I have never encountered this scenario, but will go ahead and get the discussion going… Does your client have a formal “lease agreement” that shows a fixed amount for rent, utilities, groceries, etc. (as opposed to a % of the items, or a fluctuating amount month over month)? If there is a true separation of participation (that can be proved) you may be able to argue your point to exclude the others (think of it as a stand alone apartment that happens to be located in a house). However, my guess is that there is never a true separation and you will need to include the household income.
I suspect that the others will be unwilling to share their income amounts. You will likely have to use a default calculation based upon the number of occupants with income in the home (ex. 2 other folks plus your client, each contributes 33.3%. 3 other people 25%, etc) and explain the inability to have accurate numbers.
Hopefully, someone who has dealt with this before can chime in and confirm or correct my contribution.
Have a great day,