Read every condition in the decision notice before you fix a start date. Some must be discharged before any work starts, others before the building is occupied, and some apply for as long as the development stands.
If you need to change the approved scheme, the route depends on the size of the change. The council can approve a non-material amendment under section 96A of the Town and Country Planning Act 1990 if it is satisfied the change is not material. For a bigger change, apply under section 73 to develop without complying with a condition, which gives a new permission. Section 73 cannot extend the time you have to start the development.
Understanding planning conditions
The different types of conditions attached to your permission and what they mean
Discharge planning conditions
How to formally discharge conditions before you can start building