Although they look like any other window, lot line windows, or windows on the side of the building right up against the property line, are treated very differently by the Building Code. In fact, they’re not even considered “real” windows because of fire safety issues. Ever seen buildings with boarded up windows? Those are lot line windows. By law, lot line windows have to be boarded up if the adjacent building built up to the lot line window’s height. This is because in the event one building caught on fire, flames could spread via the lot line windows to the neighboring building.
In order to be considered habitable, rooms need at least one unobstructed window. Since there’s always the possibility a lot line window could be boarded up, lot line windows can’t fulfill this requirement. Hence, they’re not considered “real” windows.
This distinction catches many people off-guard when remodeling. Since a lot line window looks indistinguishable from regular windows, people often submit plans that use it as a new room’s required window… and then their plan comes back rejected, much to their surprise. In order for rooms with a lot line window to be approved, they effectively need to have at least two windows–the lot line window and the “legal” one.