If you own a piece of property or am contemplating the purchase of property and want to build something or expand on an existing structure you will need to do a zoning check preferably with an architect in order to determine what is allowed. There are building codes rules as to lot coverage, allowable rear, side and front yards , height, parking and usage to name a few and it may be that your proposed project is not allowable under the applicable zoning.
If the project does not comply, the applicant will receive a denial letter which will list those sections of the building code that the project violates. You can then apply to the Board of Zoning Appeals for the municipality for a variance or relief from those sections and present your case to the board. You will need to notify the property owners that abut your property as well as those within a specified radius and additionally fill out a petition and various disclosures.
You will be given a hearing date to present your case to the Board which will vote on the application. A large consideration is whether the project is in character with the neighborhood as well as your personal economic hardship.