Lisa
Dont know if its different with general cleaning as opposed to window cleaning.
I keep lines of communication open regularly with the person who hired me, if you build good relationships with them , then it is easier to guage a situation before you put it formally to them.
Instead of writing to them , why not request a meeting with them and be honest about what you want.
This way they will agree to your terms 9 times out of ten or you can negotiate a compromise.
If they like what you are doing then they will normally bend a little to keep you, remember most people dont like change unless you have been doing a bad job.
Dave