A basement apartment is an apartment located below street level, underneath another structure—usually an apartment building, but possibly a house or a business. Rent in basement apartments is usually much lower than it is in above-ground units, due to a number of deficiencies common to basement apartments. The apartments are usually cramped, and tend to be noisy, both from uninsulated building noises and from traffic on the adjacent street. They are also particularly vulnerable to burglary, especially those with windows at sidewalk level. In some instances, residential use of below-ground space is illegal, but is done anyway in order for the building owner to generate extra income.