The redesign and refurbishment of JS Gibson's Grade II-listed building on Parliament Square in central London aims to create a new court and Supreme Court library.
The work will include restoring the building's original light wells while existing historic courtrooms will also be upgraded and public access enhanced.
The scheme, awarded in April last year to lead architect Feilden + Mawson, with Foster and Partners as consultant architect, has been developed in consultation with the 12 current Law Lords.
Middlesex Guildhall's existing crown court work will be relocated elsewhere in London during the construction period.
The project is scheduled for completion in late 2008.