When I was starting up I went to a big estate that was still being built. my pitch was I will do your windows for x amount a month, and the first clean is free. This really worked well for me, I got and retained most of them and I now have six days work there every month.
Remember, I was just starting out and didn't have a full round, I was just giving them my time for the first clean, it wasn't costing me any money.
Your case is different. Big houses = big money, they either have a window cleaner or are looking for one.
This type of house doesn't get cleaned by the owner. You don't want to be offering free cleans to someone that may already have a cleaner. They could easily accept your offer, and say not this time thanks to their regular guy when he came round.
New builds with new owners, yes ok if you don't have a full round, otherwise no free cleans unless it's a sales office on a brand new estate. Dai