If your plan is to expand a Manhattan townhouse in a landmark district you will need to get approval from the Landmark Preservation Commission(LPC) before getting a Department of Buildings Permit. You should hire an architect who is familiar and has worked before with the LPC so that they can best advise you. Generally, you might be able to build an extension on the back of the house that will match what the LPC has approved for neighboring properties but they may or may not let you go as high as they did. Also, materials used in the renovation will influence their decision to approve or deny including down to the types of windows and colors of the window frames.
Putting a penthouse on the roof might also be acceptable but it can not be visible from the street. So you might have 3000 square foot of air rights left on an Upper West townhouse on park block but the likelihood that you will be able to use them all will be small.
The Author- Brian Silvestry , a licensed real estate broker, has been selling residential and commercial real estate since 1999. He has sold in every neighborhood from Battery Park City to Washington Heights.