Getting a penalty clause in a building contract

[Mr Green]

I have a place abroad and when ever we get a quote for a building job they usually say it will take 1 week to complete the work but more often than not it takes 2 or 3 weeks. They give you the impression that 4 guys will be working on it but only 2 show up.
I would like to include a penalty clause in the contract/quote next time to prevent this sort of thing happening has anyone any experience of this?