buildable in small houses. (not longhalls and towers) closets would appear to be as a door but open like a container. they would have the same or slightly more space than a cupboard would, and reguardless of what floor they were built on, the limit is one closet per house. \
like cellars, they wouldnt take up any space. and like cellars they would require a bit of metal to build, say a nugget or two for the doorknob? unlike cellars you need to put in materials rather than take them out. plenty of wooden boards and stone required to build yourself a closet.
ONLY clothes armor ect can be stored in the closets. no hand held or other random items.