There are a lot of people who object to the use of Greenfield sites for new housing. Although, when asked, the CPRE don't know what proportion of the UK is built on (therefore how much it would be if increased) the figure is in the region of 6% - 8%.
So, even if we doubled the amount of built-on land, the amount of open land would go from (about) 93% to (about) 86%. Some might say that the amount of open space does not need to be kept so high to the cost of those needing homes, and to the benefit of those who already have them.
Joke:
Q: What's the difference between a Developer and a Conservationist?
A: A Developer is somebody who wants to build a house in the country. A Conservationist is somebody who's already got one.