One thing I noticed in reading a Council's
HMO regs recently was that providing communcal space allowed smaller rooms to be let out.
And, it got complicated as it varied on what the communal space was! Kitchen. Kitchen/dining. Kitchen/living. etc. The allowable room space dropped as more communal space was available.
Also what was the single and couples size was listed separately, so
by providing a living room you could find you could let some of the middling sized rooms to couples, and not just to singles. Which obviouly increases rental incomes.
I recently lived in an
HMO, (call it research!), and I noticed the kitchen/dining/living room were used early evening for eating, and rarely outside that time. In my opinion providing a bigger kitchen dining area is more important than providing a living room - people tend to relax in their own rooms.
I've still not worked out what to do with a tiny box room though - I've seen shared computing facilities provided, but I'm more convinced that offering the box room with another room to a couple for higher rent is more useful!