If a client wants to defer possession of a building and hence put back the date of completion is there any mechanism in the sbc to allow this?
Your questions actually addresses two separate issues.
So, the question you asked, can the Employer put back the date of practical completion. The answer here is no, practical completion is a matter of fact and it occurs when the works are complete (with the exception of small items/snags).
The second question is can the Employer alter the Completion Date to defer possession. Technically, the Completion Date can only be altered after an application by the Contractor or by prior agreement. However, the Contractor’s obligation’s generally run to practical completion so why an Employer would want to alter the Completion Date is not clear, once practically complete the Employer can take possession (in the sense of physically) or not, that is his choice but he cannot slow down or impede the Contractor without giving rise to a variation.