One of the most important collections in the modern British art world is being displayed in its entirety for the first time from Saturday.

On one wall four familiar faces stare from a canvas, The Beatles logo emblazoned in the middle. On another Mick Jagger looks down.

Further along the corridor of rooms at Pallant House Gallery in Chichester huge swathes of colour depict glamorous women juxtaposed with nuclear bombs.

The works are part of a collection of more than 500 paintings, prints, drawings and sculptures.

It will also be the first time Sir Colin St John Wilson, the collection's owner, will see the work hanging together in the £8.6 million extension he designed for the Queen Anne house in North Pallant, Chichester.

Over the past 60 years Sir Colin, who designed the British Library, has built up his collection acquiring work by leading British pop artists of the 60s and 70s including Patrick Caulfield, Richard Hamilton and Peter Blake, as well as major works by Lucian Freud and Eduardo Paolozzi.

Speaking yesterday in one of five new rooms he designed with his wife MJ Long and her business partner Rolfe Kentish, he said: "Once you start collecting, the collection collects you. It says, 'Come on, you cannot just disperse of me now - we are all part of a piece,' and we needed to do something to keep it all in one piece. That is why it has been fantastic to get the chance to build this building."

Built over two floors, the modern extension also includes a library, prints room, education room, lecture space, bookshop and restaurant.

Director Stefan van Raay, originally from Holland, worked as a senior curator at Glasgow Museum before taking on the Pallant House Gallery project in 1997.

He said: "The vision was to make this into one of the most important British modern art collections in the world. I think it is second only to Tate Britain. I am extremely excited and happy about the opening. I keep discovering new favourite bits all the time."

As well as providing a home for Sir Colin's collection, the extension was built to address the need for disabled access in the original gallery and allow more work and temporary exhibitions to be shown.

A courtyard garden, designed by Christopher Bradley-Hole, a five times Chelsea Flower Show gold medallist, has been created in a space adjoining the new building and the original gallery.

It opened to the public in 1982 after Walter Hussey, the Dean of Chichester Cathedral, bequeathed his art collection on the proviso the building would be restored.

Despite mainly displaying work dating from 1890-1945, a new installation for the central wooden staircase is created each year.

This year's commission, Shell, features 20,000 mussel shells, lined with red velvet. Pallant House Gallery opens to the public on Saturday. For more information or opening times visit www.pallant.org.uk .