There isn't a legal guideline for private properties but there is for social housing. Another advantage of social renting compared to buying!
In our LA they absolutely don't count bathrooms and landings as bed spaces and I'd be surprised if anyone can find official guidelines where this is allowed. There are lots of inaccurate rumours about this and the best way to be sure is checking with your own HA/LA. Here they only count living rooms, bedrooms, dining rooms and the living part of a kitchen/living area (but not a kitchen which doesn't have a living area).
A floor area of 89sqft here is suitable for one person and up to 110 sq ft for 2 people. But children under 10 are considered as half a person. It's also considered to be overcrowding if the people over 10 are opposite sex and sharing a room (unless of course they're a couple).