Is it legally permissible for an employer to take partial possession of a building so that an architect has no compunction to certify practical completion?

This is dependent upon contractual terms. Under JCT Standard Building Contract 2009, JCT Design and Build Contract, JCT Intermediate Building Contract 2009 and JCT Intermediate Building Contract with Contractor’s Design 2009, it is permitted for an employer to take partial possession of any part of the Works on the condition that the contractor is agreeable. Once partial possession has been undertaken, it is considered that practical completion has occurred with regards to those specific parts of the building.