So far £12m of the £21m cost of the first phase has been raised, £10m of it "in honour of Sir Paul Getty" from Mark Getty and the Ann and Gordon Getty Foundation. £1m has come from the Garfield Weston foundation. A further £4.5m is being sought from the Heritage Lottery Fund, and the rest from private donors. A virtual-reality fund-raising video, fronted by ITN newsreader and National Gallery trustee Jon Snow, is being used.

Director Charles Saumarez Smith says that the National Gallery feels it has been left behind by other Lottery-aided museums and galleries. "There is a danger of the National Gallery being perceived as slightly old-fashioned, frozen as a historical collection in a 19th century building. That's rather different from the idea of a collection of old master paintings that is constantly changed, invigorated, kept up to date. And the downstairs visitor facilities are pretty run-down and seedy. The idea is to open up the two doors and to create what is potentially a sensational central light-filled space which will provide a sense of architectural grandeur which is currently missing."

A few years back, an institution such as the National Gallery could have applied to the Lottery funds for the £100m full monty straight off. Those days are over. Saumarez Smith says that breaking the scheme down into smaller chunks was simple pragmatism. "My view is that once we've done the first part, it makes enormous sense to do the next part. The grand vision has a powerful logic to it."

Architect Sir Jeremy Dixon says: "This scheme makes connections that completely embrace Trafalgar Square. One of the great things about the National Gallery collection is that it is almost all on one main level. That will remain the case. The new spaces on the ground floor will be for reserve collections, temporary galleries, education and public facilities."

The new daylit public atrium in the East Wing is inspired by a great imperial building, says Dixon's partner Ed Jones. The staircase court in Sir Edwin Lutyens' Viceroy's Palace in New Delhi, begun in 1912 and completed in 1931, provided the model for reworking the National Gallery. Lutyens's example is open to the sky. But this being the 21st century and wet London, the National Gallery's atrium will be roofed with the same high-tech translucent foil as used on the Eden Project in Cornwall.