‘Nothing sharp.’ When I listen back to my conversation with interior designer Joanna Plant at her new home-cum-studio in Camden, I hear this phrase a few times. It’s not one I’ve heard before in relation to decoration, but it makes sense. Joanna and her husband, Nick, have only lived here a year or so, but the house already feels settled. Nothing shouts and nothing is sharp. This is a slow house, with places for sitting and mooching, one in which every space is used.