“Do I need a building permit to do that?” is a frequently asked question when people think about building, remodeling, and home improvement projects. You might be surprised how many construction projects require one, according to the International Residential Code. From a practical perspective, people do not obtain one when they should and building officials may not make an issue about this oversight. But as inspectors know, work performed without one is a common source of safety issues and costly repairs.

You often need a building permit for:

replacement of or major repairs to water heaters, furnaces, air conditioners, gas and plumbing pipes, and similar components,
modifications or major repairs to the electrical system such as adding or moving receptacles or lights, and adding new circuits,
installation of new plumbing, electrical, and heating and air conditioning equipment and components,
replacement of building components such as roof coverings and exterior wall coverings, Manual J 
construction of new buildings,
additions, structural modifications, and major structural repairs to existing buildings,
movement and demolition of existing buildings,
changes to building occupancy.
A change to building occupancy means changing how the building is used. Example: using a single family home as a place of business is a change in occupancy that may require a building permit and may require a zoning change or zoning waiver.

You may not need a building permit for:

building or installation of one-story detached accessory structures less than 200 square feet with no utilities,
fences not more than 6 feet tall,
retaining walls not more than 4 feet tall measured from the bottom of the footing to the top of the wall and not supporting a surcharge [a vertical load in addition to and/or above the retained ground],
driveways and sidewalks,
painting, wallpapering, floor covering installation, cabinet and countertop installation, and similar finish work,
installation of portable plug and cord connected electrical equipment,
installation of portable gas heating, cooking and clothes drying equipment,
installation of portable HVAC equipment [such as window air conditioners],
clearing of plumbing stoppages and repair of plumbing leaks and removal and reinstallation of toilets if the repairs do not involve replacement or rearrangement of valves, pipes or fixtures.
You usually do not need a building permit for routine maintenance of existing buildings, fixtures, and equipment if the building structure is not affected and if the nature and use of the electrical, plumbing, gas or HVAC system is not changed.

A permit is a good investment, even though it costs money and slows the project. It provides a comparatively inexpensive way to ensure that the project is inspected and it helps reduce your potential liability if there is a defect that causes injury or damage. The building official can require dismantling or removing work performed without one.