Every building more than 50 years old is subject to demolition review, whether it's total demolition or partial demolition (partial being defined as: "The pulling down, destruction or removal of a substantial portion of the exterior of a building or structure or the removal of architectural elements which define or contribute to the historic character of the building or structure.")
Every such application could go to the Historical Commission for review (even structures like nondescript detached garages) but in recent years the practice has been that some demolitions are administratively approved without going to NHC, if both the chair of the NHC Doug Cornelius and the preservation planner (staff person in the Planning Dept who works with the NHC) agree that no one on the NHC is likely to want to find the building 'preferably preserved.' Sometimes at a meeting Doug will say he was on the fence about bringing a house to NHC but wanted to give members a chance to discuss it. I appreciate that he does that.
Factors the NHC considers are how much a house has been altered, how much the neighborhood context has changed, as well as any historical or architectural significance specific to the house. In the case of a partial demolition/addition, it may be a factor that the addition is going onto the front of the house, so it's visible from the street. NHC is less concerned about what's not visible from the street.
For people who want to know more, here are some links.
Regards
Julia Malakie