Buyers of real estate are always looking for ways to enhance the value of their properties. In many cases buyers will add square footage by extending a building deeper onto the property, but this rarely results in the addition of rooms, just the enlargement Buyers will also try and add floors, but there are often landmark and building height restrictions as well as structural concerns that may impede this approach. Another more cost effective option that was used in the renovation of one of my current listings at 6 West 83rd street is to lower the garden to the level of the cellar.
When purchasing a townhouse, buyers are effectively only paying for square footage that is 'above grade', i.e. space that has at least 1/2 of the windows above ground. So when the city of New York is measuring the area of a property it is only counting the above grade square footage, and it is that above grade square footage that is used to calculate the price per square foot.
By lowering the garden, you are effectively making the cellar above grade by bringing a full allotment of light and air into what was previously the lower level resulting in the addition of that square footage to the total. In some cases owners will create a two-tiered garden, in others, the renovation will include lowering the whole garden. 6 West 83rd is an ideal candidate for this type of renovation as it is one of a series of townhouses on that block that has lowered the garden. So remember, when adding square footage, particularly in a landmarked neighborhood, going lower can bring you higher (prices).