Construction started this week at 6 Grace Court Alley, a carriage house on a charming Brooklyn Heights cul-de-sac. When it was a HOTD in January, we wondered if the building, which has a three-bedroom, one-bathroom apartment on the second floor and a four-car garage on the first, would be renovated into a single-family house with a single parking space. (The home was priced at $2,700,000, which is what it sold for.) Baxt Ingui Architects (which was the firm responsible for another recent Heights renovation) is now in charge of what will be an extensive interior and exterior reno on 6 Grace Court. For the exterior: “We will be using the original steel lintel on the garage door to the right and creating an equal door on the left. We are leaving the original and existing windows on the second floor as well as the arched front door… We will be stripping the façade and re-pointing the brick.” As for what’s in the works for the interior: “A penthouse and roof deck that will not be visible from the street, and a cellar. The cellar is interesting in that the house had a cellar at one point that was then filled in. The house next door still has theirs, we did probes to confirm that it was there, and found the walls to go down to the cellar level and in all areas of the probe found what appears to be a slab. I am not sure why or when it was filled in, but assume it was to deal with the weight of the cars vs. horses.” Click through for tax designation photos and exterior plans.