Hi
The link in the help guide to English censuses at the top of the London and Middlesex boards gives the guidance given to census enumerators for each census 1841-1901
For uninhabited property it is
'Where you have to insert an uninhabited house, or a house building, do this by writing in the fourth column “1U,” or “1B,” on a distinct line, taking care to omit none which are noted in your memorandum Book. When two or more houses, uninhabited or building, occur together, insert the total number, thus:-“3U,” “2B,” as the case may be.'
Regards
Valda