'I dream in Lego': How to make a giant advent calendar from building blocks
Duncan Titmarsh, a father of two and former builder, is the only person in the UK who can call himself a "Lego Certified Professional" - a title that he shares with just 12 other people around the world.
He heads a team of people that build structures and sculptures from the building blocks, and this year they have been tasked with creating a giant Lego advent calendar for London's Covent Garden market made from over 600,000 bricks.
BBC News went to meet him and followed the process from construction to installation.
Video journalist: Suraj Patel