SK53
Generally use building=X for its construction reason. Mews were not built purely as housing. By all means add building:use=house or residential (see https://wiki.openstreetmap.org/wiki/Tag:building%3Dapartments#Converted_Buildings_used_as_Apartments). Reason for this is that every converted pub or church would then be building=house or building=apartments. A decent typology of buildings requires discrimination of classic types, and mews are a very distinctive form. building=house is already far too generic: typically in UK we might want to see a minimum of terraced houses, semis, bungalows, detached houses, townhouses (in modern UK sense). UKMap, a commercial firm, have a good typology so I know this works, but obviously cant refer to any of this for OSM