Converting your basement into added living space can be a difficult process. Typically, you will be required to convert the small, stock windows of your basement to egress windows to meet local building codes. Though this can be a formidable expense, it is also a sound investment with all the added light and safety for anyone living down there. Remember, basement fires are common, so this is more than just an issue involving strict local regulations.
One egress window is ordinarily required in every room used for sleeping purposes. Typically older basement windows are not built to code, as they'll have to be a specific height and width to accommodate safe exit from the house. When renovating a basement in to a living area with bedrooms, you'll typically have to add one egress window per room to adequately adhere to your local codes. Though this can be a substantial expense, typically the added living space will more than make up the cost of your investment 10 to 20 times when it comes time to sell your house.
Local regulations vary from area to area, so it is important to check with your local code administration office. Likely you'll find that your basement windows are not up to code for bedrooms if your house wasn't built recently, or built with basement bedrooms. It is also important to note that any rooms in the basement that are bedroom-sized with a closet will likely require egress windows, regardless of how your blueprints label the space. This means that you aren't going to be able to avoid building codes by labelling a small room a den, as this is a safety code as much as it is a building code and your local housing officials have surely seen plenty of this in their line of work.
International Residential Code
Under International Residential Code (IRC) all bedroom windows must have a minimum opening width of 20 inches, a minimum opening height opening of 24 inches, and a minimum net clear opening of 5.7 square feet or 5.0 square feet for ground floor with a maximum sill height above the floor set at 44 inches. If the window is located under a deck or porch, an additional 48 inches is needed from the top of the window to the bottom of the deck or porch joists.
The basement windows must also provide at least 9 square feet of floor area with a minimum dimension of 36 inches in width and length. This typically means constructing a window well if the basement area is far below ground level. If the window well depth exceeds 44 inches in depth, it is required to contain a permanently affixed ladder or steps for climbing, which cannot be obstructed by the open window. The ladder must be at least 12 inches wide and must project at least 3 inches from the window well.