More space please. And another bathroom would be great. This is the dilemma for many homeowners in row houses. And the answer unfortunately is “it depends.”
Digging out a basement used to be a very expensive, difficult project. Today, the cost of underpinning, (the construction process of digging out under a structural wall), has come way down and it is done a lot more often. For this house, we started with a first floor only (tiny crawl space below) and dug down 11’ to give enough room for all the new structure plus a 9’ ceiling. For multiple reasons tied to zoning and historic regulations, this was the best way to add space to the old house.
Views from an alley or street are super important to the historic review process. Letters from neighbors are also very helpful. For zoning, we need to stay within the lot coverage allowed for the property.