The most similar thing to B & Q that I've found in Switzerland is OBI. In the Lake Geneva region, there are stores in Renens (5km west of Lausanne) and Aigle (10km south of Montreux) see their map
here. I've been to the one in Renens a lot, they have everything that you might need for minor renovation work at a fair price, but I've no idea whether they'd have all the stuff that you'd need for a big job.