Zoning may or may not be involved in a given project. If you are to build an addition on an existing home and the building parameters—building height, side- and rear-yard setbacks—fall within what is permitted in the zoning laws, you should only need a cursory zoning review. If features of the project exceed what is allowed in the zoning rules, the building will have to be designed to fit within the restrictions, or an application for a variance submitted requesting that the rules be modified for this project. The variance process is generally time consuming and the outcome is rarely assured.
EOB, Planning/Zoning Notes