It's an odd definition of "middle class" that includes a place in the country and an acre of roof-space. Maybe it's just because I'm a dirty sooty northerner but, I've always assumed myself to be (just about) middle class and yet I live in a flat. My parents definitely are, and it's true that their house is quite large, but it also has eight people split across two families living in it.
The definition of middle class I see bandied about in the media seems very southern-centric. Up here, if you're so rich you can afford a country pile, you're a toff and possibly a banker, whereas it seems the home counties would consider you working class if your yacht is shorter than 110 feet and you only have one land rover.