through-the-wall air conditioner

I've seen post-war buildings where the holes were there but plugged up, and I was told that there would be no problem opening them again. I think there's usually a board-approved A/C company that would do it. Is that what you're talking about?

I installed them in my apartment. It will cost an arm and a leg. Approximately $2,100 each for standalone AC unit. If you want a HVAC unit (if heating is connected), get ready for $4,500 each unit. It is a bit messy, and it take 3 days. One day to drill the hole, one day for cement to dry and one day to slide the AC unit in. Talk to the super. There are approved companies, usually one. You will probably have to submit some renovation package to the building and the contractor needs to provide certificate of insurance.