The practice is currently revamping the Grade I-listed castle in a £6.5 million project, which will include the creation of the new Museum of Somerset (pictured above) that will 'tell the story of Somerset'.
FCBA will be working alongside museum designer Event Communications, which worked with Daniel Libeskind on the Imperial War Museum North in Trafford Park, Manchester.
The news follows FCBA's other recent HLF success with its redevelopment of Valance House, a Grade II-listed estate in Dagenham, east London.
Valance House is set within a protected park, and according to FCBA is 'the only surviving reminder of rural Dagenham'.
The practice is working towards turning the 17th-century house into a local museum, and will include a new visitor centre.
Work on both projects is expected to start on site in 2008.