Solution
The neighbouring buildings provided the crucial context. They had full mansard roofs, which made it seem likely that the council would allow us to match them. The large extensions to one of the neighbours not only provided a precedent, but they also created a situation where the problems of overshadowing and overlooking between the properties would actually be reduced if we were able to match what they had done.
Armed with this knowledge, we were able to start working on an ambitious design that would increase the size of the building on every floor while stepping back at each level it went up. The basement would be expanded to add crucial space for the ground-floor business. Then there would be the large double-storey rear extension covering the entirety of the backyard. Because this was an infill extension, we were sure to include windows all along the rear wall and a skylight on the ground floor.
On the first floor, we placed a one-bedroom apartment with an open-plan kitchen-living-dining area flowing seamlessly into the double bedroom. On the second and third floors, we took advantage of the triple-storey rear extension and the new mansard roof to design a spacious maisonette. The two bedrooms and the bathroom are on the second floor, with the open-plan kitchen-living-dining area up in the mansard.
And so, despite the archaeology area and the limitations that could have meant for the basement, and the conservation area and what that could mean for the rest of the building, we were confident that what we had designed would be a clear improvement on what was there before both in practical and heritage terms. We backed that up with a carefully written heritage statement we prepared in-house. The council were convinced by both our design and the case we had presented, and granted planning permission.