First, he is an American, and so liable to be regarded as an interloper in this most British of locations. Second, his plans will follow a previous scheme that was carbuncled out of existence with the help of Prince Charles.

Finally, he will be revealing his design while the critics are still fresh from their mauling of the new Tate extension, the Clore gallery. That means Venturi will have to do better than the Tate's architect, James Stirling, widely regarded outside Britain as the world's best architect.

A portly 52-year-old with rumpled grey hair, Bob Venturi lives and has his work base in Philadelphia where he teaches at the university. He has something of a reputation as a "pop" architect. In 1967 he published an architectural treatise which has become the bible of the post-modem movement. In 1972 came Learning From Las Vegas, which found a rich vein of architectural symbolism in the apparently barren environment of roadside ribbon developments.

The logical outcome of this was what he called "The decorated shed", and in 1977 he duly designed a discount store in Pennsylvania covered with a giant wallpaper pattern of red and white flowers. From here to the National Gallery seems a giant step, and when Venturi was chosen as architect for the new building by a group of 14 gallery luminaries and advisers who travelled the world, jokes were made about Sunset Strip in Trafalgar Square.

Visitors to his office, a converted 19th century warehouse in a run-down suburb of Philadelphia, at first find these fears redoubled: they have to negotiate a disconcerting garage-like downstairs lobby filled with flashing neon signs of the Las Vegas variety. But upstairs Venturi is to be found at his drawing board with books on English architects of the past spread out all round him.