Under the contract, Skanska will build a 19,000 sq m unit as well as demolishing and refurbishing a further 1,500 sq m of existing buildings.
Skanska will carry out the work under three independent phases. Laing O’Rourke and Miller were unsuccessful bidders for the contract.
Carstairs provides care and treatment for patients with mental disorders.
Construction will begin in March and completion of the total scheme is set for summer 2011.
Lead architects on the job Macmon, Rybka are M&E consultants and Faber Maunsell are civil and structural consultants.