If this is a proper firm (as opposed to a small builder) and you have not worked for them before I would suggest that you are being used in an exercise to reduce costs by using your quote to force down the rates of their existing cleaners.
If a small firm then you will need to understand their requirements. Most builders cleans are carried out in three stages, each of which should be quoted and charged for independantly.
Worst part is always the windows, but then it is all pretty bad