IHTM33111 | Loss on sale of land: adjustments to sale price: introduction
From HM Revenue & Customs · Inheritance Tax Manual
In straightforward cases the relief for loss on sale of land operates by replacing the value of the property at the date of death (IHTM33100) with the sale price (IHTM33072).
But there are special rules that adjust the sale price
to ensure that the date of death values and the sale prices are compared on the same basis, and
These adjustments provide the sale value (IHTM33073) on which the relief is based.