You need to check whether they want a builders clean, or just a window clean, as I have found recently they are happy to have the windows cleaned with wfp without any scraping work at all.
If they do want a builders clean, you will need to check their policy on ladder work, as I was under the impression that ladders were banned on building sites, if that's the case you will need to do the windows when the scaffolding is still up.
I have had a couple of site agents suggesting that we should lean out of the windows, suggesting that was how they had done it, needless to say I walked away from those jobs.
Take plenty of pictures to cover your back, especially if you have to clean baths and their is plaster or cement in them.
Watch out for your payments towards the end of the site, as this tends to be the time when payments dry up, and your standards are questioned.