What is checked between the units of a duplex for fire safety?

The wall and floor assembly separating the two dwellings — and, most importantly, whether that separation actually continues into the attic. The International Residential Code at R302.3 requires dwelling units in a two-family dwelling to be separated from each other by wall and floor assemblies with not less than a one-hour fire-resistance rating, with rated assemblies extending tight to the exterior wall and, for walls, from the foundation to the underside of the roof sheathing. R302.3 allows the wall not to continue through the attic where the ceiling is protected by 5/8-inch Type X gypsum board, the supporting framing is protected, and an attic draftstop is provided above and along the separating wall. That draftstop is where the defect lives. In converted duplexes across Iowa and Minnesota — single-family homes split into two units, often decades ago and often without permits — the attic is frequently one continuous open volume spanning both dwellings, meaning a fire starting in one unit enters the other unit's attic unobstructed. It is a genuine life-safety finding and it is invisible from inside either unit. The adjacent findings we look for are the ones that quietly defeat a separation that was built correctly: plumbing vents, wiring and bath fan ducts cut through the rated assembly and never firestopped; a separation opened up for a later duct or HVAC run; recessed can lights set into a rated ceiling; a pull-down attic stair breaching the assembly; an unrated mechanical room or an open stairwell tying the shared basement to both units. Note that a townhouse is held to a different and generally higher standard under IRC R302.2 than a two-family dwelling under R302.3, so the two are not interchangeable. What we report is what we observed against the standard; whether a given unpermitted conversion can be legalized is a question for the local building official.