Every project should have a contingency allowance for both time and budget as there is always a tendency to under-estimate tasks, particularly when under pressure to deliver as soon as possible. Indeed, in practise, many project managers are not given the luxury of estimating the total time required but are likely to be given a fixed end date and the project tasks have to be fitted into the time available.
The tasks need to be scheduled in the correct order and dependencies between tasks factored in. In complex projects several tasks will be performed in parallel to maximise the total project time. The plan will also take account of the project budget.