His pleasure did not last long. On Friday Building Design magazine revealed that he had won the award on the back of a mere 60 votes, such is the lack of interest in architecture among the British "people".
The idea of the People's Choice is to let members of the public say which buildings they really like. Pity then that so few bothered to vote.
To make matters worse, it has now emerged that a fair number of those who did vote actually work for Foster and Partners. Staff were sent an e-mail by the management informing them of the competition and explaining how to vote. It is not quite ballot-rigging as there is nothing in the rules to prevent it, but Riba is looking into the matter and insiders believe the rules may be changed.
As for the people of Berlin, or at least those who have to use the Reichstag, they are less than impressed with Lord Foster's work.
Take the bartender casting a contemptuous glance at the mural above her head. "It belongs to a museum," she says. "Not here." Since its opening this year, the building has become a big tourist attraction. Visitors flock to its glass dome, admiring on their way up the vast selection of modern art on the sandstone walls.
But some of the people working there are uninspired by their environment, and complain of irritating details. "I think what Foster has done here is a work of genius," says Antje Vollmer, deputy president of the German parliament. "But there are a few things that, from the point of view of the daily workings of parliament, are very difficult."
MPs would quite like the privacy of some telephone booths, for instance. "When you make a call, everyone around you can hear every word," Ms Vollmer complains. "For politicians it's simply impractical. Foster creates big difficulties, because he says he does not want the artistic concept disturbed." So no phone booths, says the builder.
There is a problem with the library. "It is like a tomb; a very, very dark blue," Ms Vollmer continues. It cannot be repainted because "the colour is part of the artistic concept".
MPs moan about the panels in meeting rooms. "Especially the black and white ones, which produce too much glare for the eyes," Ms Vollmer says. "When you sit and stare at them all day, you get a headache. MPs wanted to hang pictures on them. Foster says it's not allowed."
The Englishman has slapped "verboten" signs on many other things. He is down on greenery, for example. A gift of two ficus plants from the Dutch parliament could be displayed only with special dispensation from the architect. A group of bright red sofas in the western wing are still being fought over. Lord Foster prefers anthracite.
There are problems with seating in the chamber. The architect has allowed for 15 chairs for visiting prime ministers of the federal Lander. As most German children know, there are 16 Lander, but the unlucky late arrival from some far-off outpost like Saarland cannot just drag in an extra stool. That would mar the aesthetics.
The seats in the debating chamber are a sore point. Lord Foster wanted them in dark grey, but the politicians vetoed that plan, and are now reposing on the colour of their choice: electric blue. It looks terrible. But give- and-take between the artist and the masters of the house has led to improvements. One need look no further than the famously leaky dome, conjured up by a committee. "The dome is the outcome of a discussion with parliamentarians," Ms Vollmer says. "It is in the tradition of all good art that the proprietor has disputes with the artist. That doesn't mean that they can't create good art."
Lord Foster is not the only creative soul having problems with the people who sign the cheques. Ms Vollmer, a Green MP and admirer of modern art, can find no other word but kitsch for the idea of putting earth from each Land into a wooden trough sculpted by Hans Haacke, one of Germany's most prominent artists.
Mr Haacke's idea is that all MPs should bring a little bag of soil from their constituencies and pour the contents into the trough, there to mingle with the rest of Germany. No one knows who commissioned the work and how much it is costing, but Ms Vollmer is hurriedly scouring art collections for a substitute. Enough is enough.
Artistic power at the Reichstag, it seems, has reached its zenith, and the politicians are fighting back. "I think politicians have a choice," says Gregor Gysi, leader of the post-communist Party of Democratic Socialism. "They should either enjoy the art here, or they should keep their mouths shut."