I can't speak for the middle classes (my lot are mostly dead common) but I have found some shockingly overcrowded working-class households on the 1921 - 9 or 10 in a supposed 3-roomed house (kitchen not counted, I assume).
More relevant for you, perhaps, is the house I found where there are (all on one page so squeezed in) Twelve people in Four households in a six-roomed house.
I mustn't give the 1921 page, but the households are:
Single man, 51, broker and general dealer, works from home (where does he find the space?)
Married couple, 38 and 34, he also "Head", copper tube sawyer.
Widow 46, with her daughter, 14, son 9, and grandson,1. Not called "Head" only "Wife" but she's clearly head of her own household because she is not related to the others, no occupation.
(After a line across) Man 60, wife 45, two stepsons 17 and 14, son 2. He "fettler".
Perhaps the widow is the single man's partner, but why below the other couple except to confuse? She's no relation to anyone in the other households - I checked MMNs of her children, though no marriage found to their father. Likewise, the grandson is a son of her illegitimate daughter using her married name - but again, no marriage found corresponding to his birth.
The final couple are not married either, presumably because she is still legally married to the father of her elder sons (these are my relatives). Her husband is elsewhere, having remarried presumably because his wife vanished a decade earlier - not found on the 1911.
The only conclusion that make sense is that each household is living in one of the four bedrooms - so four and five to a room in two cases. This in the century I remember.