Many design and build contractors are usually given a 'fitness for purpose' contractual duty, but this does not apply under a JCT Design and Build Contract. Why not?

When a contract obligates a contractor to design and construct a building, the liability of the contractor is simply to ensure that the building is ‘fit for purpose’ once completed. The JCT design and build contracts have incorporated a unique clause (JCT Design and Build Contract clause 2.17.1) which explicitly states that contractors have the same liability as architects and professional designers.