Demolished during the 1950s, the only sections of Sundorne Castle to survive are a Grade-II listed gate house, a stable block and a buttressed chapel.
The castle is in fact a Georgian house, which was later altered to look more like a fortress.
The £4 million project includes the refurbishment of the existing buildings as well as the construction of a new two-storey extension on to the south west edge of the 18th century chapel.
Due to go before planners in July, the 1,200m 2scheme aims to 'integrate the historic landscape with the new working environment'.
According to the architect: 'The castle was built as a defensive structure, but it would be inappropriate to build in a defensive manner in this context now.
'The new buildings are seen as transparent, accessible, inviting and bright- while being climatically responsive and visually respectful to the existing buildings.'
The practice hopes to start work on site later in the year.