Well we were given a 4 bed house in April 2012 because the local authority accepted the medical evidence (and we have plenty) that our children cannot share.
The same local authority tried to then make us liable for £31 a week rent because of 'bedroom tax' in April 2013.
So why is our house 'too big' after being medically necessary a year earlier? (And we waited YEARS for this house).
It was only 3 weeks before the tax started that parliament made the concession that if you have the evidence we do you don't have to pay it. After six months of hell working out which therapies to cancel so the kids DLA would have to pay bills we no longer could because we were paying the shortfall from our carers allowance/income support.
I hate to say it but most situations are like ours. Authorities haven't housed people in inappropriate units for quite some time, the space has to be deemed necessary. Apart from extra rooms for disability the second biggest group is pensioners who are in 3 bed homes on their own, and they're exempt.
The number of single people of working age without disability in a house too big for them is really quite small. And even then there's nowhere for them to actually move to.