By using this old route as a new way in - straight off Trafalgar Square's traffic-free Norman Foster-designed upper piazza - the architects have carved a way into the rabbit-warren underworld of the gallery. Cafe and restaurant (by Din Associates) are on your right, shop on your left, orientation point straight ahead - from where daylight leads you inexorably to the new covered courtyard, the public promenade space that the gallery previously lacked. Dixon and Jones are expert at dealing with such raw material . So the old lightwell was tall and narrow? Fine - they make a virtue out of tallness and narrowness. Instead of going the high-tech route of flinging a look-at-me steel-and-glass roof over the space, they modify its proportions by giving it a steeply-pitched ceiling. This culminates in a deliberately narrow light slot - filled with an inflated cushion of translucent foil of the kind used on the Eden Project. That is plenty bright enough: the device makes the space feel more domestic and intimate than a Crystal Palace-derived roof would do.

Intimacy is all relative in this context, of course. This is the domestic intimacy of a mansion. Where at the National Portrait Gallery the architects put in a long escalator to take you from ground level right up to the top, here the solution is more conventional. A grand staircase, lined with richly veined grey-black polished marble, takes you up from what is essentially a big hallway to the gallery level. There is no getting round the intractable problem of the National Gallery - that you have to travel vertically quite some distance before you get to the contents. The original architect, William Wilkins (many have had a hand in the place since), designed it in the 1830s in true neo-classical fashion, with a pianonobile main floor perched up high above a raised basement level that was - and still is - used for offices and storage. Raising the building in this way gave the building presence on what was then the newly-created Trafalgar Square. Not that Wilkins carried this through - he gave it a ridiculous pimple of a central dome. But then nor did Wilkins have to worry about accessibility issues. If you couldn't make it up the steps, someone would carry you.

Today, lifts do the job. Dixon and Jones, presumably in order to give disabled or step-averse visitors something of the experience of the great staircase ascent, have introduced a quirky design element into the new court. A large semi-circular landing projects sideways out into the space near the top of the stairs. Here the lift emerges. It is a generous space to sweep round in a wheelchair, but from below, with its bulky walls, it is too, too solid. Oh that it would melt. Perhaps it should have had a glazed balustrade: perhaps it should have been smaller. However, on the upper level it makes perfect sense. Up there, it is clear that it is not just an exaggerated accessibility device. It is a viewing platform, a fine place to pause and view the bustling pond-life down below. So the way you respond to the thing will depend very much on whether you first see it from below or above.