Basement windows need to be large enough for a person to crawl through.
An egress from a basement can be either a walk-out door or a window. A basement without an egress is a basement without a proper method for existing to the exterior of the home. The only way to leave the home from this type of basement is to go back upstairs through the home and out the door on the main level. A non-egress basement is dangerous and not permitted under most modern building codes.
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The definition of a non-egress basement is determined by building codes. Essentially, governing bodies deem it unsafe for anyone to live in a basement they can't get out of easily in the event that a fire occurs. Therefore, to have a bedroom or other type of living space in a basement, there typically must be a means of egress.
Any basement that has people living in it or any bedroom in the basement must have an egress window in it. This means that it must be big enough for most people to fit through easily to escape the basement. To count as an egress window, the International Residential Code states that it must be at least 20 inches wide and 2 feet tall. The window also needs to be able to open fully so that a person could crawl through it.
Any bedroom in a home needs an egress window, including any in the basement. It is important to recognize that the definition of a bedroom is outlined in the building code and not simply reliant on someone living in that room. A closet is often one of the defining characteristics of what is labeled as a bedroom under the building code. If you have a room in the basement with a closet, it will probably need an egress window.
Older homes with unfinished basements often cause difficulties for people who decide they want to finish the basement. The problem is that if you want to convert the basement to livable space and find that your basement is non-egress, you may need to install egress windows. The exact regulations that dictate when an egress needs to be installed vary between locales. Check with your local building codes and regulations office before you start any renovations on a basement so you understand the egress requirements.