THE last time I was in Edinburgh, I noticed a Georgian building with eight real windows and four blocked-up ones by their side. They turned out to be an architectural remnant from the notorious window tax introduced by William III in 1696, which slapped a charge of four shillings on properties with 10-20 windows, and twice that for those with more than 20. This daylight robbery seemed like a good idea at the time, so much so that it was repealed only in 1851.

Doubtless the current Coalition's "bedroom tax" seemed a good wheeze to David Cameron. Faced with an annual Housing Benefit bill of £21 billion, the Government has decided to "encour-age" council-house and housing-association tenants who have a spare bedroom to move to smaller houses, in order to free larger homes for larger families. Either that, or pay a bedroom tax - of an extra 14 per cent in rent on one spare bedroom and 25 per cent on two.

This would have the double bonus of getting big families out of cramped housing-waiting-list accommodation and/or slashing the amount paid out in benefits. And after all, the free-market ideologues argued, people in the private sector have to match where they live to what they can afford.

So, from April, Housing Benefit claimants who have more bedrooms than they reasonably need (which is estimated to be about one third of claimants) will either have to move, pay more rent, get a lodger, cut their spending, or earn more. If only life were so simple.

There are a number of problems with this. For a start, the definition of "under-occupying" , to use the bureaucrat-speak, means that siblings are not allowed separate bedrooms until the age of ten for brother and sister, or 16 if they are the same sex. The rule has no flexibility for areas of the country with higher unemployment, or where smaller houses are not available - which means that there is nowhere to downsize to, nor the means of earning the extra rent. In Chester-le-Street, for example, there are 600 people under-occupying, but only 41 one-bedroomed council properties. Stockton has 153 under-occupiers, and no one-bedroomed flats.

In Wales, of the 40,000 people affected by the changes, some 4000 will have to pay more or move into the more expensive private sector, as there is no smaller social housing. Charities and housing associations there are warning of an impending explosion in homelessness among families who already have difficulty making their paltry weekly budget balance. Telephone advice hotlines are taking hundreds of calls a week.

The truth is that no one is sure how this will play out, which is why one Labour politician has called it a "grotesque experiment" and "a callous piece of public policy", which will put people into debt and fracture communities. It is hard to disagree.

No exemptions have been made for the disabled, apart from those who need an extra room for a dialysis machine. Foster carers have found that foster children are not counted as part of the household for bedroom entitlement; the Government has said they can apply for a share of the £5 million extra discretionary housing funding from their local council, but carers say that councils have been using the money for other purposes. The rules also ignore the needs of divorced parents with part-time access to their children.

The news that David Cameron has appointed the controversial Australian political consultant Lynton Crosby to run the Conservatives' 2015 General Election campaign has brought claims that the Tories will once again become "the nasty party". Those poor people about to be hit by the bedroom tax may feel that they already are.