Discount for assets deriving from another estate

I’m dealing with an estate which has in it (inter alia) the house of the
deceased’s late mother who died in 2003. She left no Will and her estate
passed to her only son, my deceased. It was never his home or his only or
main residence. No action had been taken by him to vest in himself the
ownership of the property, nor had he taken out a grant to her estate.

In the son’s estate we declared a value for the house at its full market
value at the date of the mother’s death, less a 15% discount for the fact
that it was still in another’s estate. The District Valuer is disinclined to
agree this and says he has not come across this. I cannot find chapter and
verse to quote to him. Am I right?

Julian Cohen, Solicitor