The design of the facades springs from the fact that, the higher you go in a building, the less weight the walls have to carry: so Hopkins' Derbyshire gritstone piers diminish in width as they rise. This is balanced by the fact that the bronze air ducts from the offices have to widen as they rise towards the roof and more and more air from successive floors is fed into them: so these are wrapped either side of the stone piers. Finally the ducts track their paths visibly across the roof pitch to the chimneys. Stale air is shot out of their tops: fresh air is drawn in round their bases. All this surface tracery is calculated to leave just the right amount of the facade for some exceptionally well-proportioned windows. You are therefore looking at the built equivalent of bones, lungs, and eyes. The building even has its own fluid intake, drawing cold water from a 450-foot borehole, using it to chill the rooms for free, and then excreting it into the Thames. No wonder some people find the place disturbing. This is fundamentalist eco-architecture. It may have cost a heck of a lot to build, but it's meant to be cheap to run and maintain.

Nervi-like courtyard roof

Most MPs will care little for all this. They will want to know - how important will it make me feel? Very, is the answer. The big courtyard at the heart of the building is covered with a dramatic glazed timber-latticework roof and diamond-shaped stainless steel light reflectors. "We wanted to pay homage to the marvellous medieval timber roof in Westminster Hall, using modern techniques", says Hopkins. Maybe - and maybe it shows more of the influence of the great mid 20th century Italian engineer Pier Luigi Nervi, who built audacious hangars and sports halls. Around the edges of the court are enormous low arches, transferring the weight of the building onto just six vast columns to avoid the underground lines. The District and Circle lines pass diagonally beneath the courtyard and the building sits above the interchange with these and the new Jubilee Line extension - this station also designed by Hopkins. A mighty concrete raft separates the two: what with that and the blastproof entrance and the solid walls, a modern-day Guy Fawkes would get no joy out of Portcullis House.

Exploded diagram

On the Portland stone courtyard floor there are six big low rectangular stone troughs with built-in stone benches round them. Two in the centre will have pools in them, four at the edges will be planted with 12 controversially expensive rare fig trees, leased for five years for £150,000. A standard commercial contract, insists Patti Hopkins.