an institution that is the sole or main residence of at least 90 per cent of its residents (and does not fall within any of the descriptions below of buildings not treated as dwellings below)
sefydliad sy’n unig breswylfa neu’n brif breswylfa o leiaf 90 y cant o’i breswylwyr (ac nad yw’n unol â'r un disgrifiad isod o adeiladau na ystyrir fel preswylfeydd isod)