The new extension by Sir Jeremy Dixon and Edward Jones (better known for their leading role in the Royal Opera House redevelopment) does something so obvious, it is positively diagrammatic. It fills the lightwell with new entrance and gallery space - rising sheer on one side up three full storeys. It provides a way into the heart of the building, bang on axis with the entrance. It makes a great deal more sense of the whole place, and gives it 50 per cent more space. And it still brings daylight right down to the centre of the building, only this time it is daylight pressed into the service of art and architecture.

So it might seem a design no-brainer, but it was difficult to achieve for two reasons. First, delicate negotiations had to take place with the National Gallery. The NPG gave the NG a corner of its building in exchange for the right to fill in the lightwell. Then came the matter of reconciling the curatorial need for light-controlled gallery space with the tectonic need to make a memorable space. And at the same time, people had to be drawn to the upper levels of the NPG. The existing staircases did not exactly entice you upwards: many a first-time visitor left unaware that anything lay above the ground-floor contemporary galleries. But the NPG covers the history of British rulers, aristos and other celebs from Tudor times onwards.

Dixon and Jones resolve these conflicting demands with a confidence born of years of ultimately successful struggle at the highly complex Royal Opera House. They clear out the clutter and make a big new internal entrance courtyard, paved with polished limestone. Light pours down from three floors up. A long escalator runs up this Piranesian wall, taking you up past the suspended balcony gallery with its angled walls to temper the light and allow glimpses in. At the top, you plunge suddenly from light into semi-darkness. You are right at the start of the sequence: in the new Tudor Galleries, staring at Holbein's giant "cartoon" of Henry VIII. From here, you can progress down chronologically through the rest of the building, ending up with the newest stuff such as the Miller portrait at the bottom. Or, from this same point of elevated arrival, you can climb one more level and find yourself in the rooftop restaurant.

Restaurant

This is quite some belvedere. The restaurant with its angled sun-shading roof peeps over the previously unseen roofscape of the National Gallery, past Nelson on his column, right down Whitehall to the Palace of Westminster. For Dixon, this view informs the gallery: you are looking at the powerbase of so many of the people portrayed inside, from Cardinal Wolsey to John Major. If this restaurant does not become one of the hottest lunch tickets in London, I'll be very surprised.