And don’t forget to jot down the voltage and amperage of the electrical plug, along with the capacity of the outlet the a/c will use. If you’re buying a unit that requires more electrical power than your existing outlet can handle, you’re going to need an electrician to upgrade your wiring. And that will require lots of permits along with permission from your landlord, or co-op or condo board.

Arline Kob, a principal at Key Real Estate Associates, which manages co-ops in New York, says that she’s “never known a co-op board to allow the installation of a through-the-wall unit where none exists.” The board would be concerned about façade uniformity and breaking the seal of the building’s exterior.

If your board is through-wall-friendly, you will be expected to install an exterior grille that matches existing ones and lines up to them, says property manager Michael Wolfe of Midboro Management. The grilles will also need to be flush with the building's facade and the building's architect or engineer will need to review the proposed installation.

In addition, says Kob, "the resident and future buyers, not the co-op, would be responsible for maintaining the newly installed sleeve and the wall into which it was cut. That includes damage to surrounding apartments that may result.”

If you live in a landmarked building or district you’ll have to get the go ahead from the Landmarks Preservation Commission too. Good luck with that, especially if the altered wall will be visible from the street. “Landmarks would never allow it,” says Kob.