Who is the “AHJ” and what authority do they have on a project?

AHJ refers to “The Authority Having Jurisdiction”. The AHJ has final authority for the interpretation of fire and building codes (NFPA, IMC). Although model codes are interepted fairly consistently, a number of municipalities may have variations or interpret certain elements of the codes differently. In some cases there are additions not covered in the model codes such as Massachusetts requirement for CO monitors in commercial kitchens. If there is any question about interpretation of any aspect of the code, it is a good idea to get a copy of the local code or ask for an interpretation of the section in question.