The UK practice has completed a conservation management plan as well as a master plan for the Grade II listed theatre redevelopment.
The £50m phase I will transform the main entrance with a river view walkway and clearer access to the refurbished foyers.
The service yard in the north-east will be relocated to the back of the building to provide more space for new cafes near the river bank while the ‘anchor stair’ will be also be refurbished.
The Cottesloe Theatre foyer will be enlarged to accommodate two new education and participation spaces near the existing workshops adjacent to the foyer, as visitors can view the activity in the workshops through the glazed façade.
The finalised designs for phase I are expected to be complete by the end of the year and planning applications will be submitted in 2010.
The National Theatre was originally built by English architect Denys Lasdun.