London-based firms Piercy Conner Architects and Moxon Architects - both AJ 40 under 40 starlets - join fellow Brits Studio 8 Architects in the six-strong final.
The shortlisted practices, which were chosen from more than 50 entrants, were asked to come up with an 'exceptional piece of architecture which would have a strong visual presence as well as providing a high-quality office environment.'
According to an RIBA architect adviser and competition judge, the standard of entries was 'very high.'
He said: 'It was a pleasure to review such a varied and diverse response to the competition, which provided an enlightening 'snapshot' of current architectural thinking.
'Entrants engaged with the challenges set down by the brief with intelligent, considered and often inspired approaches.
'It was also encouraging to see the number of submissions that carefully considered the environmental and ecological aspects of the brief,' he added.
All six architects will be invited to make presentations to the jury panel, which includes architect Ian Simpson and Nick Johnson of Urban Splash. An eventual winner will be announced in mid October 2007.