The London-based practice had initially proposed a 4,000m2 extension to the Grade II-listed building, providing new office and public space.
But the practice was forced to reconsider its plans due to respond to the needs of local residents as well as the overall ‘sensitivity of the site’.
It is now working on a 2,600m2 extension that will ‘conserve the secret garden’.
However, despite being nearly half the size the budget remains at the original £13 million. According to the practice this is due to an ‘inflation of costs’ since the beginning of the project.
The building is expected to open by 2010.