Even when the windows are open, we tend to think that they reflect invisible planes, separating the interior from the exterior.

But some interior design firms push the limit, build window seats, open cramped attic ceilings or even extend to become on demand balconies.

Each inch counts in cities with high cost of living — floor space can cost hundreds (or even thousands) of dollars per square foot.

This collection of “More Sky” windows, designed by architect Aldana Ferrer Garcia, calls into question dichotomies indoors / outdoors.

In accordance with Brooklyn’s building regulations, she designed a Hopper, Casement, and Awning Window, each offering distinct ways of expanding space, as well as exposure to sunlight and views. Both projects are experiments for now, but commercial similarities still exist.

Companies such as VELUX and Fakro are selling “roof windows,” which are folding up and locking to form exterior awnings. Below, extendable railings help to enable space under steeply angled ceilings otherwise difficult to use.

Easy and mechanical, these window systems are designed to open and close like any other device. They too act as traditional skylights when closed.

The designs of Bloomframe are even more creative, folding off from fairly vertical configurations to form complete additional balconies. Developed by architecture firm Hofman Dujardin based in Amsterdam, these contraptions deploy in less than a minute by remote control.

The principle of forcing out interior space is nothing new— bay windows have long been doing similar work. But these kinds of solutions help to expand the toolkit for architects who want to push the boundaries of built spaces for existing structures. Meanwhile, no survey of stunning windows would be complete without considering the tilt-and-turn European