Green fields equivalent to double the size of Milton Keynes will need to be used to meet the government’s housebuilding pledge, according to one of the country’s leading urbanists who has said planners should take “confident bites” out of the green belt. The demand for land resulting from Labour’s promise to build 1.5m homes in its first term could be even higher if previously used brownfield land is not exploited, according to David Rudlin, an expert in new towns who has been in talks with the new government. If Labour stayed in power for two terms, land the equivalent of one-and-a-half Birminghams would be taken up to build 3m homes even if 60% of homes were built on previously used land, he said. If no previously used land is found, that rises to three-and-a-half Birminghams. The figures reveal the scale of the challenge facing the government as it attempts to boost housebuilding and tackle the housing crisis in the face of voters’ resistance to building on green fields in their own areas. The last government’s attempts to loosen planning rules to build more homes on open land fell foul of nimby (not in my back yard) rebellions. Rudlin and his team were awarded the £250,000 Wolfson economics prize for their vision of a new kind of garden city in 2014. He is now calling for Labour to switch from the development of car-reliant housing estates spread around the edges of towns by private developers to a more strategic approach in which local councils decide on the best sites and “take ‘confident bites’ out of the green belt by allocating a few large urban extensions”. Each would have about 3,000 to 5,000 homes, dozens of shops at subsidised rents, and schools, creating a sense of a unique place. They would be surrounded by open land or woods so people in the existing neighbouring settlement would not feel they were being overwhelmed by sprawl. “[The green belt] shouldn’t be immutable,” Rudlin said. “[We need] the ability to loosen it occasionally, to redraw it. It shouldn’t be abandoned or eaten away.” On Monday, the chancellor, Rachel Reeves, announced changes to the planning system to boost housebuilding, including the restoration of mandatory local housebuilding targets and the relaxation of restrictions on building on parts of low-quality green belt, which Labour has termed “grey belt”. Green belt is land protected under planning rules to prevent urban sprawl, whereas green fields are open land. The government said half of all homes on such sites should be classed as affordable (typically 80% of market rent or less) and it would require plans to improve existing green spaces and create new ones. County councils immediately pushed back, saying “housing targets should not be overwhelmingly allocated, or re-allocated, to county and rural areas” with a warning about excessive pressure on roads, schools and health services. Hinting at disputes to come, the Conservative Wiltshire councillor Richard Clewer, housing and planning spokesperson for the County Councils Network, said national targets “cannot be an effective substitute for local decision-making” and “there should be a fair distribution of housing across England”. As part of its housing push, Labour has pledged to build a generation of new towns with “beautiful buildings and tree-lined streets” and previously said a new town commission would be set up within six months of an election win, with a list of sites decided on within a year. The last successful new towns initiative stalled in the 1970s after the construction of places such as Stevenage and Milton Keynes. Attempts by Gordon Brown and David Cameron to build, respectively, ecotowns and garden cities largely failed. But Rudlin said any new towns were unlikely to be anywhere near the scale of Milton Keynes, which has grown to 287,000 residents over the past half century. “It is no longer possible to build a garden city from scratch,” he said. “We just don’t have the ambition to build another Milton Keynes, so we end up proposing free-standing ‘garden cities’ that are small and unable to support local facilities.” Nicholas Boys Smith, the interim chair of the government’s Office for Place, also said that “very often” new towns should be urban extensions. “New towns should be in areas of high demand [and] they should make use of existing, possibly enhanced, infrastructure such as trains,” he said. Boys Smith runs the charity Create Streets, which last month published a “new towns code” for Labour’s use. The code suggested designs with Victorian- and Georgian-style mansion blocks and terraces, which deliver more homes per hectare than current volume housebuilders normally do. “These scenarios can deliver more homes on less land than the conventional volume housebuilder model, thereby reducing the impact of development on the environment,” it said.