In the two years prior to embarking on the reno, Ms. Farrell tallied up things they’d need for every room, hallway, and staircase, from the mundane, such as baseboards and switch plates, to decor items such as curtains or lamps, to big-ticket items, such construction costs, permits, and architect’s fees. ‘So when it actually came to it,’ she says, ‘I had it all ready to go.’Carla Manya Weinberg