The term ‘usual domestic offices’ is not. I believe, in common currency in the 21st century. Today, it might be construed as meaning a utility room or perhaps a home-working studio or nook for the self-employed. Before the mid-20th century, most people would have taken it to mean the place where the work of the home was carried out: the kitchen, back kitchen or scullery, pantry, brewhouse and dairy. In the days before labour-saving devices when many people employed a servant or daily help, the operations needed to prepare food and drink were often time-consuming and repetitive tasks requiring considerable effort and labour, and these rooms were where they were carried out.
Elsewhere in the house, the equivalent of today’s ensuite bathroom would be a wash stand, bowl and a thin towel for morning ablutions, and a bath tub in front of the fire periodically for a ‘proper soak’ and all over wash. Hot water was heated in a copper (a metal pot encased in brick with a fire beneath it) and carried in pails through the house.
Of course many people had to make do with one or two rooms in which all household tasks were carried out that could not be done outdoors. For them, the functions of the kitchen, dining room and drawing room of the wealthier, plus the nursery if there were young children, would be confined to a single parlour.
In 1911, the census that year asked people to state the number of rooms in their house including the kitchen but excluding the scullery, landing, lobby, closet, and bathroom as well as warehouse, office or shop. In the two previous censuses of 1891 and 1901, respondents were asked to provide the number of occupied rooms in their property if less than five. The responses to these questions combined with the number of people in the household, occupation and location enabled greater demographic analysis of how people lived than had previously been possible.