http://www.legislation.gov.uk/id/asp/2013/11/schedule/4/enacted

SCHEDULE 4Relief for certain acquisitions of residential property

PART 4Acquisition by property trader from individual where chain of transactions breaks down

10Partial relief

Where qualifying conditions (a) to (g) but not (h) are met, the chargeable consideration for the acquisition is taken to be the amount calculated by deducting the market value of the permitted area from the market value of the old dwelling.