The Rugby Football Union is to spend £54m extending the stand outwards to add six levels for hospitality and conference events.
Designed by Mott MacDonald, the redevelopment plan will double the available space on the east stand to around 220,000 sq ft but not affect the existing bowl infrastructure and seating.
Mace was originally brought on board as project manager and is now to take the main contractor role.
Construction has started with the project aiming for completion in June 2018.
