In some circumstances planning permission can be granted for a temporary period which can be because a temporary structure or use of land is being applied for.
Majority of permissions
In the majority of cases permission is granted for a 3 year period which commences from the date that the Council grants planning permission.
Making a start
It is only necessary to make start (i.e. to ‘implement’) the development within the 3 year period and there are no limitations on when the work has to be completed.