In residential areas, local rules commonly restrict artificial (constructed) backyard fences to a height of six feet. In front yards, the limit is often four feet.
Height restrictions may also apply to natural fences -- fences of bushes or trees -- if they meet the ordinance's general definition of fences. Trees that are planted in a row and grow together to form a barrier are usually considered a fence. When natural fences are specifically mentioned in the laws, the height restrictions commonly range from five to eight feet.
If, however, you have a good reason (for example, you need to screen your house from a noisy or unsightly neighboring use, such as a gas station), you can ask the city for a one-time exception to the fence law, called a variance. Talk to the neighbors before you make your request, to explain your problem and get them on your side.