IHTM09732 | Related property: how the rules work
From HM Revenue & Customs · Inheritance Tax Manual
Under IHTA84/S161,
you calculate the ‘appropriate proportion’ (IHTM09735) of the value of that combined unit to give you the value of the property in the deceased/transferor’s estate.
Example
T owns 40 out of the 100 issued shares in X Ltd. T’s spouse owns 30 of the remaining shares. These shares are related property. Valued on its own T’s holding is worth £8,000. The value of the combined holding of 70 shares for Inheritance Tax (IHT) purposes is £28,000. Under the related property rules, the value of T’s holding for IHT purposes is not £8,000, but four sevenths (the appropriate proportion) of £28,000, that is £16,000.