Only C/AS2 has specific text in para 4.6.11 allowing an individual household unit to have upper floors. There is no corresponding paragraph in C/AS1 allowing household units to have upper floors, but is commonly thought to. But if C/AS1 was only for household units with a single floor level then all multi-storey single household unit detached dwellings in NZ would be Risk Group SM and not Risk Group SH.
What could be a catch is the independence of escape routes. An escape route terminates at a Final Exit. A Final Exit is the point that gives direct access to a Safe Place. A Safe Place is an open space where people may safely disperse after escape the affects of fire. Therefore; if external escape routes from separate household units need to converge before reaching a Safe Place (like down a narrow walkway or driveway), then the escape routes wont meet the definition of Risk Group SH, even though the building will in all other respects.
Also note that there is a limit of application on fire fighter access to multi-unit dwellings. NZBC C5.3 to C5.8 is limited from applying to detached dwellings and WITHIN household units of multi-unit dwellings. So, although C/AS1 and C/AS2 require a 75m fire hose run from the hardstanding location, the Building Code doesnt have any such performance criteria for fire hose coverage for Dwellings and within household units of Multi-Unit Dwellings.
Even better, since a Final Exit is not located immediately outside a building, the external escape route between the building and the Safe Place would meet the definition of an Exitway as it is protected by distance or fire rated construction. NZBC F6 and F6/AS1 require Exitways to be provided with emergency lighting. There is a limit of application in NZBC F6.3.1 that exempts F6 from applying WITHIN household units of Mult-Unit Dwellings, but there is no such exemption outside of household units of Multi-Unit Dwellings. Therefore; this would imply all external escape routes from Multi-Unit dwellings require emergency lighting from unit doors, over footpaths, carparks and driveways until they reach a Safe Place.
The best snag is the measurement of escape route lengths on upper floors in Risk Group SM compared to Risk Group SH. There is no exemption from the upper floors in household units in Risk Group SM from being treated as intermediate floors, therefore the escape route length on upper floors within the same firecell in Risk Group SM is factored by 1.5. This makes achieving a complying escape route length in Risk Group SM very difficult without a NZS4512 or NZS4514 smoke detecting fire alarm or NZS4515 sprinkler system. Risk Group SH is of course silent on how to measure escape route lengths on upper floors, as Risk Group SH is silent about even having upper floors.