I did this recently, and this is how it worked for me:
The building company contracted the 'general' cleaning (including window cleaning) out to a cleaning company, and this cleaning company would clean the windows before the scafolding was taken down from each new-build; so no window cleaner was required.
Unfortunately this didn't happen and the scaffolding was taken down before the cleaners got to do the windows, so they hired me. They paid me directly and then 'claimed' the money from the builders. (The building manager checked with me how much I was charging since he thought the contracted cleaners were adding some on

).
After three house cleans they said I was too expensive (I was charging £60 a house), tried and failed to get someone else (another window cleaner arranged to quote but failed to turn up), so they asked me to do tops only.
So I did the first floor windows at £30 a house, but it was only five or six windows; so still a winner.
It's not guaranteed regular work, so go in there with a good price.
So in answer to your question, a window cleaner may not even be required for this job; but a shy child gets nowt, as they say.
I'd phone the builders first; since the show home office will be more concerned with prospective buyers.