I would call the builder immediately if you haven't already. Make them aware of the problem before your warranty is up. Have them send someone to check it out. Sewer smells can be as simple as a p-trap that has just dried out; or a major problem, i.e. broken drain line, trap, vent stack, etc. inside a wall or under a slab.Why does my bathrooms smell like sewer? this is a brand new home I need help please my 1 year inspection is up in July thanks 4 all your help.