I think some of the problem is that people think houses = money.
If you can sell your house and buy a comparable one in the same area for less, then yes - you have disposable income.
If however, you can sell your 3 bed semi in Clapham and only afford to buy a 3 bed semi in Clapham with the proceeds then you don't profit and thus don't have disposable income.
We bought our house for 80k, it's now worth 180k - I don't have twice as much money, we still have the same car, our lifestyle hasn't changed - it's monopoly money & doesn't bear any resemblance to real life.
Some people are saying "move away then" - why? If you start taxing the poor out of certain areas then you start to create "ghettos" where only the poor live. "Financial Cleansing" of desirable areas is not the way forwards surely?
Rather than taxing those who live in an area that has suddenly become "fashionable" why not make it illegal to give a mortgage for more than 3 x income? Tax the shite out of second homes that are let, tax the shite out of holiday homes. Until more people have 1 home, why should some people have 2? That makes more sense to me.
If no one can afford to buy a house at stupid prices because they're limited to 3 x their wage them maybe overall house prices will drop. Before people start crying about "negative equity" - that only applies if you sell your house. If more people lived in their houses instead of seeing them as "get rich quick" schemes things would be tidier all round.
There are bin men, cleaners & all sorts of public sector workers living in "mansions" in London - they're not rich, they just happened to be born there!!