Delay to construction work is a common feature in the industry and nobody has yet worked out a way to avoid it. A definition of delay is when the planned / contract completion date is earlier than the actual completion date. The reasons for this are manifold but the most common ones are:
The quality of construction planning is generally dire because the people putting together the schedule do not understand the construction process.
The MD says “That’s too long we will never get the job with that duration – lop 3 months off” so the planner does.
Most contracts have a mechanism for changes in the design or specification. Changes cause delay. There should also be a mechanism for changing the contract completion date which is invariably ignored.
These are routine in the complexity of a construction project and the contractor invariably tries to cover them up causing more delay.
Planning Authorities decisions – English Heritage – Statutory Authorities all have the entitlement and the ability to delay the works. Which they exercise gleefully.