Green belt land may have been torn up for housing unnecessarily, campaigners believe, after the 2021 census suggested population growth in many areas has been overestimated – in some cases by tens of thousands of people. But the census also revealed other estimates were far too low – by up to 16% – meaning local politicians now face pressure to allocate more land for homes than previously anticipated. The discrepancies matter because official population estimates are used to project housing need and building targets. They are likely to be seized on by campaigners for both more and less new housing. About 340,000 new homes are needed each year in England, according to one estimate, while 216,000 were built in 2019-20, the last year for which full figures are available. In Coventry, the ONS had estimated the population at 379,387 but the census recorded only 345,300 people – a 34,000 difference. Campaigners said this meant some of the recently built housing around the city was in effect being planned for “ghosts”. “The best remaining pieces of unspoiled Arden landscape in Warwickshire have been sacrificed for no good reason,” said Merle Gering, a campaigner. “There was enough brownfield land in the city to cater for all the likely growth to 2031.” Guildford was estimated to be home to 150,352 people in June 2020 but the census showed it had almost 7,000 fewer the following March. The council had based a housebuilding target of 562 homes a year on ONS population projections, carried out before the census. “My concern is that large areas of the green belt around Guildford have been allocated for development based on inaccurate and misleading statistics,” said Niels Laub, a member of the Guildford green belt campaign group. “Housing targets should be reappraised.” The census will also increase pressure for higher housing targets in some areas. Before the population count was published, Cambridge city council and South Cambridgeshire district council launched a joint plan to build 33,500 additional homes by 2031. But the census showed that the 2020 ONS population estimate for Cambridge city of 125,063 was 20,637 people short, suggesting many more homes may now be needed. The council said: “It is vital for more good quality, sustainable housing to be built in the area, which is why this continues to be a priority for both councils as they prepare their next joint local plan.” The ONS estimates for Peterborough also missed more than 13,000 people, and in Oxford more than 10,000 people were missed. A spokesperson for ONS said it expected differences between the census and its estimates and that it would “rebase” its figures using the census data. It said it was also planning a new approach using different data sources. “The population continues to change and we recognise the need to understand those ongoing changes in a more timely and frequent way than ever before,” they said. In 2020, MPs in Warwickshire called for an inquiry into the ONS estimates, telling Sir David Norgrove, the chair of the UK Statistics Authority, that “bad decisions – to irrevocably destroy historic countryside – are being made on the back of bad data.” In 2021 the Office for Statistics Regulation concluded: “Population estimates for some cities such as Coventry did seem to be inconsistent with, and potentially higher than, local evidence would suggest.” The same problem was seen in “a number of smaller cities with large student populations”, it said.