My husband has passed away. In the inheritance tax return, the notary applies a 20% allowance to my late husband’s main residence, which I still occupy. Is this allowance mandatory?

Yes. Article 764 bis of the French Tax Code provides for a mandatory 20% allowance on the fair market value of the property that constituted the deceased’s principal residence at the date of death, provided that, on that same date, the property was also occupied as a principal residence by one of the listed persons, in particular the surviving spouse.