Most alteration to an existing building in New York City requires a permit from the Department of Buildings (DOB). The DOB divides construction alterations into three types: 1, 2, and 3. Most alteration permits require the involvement of a New York State licensed professional engineer or a registered architect.
This application is required when the planned renovation will require a change to the Certificate of Occupancy of the building, for example the conversion of a commercial-use building to a residential-use building or vice versa. Such rezoning may require a Uniform Land Use Review Procedure (ULURP). (citylab, 2011)
Structures built before 1938 did not need a Certificate of Occupancy, so any renovation to an older building will require a Type-1 permit. Other reasons for a Type-1 alteration permit include any vertical enlargement, increasing the square footage by more than 110%, or increasing the number of apartments in a building. (nyc.gov, 2020)
Type-2 permits deal with any alteration that does not involve a change in occupancy. Alterations of this type usually deal with interior renovations such as adding a wall, bathroom or extra bedroom. It also is required for the rerouting of gas lines and electrical work.
These are more minor alterations that only involve single, specific projects like a curb-cut or a construction fence. In many cases, Type-3s do not require detailed plans or the oversight of a professional engineer or a registered architect. (citylab, 2011)