I don't know what state you are in but, you have to 1st find out the exact zoning. Example many times it is "commercial" but, it will ussually have some kind of designation C-1, C-2, R-1, etc. The zoning in PA is done by the local municipallity so, C-1 in one town can allow totally different uses as a C-1 in the next town. Next you need to get a list of permitted and conditional uses from the local zoning officer and see if it is a permitted or conditional use. I don't know if you have gone this far yet. Many times a so called "commercially" zoned area will allow residential uses. The only way to know is get a copy of the permitted uses. Conditional use ussually needs the local municipally approval.
If not you will need to do a nonconforming use application which will take some time money and energy or a zoning change which is very hard to do as "spot zoning" is illegal.
If you find residental use is not a permitted or conditional use. I beleive at that point you will need to talk to an attorney as to best course of action in your state.
Jul 15, 2010