Taxing problem

Vince Cable has this thing about a "mansion tax" doesn't he - keeps coming back to it.  Problem is - and this is a bit strange coming from a LibDem - that such a tax takes no account of the ability to pay.  You can live in a big house worth a lot of money, and still be hard up financially.  A better idea which I heard suggested the other day, would be to make the sale of property subject to capital gains tax on any gain of say over £100,000.  Not sure whether the amount it would raise would be worth the cost of administering it, mind.