The Conservative Party manifesto will be launched today, with a headline pledge of extending Right to Buy to housing association tenants.

The extension of the flagship Thatcherite policy is expected to see housing association tenants qualify for a discount on buying a housing association property that will be capped at just over £102,700 in the capital and £77,000 for the rest of England.

However, Ruth Davison, director of policy for the National Housing Federation, told Radio 4’s Today programme that the organisation had previously researched such a move and come up with a conservative cost of £5.8bn, as housing associations would have to be fully recompensed for any shortfall. However, she added that briefings in today’s newspapers said it could cost £20bn of taxpayers’ money.

“It won’t help the millions of people in private rented homes who are desperate to buy but have no hope of doing so, nor the three million adult children living with their parents because they can’t afford to rent or buy,” Davison told the Guardian.

“To use their taxes to gift as much as £100,000 to someone already living in a good quality home is deeply unfair. Little wonder then that 60% of the public believe that it would be unfair for social housing tenants to get a discount to buy their home while private renters do not.”