In England, it's over 9% if you take out gardens. And I wasn't including parks and playing fields in 'built up'.
And gardens aren't countryside. It's the countryside, our productive agricultural land, that is being built over. A couple of years ago our local FB page had a picture of a combine taking the last crop off a large field that is now being built all over. The last crop, ever, from a piece of land that had probably been in production for over a thousand years.
And with an increasing population comes not only houses but huge energy projects (like the massive pylons planned to built across East Anglia, complete with huge substations and so on), road widening and road building schemes, more hospitals and other community facilities, more warehousing.
As a nation, we need a serious discussion about whether we want to keep on increasing our population (and reducing our food security and putting more pressure on our water supply), or go through the economic pain now of dealing with an aging society so that we come out ahead when the Ponzi scheme of an ever-increasing global population finally grinds to a halt.
'Build more houses' is a facile answer to a very complex issue.