Before the plumbers started work, I got a crash course in how design decisions affect even things as prosaic as heating systems. I was under the impression that we were just going to re-use the old radiators that had come with the flat (they've been sitting out on the balcony since we demolished the walls, but they're still functional, apparently). But the key design feature of the renovated flat is the double-height ceilings and opened-up spaces. Those features mean that we'll need more heat than if we had just kept the small, boxy spaces--so apparently we have to buy radiators that are more powerful than the old ones in order to keep the greater volume of space warm in winter.
Meanwhile, André hadn't liked the old radiators anyway because they're big and clunky...even I can see that they would sort of detract from the design of the space. I'm sure he was secretly somewhat pleased that we have to get different radiators, because it gave him the opportunity to choose ones that are lower to the ground and somewhat sleeker...