But the bookmaker, which has Chipperfield's America's Cup Building, OMA's Casa da Música, and Foster & Partner's Dresden Station as joint 3-1 favourites, was philosophical about the potential of a costly payout.
William Hill spokesman Rupert Adams explained that the odds for the shortlist were determined by a 'blind-tasting' mechanism, in which various directors and members of the William Hill press office vote according to the buildings' 'wow factor'.
Adams said: 'We send the images to the head of trading in Leeds - Tim Packayert, another is sent to head of horse racing, Wendy Roberts, and we have a look at them in the press office.
'We normally know pretty quickly when the prices go out whether we are close or not, as money goes on from people who know what they are talking about.
'Not one architect looks at the images for us, but generally we have done well. It is always well supported, but generally people never bet more than £2,000,' added Adams.
The remaining buildings' odds at the time of writing are: David Chipperfield Architects' Museum of Modern Literature at 5-1, Glenn Howells Architects' Savill Building at 5-1 and outsider, the Young Vic Theatre by Haworth Tompkins at 7/1.