The Conservatives have come under fire after David Cameron pledged to give people the chance of a “good life” by extending Margaret Thatcher’s right-to-buy scheme to 1.3 million families in housing association properties.

The Confederation of British Industry and the blue chip Jones Lang LaSalle (JLL) property firm joined many housing associations, warning that the Tory plan would not address the chronic shortage of housing. The mayor of London, Boris Johnson, said last month that the proposal would involve massive subsidies.

Adam Challis, head of residential research at the Chicago-based JLL, which dates back to the 18th century, said: “The expansion of right to buy may be good politics, but represents terrible policy. This is exactly the kind of short-termist thinking that the country’s 4.7m households in social housing don’t need, not to mention the same number again of aspiring owners in private renting.

The sharply worded criticism came after the prime minister sought to revive the Tories’ election campaign, which has been criticised for being overly negative, by giving tens of thousands of housing association tenants a discount to buy their home.

Looking straight into the camera at the launch of the Tory manifesto in Swindon, where he promised on 10 occasions to help people lead a good life, the prime minister said: “So let the message ring out from this generation of Conservatives: you’ve worked hard, you’ve saved, the home you live in, it’s yours to buy, yours to own – the dream of a property-owning democracy is alive – and we will fulfil it.”

In the biggest fresh announcement of the day, the Tories pledged to double to 30 hours a week free childcare for working families. The proposal, which will be available to working parents of three- and four-year-olds, will represent a saving up to £5,000 a year. This exceeds Labour’s offer of 25 hours a week.

In an attempt to answer criticism that the Tories champion the rich, the prime minister rebranded the Conservatives as the “workers’ party” as he pledged to legislate that workers on the minimum wage would be exempt from paying income tax. It will only apply to those working no more than 30 hours a week.

The Tories scrambled to shore up their housing pledge after warnings that the policy was unworkable. Gavin Smart, the deputy chief executive of the Chartered Institute of Housing, warned that the proposal could make life more difficult for people on lower incomes, adding: “We fear the figures simply won’t stack up.”

David Cameron launches the Tory election manifesto. Guardian

The National Housing Federation said the policy could cost up to £5.8bn a year because compensation would have to be paid to housing associations for forcing them to offer housing stock to tenants at below-market rates.

Under the Tory plans, councils would be forced to sell off their most valuable homes, to be replaced by affordable housing in the same area. The funds from the proceeds of the council house sales would be used to pay for the extension of the right-to-buy scheme to housing associations. A further £1bn from the sales would be used by the Tories over four years to prepare brownfield sites for house building, with the aim of creating 400,000 extra homes.

Senior Tory sources said the £5.8bn cited by the National Housing Federationn assumed that everyone took up the option to buy their housing association home immediately. “That is not going to happen,” said one source, who pointed out that the policy had been costed by officials in the Department for Communities and Local Government and in the Treasury.

The Tories said around 15,000 council properties that are in the top third price bracket – around 0.4% of the stock – become available every year. Under the new rules, councils would be forced to sell those homes, raising £4.5bn net a year. This would be used to fund the building of cheaper, replacement properties.

“That is more than enough to pay for one-to-one replacement of the council houses sold, discount for the right-to-buy housing association property, one-to-one replacement for the right-to-buy homes and this £1bn over four years fund to deal with brownfield sites,” the source said.

The Tories insisted they were not stoking housing demand and rejected Labour suggestions that it would add £4bn to the housing benefit bill. The source said: “This is going to reduce housing benefit bills because it is going to increase housing supply, it is going to reduce waiting lists for social housing. This is a massive housing supply policy. You sell a £1m house in London and you can build a dozen new houses.”