The Conservative party has pledged to change planning rules to boost the number of affordable homes in the countryside – but would still open up the right to buy to rural housing association tenants.

The party’s rural manifesto says it will provide more affordable housing by extending the exception sites policy. Exception sites only get planning permission on condition that all the housing built on them is affordable and remains so in perpetuity.

Shadow agriculture secretary Jim Paice said the Conservatives would amend planning guidance to promote the reuse of former agricultural buildings for commercial purposes or for housing. “There is little doubt that with the current changes there will be many redundant farm buildings,” he added.

To tackle what the party sees as “over-development”, it will abolish the regional planning system. Paice added: “We will give local communities a much stronger voice in deciding the right level of development in their areas.”

However Justin Roxburgh, chief executive of Falcon Rural Housing Association, said: “There is a conflict between the exception sites policy, which should secure affordable housing in perpetuity, and the right to buy. Most rural units lost under the right to buy in the last 15 years have not even been replaced yet. Introducing right to buy to housing association units will be the end of it.”