An egress window must satisfy four International Residential Code (IRC) criteria:
Minimum width of opening: 20 inches
Minimum height of opening: 24 inches At first glance, you might assume that a 20 inches by 24 inches window would be acceptable for egress. However, those dimensions would yield a net clear opening of only 3.3 square feet. To achieve the required net clear opening of 5.7 square feet, a 20 inches wide window would have to be 42 inches high. Likewise, a 24 inches high window would have to be 34 inches wide.
Minimum net clear opening: 5.7 square feet (5 square feet for ground floor) Refers to the actual free and clear space that exists when the window is open. It is not the rough opening size or the glass panel size, but the actual opening a person can crawl through. Nearly all window manufacturers specify which of their windows meet national egress requirements right in their catalogs and list the net free opening of their windows. The salesperson who sells you the window should be able to help you select a code-compliant window, provided he/she knows you need an egress window.
Maximum sill height above floor: 44 inches
The window opening must be operational from the inside without keys or tools. Bars, grilles and grates may be installed over windows but must be operational without tools or keys and still allow the minimum clear opening.
Increasing an Existing Window Opening
If you're replacing a smaller window with a larger one that meets egress requirements, bear in mind that enlarging the height of the opening takes less structural work than enlarging the width. Increasing width might mean installing a larger, beefier horizontal structural header over the window opening - a major project. Increasing height is often only a matter of lowering the height of the sill below the window.