PM paves the way for greenfield building The Prime Minister also backed the search for shale gas, which opponents claim could despoil large parts of the country. Mr Cameron insisted to MPs that more house-building and "fracking" to release shale gas were needed to provide for Britain's economic needs. He backed the argument of Nick Boles, the planning minister, for more building in the countryside. Mr Boles said nine per cent of England had been built on, and suggested it should be 12 per cent. Mr Cameron denied setting a target for developing rural land, but said Mr Boles was right in that it would be necessary to build on some green spaces. "He was making a point, that we are not building enough houses," the Prime Minister told the Commons liaison committee. If we want to build more houses, we will have to build on some greenfield sites. He said he was keen to be part of a shale gas "revolution," adding: "If we ignored it completely, you could be giving your economy much higher energy prices than is necessary."