To be honest I don't see the point in extending. Once you've spent the money, you may as well have moved as its not always necessarily cheaper to extend. You'll never get your money back if you wanted to sell either as it'll be the same house just a bit bigger in the same location.
Sorry my friend but that is nonsense on so many levels. If your house is in a desirable area and you extend sensibly you will not only get the money back but you will add value.
It all depends on what sort of house in what sort of area you are talking about. If are talking about spending £30k on a £75k terraced house in the North then you might well be right.
However you could easily spend £250k on some houses and add loads of extra value.
In my own case I love my location, I don't want to live anywhere else except the very street I'm on. I just want a bigger house and so will (and have) extend.