Close to half of affordable homes in some of the most expensive areas of England will not be built if ministers proceed with changes to the planning system, analysis by councils suggests. The government’s proposal to scrap the duty of developers to build affordable housing on sites for up to 40 or 50 homes, would have led to 30,000 of such homes going undelivered over the last five years, according to the Local Government Association (LGA). Some areas likely to be most affected are the least affordable and under greatest housing pressure, the cross-party grouping said. Elmbridge in Surrey, where the average house price is over £760,000, has 486 affordable homes either built, under construction or with planning permission over the past five years. This would be reduced to 271 if the proposed 40 or 50-unit threshold were introduced, the LGA said. Lewes district council in East Sussex could lose up to 37% of its affordable homes, based on past trends. Council leaders in Cornwall have complained the change could result in 300 fewer affordable homes in the county every year. The warning comes as several Conservative councils voice objections to the planning changes, which they also complain will limit local power over developments. Robert Jenrick, the housing secretary, defended the plans in an interview with the Guardian, telling rebellious shire Tories their party had a “moral mission” to build more homes. The government admits lifting the threshold at which affordable housing must be included from sites with more than 10 homes to those with over 40 or 50 could cut affordable housing delivery by between 7% and 20%. But its consultation on changes to the planning system states: “We anticipate that raising the threshold would make more sites viable for [small and medium-sized] developers and would increase the pace of their delivery as the need for negotiation would be removed.” The LGA’s housing spokesperson, David Renard, who is the Conservative leader of Swindon borough bouncil, said the proposals were “of huge concern”. “We need to build homes that are affordable to local people and help to reduce homelessness, rather than contributing additional funds to developers’ and landowners’ profits,” he said. “These current proposals risk allowing developers to game the system by only putting forward schemes for fewer than 40 or 50 homes, and so avoid building any affordable homes at all.” Karen Randolph, portfolio holder for planning at Elmbridge, said: “We are strongly opposed to this approach. There is a significant need for affordable housing in Elmbridge, which coupled with limited land supply and reliance on small sites, means that every opportunity must be taken to secure affordable housing.” She said the average size of the development site in the borough was 0.11 hectares – around a quarter of an acre. William Meyer, the Liberal Democrat cabinet member for housing at Lewes district council said: “We have massive land shortage in Lewes and if small sites are going to be excluded it will make a difficult situation worse.” Lewes town is in the South Downs national park, which has a rule that half of any developments of more than 10 homes must be affordable. But the change would affect other settlements in the district outside the park boundary, including Seaford and Newhaven. Meyer said building council housing rather than relying on private developers was the answer and that a cut in March to the cost of central government loans for that purpose had helped. A spokesperson for the Ministry of Housing, Communities and Local Government said the LGA was overstating the risk, because its proposal to lift the threshold for affordable housing contributions was planned to only be in place for 18 months. They added: “Our proposals will bring in a new, simpler infrastructure levy to ensure developers pay their way and deliver at least as much, if not more, onsite affordable housing than today.”