When can they start?

When everything and everyone is ready!

You need your planning permission, to have appointed your builder, advised building control. There is no point in trying to start too early or rush as doing this will most likely ony cause delays later.

When trying to work on a start date, its always worth trying to look forward to a date when you need to have the work completed, get an idea of the timescale from your builder or sit and work on a programme with them so it can be agreed and is reasonable that the work can be carried out in that period of time, and work backward to schedule in a start date.