Explain?
If all house prices fall they will only be in the same situation, as their current house price would have fallen, and so would the house price of the the one they are looking at. So they would still have about the same percent deposit for their new house, But they would have to take out a smaller mortgage as the prices would be less.
previous their house £200,000 wanting to buy a £400,000 house they still need £200,000 on a mortgage
new prices their house £150,000 wanting to buy a £300,000 house they now only need £150,000 mortgage so they are better off
It works on a sliding scale, so the people at the lower end of the run gain the most. And the people who can afford to lose a little more when upgrading can do.
But their house in the future may become more accessible to first/second time buyers, and making it easier to sell when the price is more realistic.
The part you've forgotten is the size of their existing commitment on their current house, because very few trade ups are done with a fully paid of mortgage. The numbers suggest we're looking at taking house prices back to mid 90's levels, a very big drop.
To use your example a bit more.
House value £200k, Outstanding mortgage £100k, new house £400k.
Equity in house £100k, remainder to be found £300k
After drop
House value £150k, outstanding mortage £100k, new house £300k
Equity in house £50k, remainder to be found £250k
still looks ok so far?
Now lets look at the sort of figures we're actually talking as a drop in average house prices. I'll use the most generous figure from earlier (the 2.5 times joint average income), the £200k price is close enough to the peak price for now.
House value at peak £200k, outstanding mortgage £100k, new house £400k
Remainder needed £200k
House value after drop £81997, outstanding mortgage £100k, new house £163k
Equity in house = -£18003, remainder needed = £181003
Suddenly, we see the crux of the issue, the amount that needs to be found now exceeds the value of the new house, just to move, this person needs to find £20k, so they haven't benefitted at all from the change in house prices, in fact they've been significantly disadvantaged by it.
This is the reason why this sudden approach by the FSA is so stupidly misguided, it's completely the wrong time to attempt such a change in practice, and the likely consequence is to ensure we get a full blown depression rather than just a recession. Forget an economic recovery, no matter how much money is pumped in to other areas of the economy.