 500 paintings, drawings, prints and sculptures by the leading British Pop artists of the 60s and 70s such as Peter Blake, Richard Hamilton and Patrick Caulfield, along with major works by R. B. Kitaj, Eduardo Paolozzi, Lucian Freud and more, art which Wilson has accumulated over almost 60 years. Hence his excitement at viewing a lifetime’s obsession in a setting over which he was an influential adviser.

Sandy Wilson will always be associated with the British Library, which absorbed 36 years of a distinguished career (which also included a professorship at Cambridge University), but his first and constant ambition was to be an artist. If he was to be frustrated in this dream he has spent his life among artists and “got works hot off the easel and out of friendship as much as for cash.”

That is the way it started back in 1947 when browsing through a London gallery. “I was admiring a collage when a voice said ‘you should have that’. It was Eduardo Paolozzi. I said I would give him all the money in my pockets which amounted to 37 shillings and 6 pence. When you get works of art that way you can never sell them.” The collage will be on loan at Pallant House Gallery, along with such landmark British art works as Richard Hamilton’s Swingeing London ‘67, Peter Blake’s The 1962 Beatles and a Kitaj family portrait of Sandy and MJ working in their studio.

The passion for art and the profession of architect have nicely blended over a long career, not least through the commissions that Wilson, Long and Kentish have received to design artists’ studios. They have created three work spaces for Peter Blake; and Frank Auerbach, R.B. Kitaj, David Inshaw, Antony Gormley and Paul Huxley are among a côterie of leading artists who have looked to the team for new studios. Often the payment included another work of art.

From small artistic spaces it was a natural step for Long & Kentish to create the National Maritime Museum in Falmouth. The original plan for Pallant House Gallery had been to build over the garden but in what Wilson describes as a “quantum leap of the imagination” the Trustees, inspired by David Hopkinson, decided on a major building initiative which would expand the exhibiting space six-fold. The fact that Wilson was “finding the ownership of the collection a major responsibility”, and was wavering between Chichester and the Fitzwilliam in Cambridge as its permanent home, helped to speed up the decision process.

Wilson says that his artist friends are delighted with the display of their work at Pallant House Gallery, and through his contacts, the Gallery has already secured additional gifts from Kitaj, Huxley and other artists. There is a resurgence of interest in the art of the period and it is very sympathetically displayed in the clean, naturally lit, galleries which are something of a Long, Kentish and Wilson hallmark. Although a minnow to a whale when compared with the British Library “out of the corner of the eye you can see one or two connections”, says Wilson, most notably the practical use of natural light from above.

Pallant House Gallery also shares a more frustrating characteristic with the BL– a long delay in its completion. For the architects, the local pressure groups who protested at the demolition of a pastiche Georgian building of the 1930s next door to create room for the extension, were an expected hurdle in such a conservative town as Chichester, but the dire condition of the partition walls and the discovery of crumbling foundations, plus a medieval crypt, caused unexpected and much greater problems.

“It could not have been more difficult” says Kentish, “working in a conservation area next door to a Grade I listed building in a largely residential district. But the Trustees have been bold and we have got the design we wanted”. The new windowless brick façade is dull to some eyes but in the original plan it was to have been white, which would have raised even more hackles, and the partnership believes strongly that art can only really be shown successfully against blank plain interior walls. They are no great fans of glass exteriors.

The delays in the development took a toll but the support of the Pallant House Gallery director Stefan van Raay has pushed the building through to a successful if much delayed conclusion at a cost of £8.6m, with much of the money coming through lottery funding. The idea has been to work in sympathy with the protected 1712 building, although there is one fairly sizeable gallery in the extension, many of the rooms there balance in hanging space with those in the original house so that works can be moved around easily. Everything has been planned to the highest international environmental standards so “if Stefan wants to show Rembrandts, they can safely hang here”. The museum will now be entered through a new ground level entrance into the extension to make things easier for disabled people and the Gallery is also topically “green” through the introduction of an innovative underground geothermal heating and cooling system.

All the boring engineering plant facilities are buried in the basement, and the public spaces, such as the shop, café and the educational area are located on the ground floor so that the galleries above are flooded with natural light, which is reflected rather than direct to assist conservation. A lift connects between the new galleries and the old house which has also been given an interior facelift. The lecture theatre has been made large enough to ensure that most of Pallant House Gallery’s holding of art can be always on public display, even if it must be packed densely on the walls. The café, which leads on to the newly designed garden, seems set to become a favourite Chichester meeting place. Everything is neat and clean cut and carries the Long & Kentish hallmarks of providing plenty of wall space to display art in a natural light.

The practice is now helping to create an extension for the Jewish Museum in London and on other cultural projects, such as visitor centres along the Dorset coast. For Sandy Wilson, who operates as an associate with Long & Kentish, the big challenge is to help modernise one of the most sensitive buildings in the country, the Royal Academy in Piccadilly, transforming the bleak underground accommodation which houses the RA School and improving access across the site. Having successfully overcome local opposition, and a difficult site, in securing Pallant House Gallery as the leading centre for Modern British art in the South of England, there is a reluctance to just walk away. Through generous gifts over the years the museum has a collection of 20th century British art unrivalled outside London.