A widow’s executors can nearly double her Inheritance Tax threshold with one form. The share can sit unclaimed for decades, because nothing was paid at the first death, and nothing being paid is the point: an estate left entirely to a spouse uses none of its allowance, and the unused share waits for the second death.
How the transfer works
GOV.UK says that if you are married or in a civil partnership and your estate is worth less than your threshold, any unused threshold can be added to your partner’s threshold when you die (GOV.UK, How Inheritance Tax works: thresholds, rules and allowances: Overview, checked 1 November 2026). The threshold is £325,000, and the home addition of £175,000 transfers on the same basis.
A husband who leaves everything to his wife pays no tax, because a transfer to a spouse is exempt, and uses none of his threshold. His widow’s estate then has her £325,000 and his, and the same again for the home allowance if the house goes to the children. Our Inheritance Tax hub covers the thresholds themselves.
It is a percentage, not a sum
The share that passes is a fraction, applied to the threshold in force at the second death. GOV.UK’s guidance on the home allowance says it is the unused percentage that is transferred, not the unused amount, so that if the maximum increases over time the survivor’s estate benefits from the increase (GOV.UK, Transferring unused residence nil rate band for Inheritance Tax, checked 1 November 2026).
The £325,000 band works the same way. A husband who died in 2005 leaving £52,000 to his children, when the threshold was £275,000 (GOV.UK, Inheritance Tax thresholds and interest rates, checked 1 November 2026), used 18.9% of it, so 81.1% transfers. Applied to today’s £325,000 that is £263,575 for his widow’s estate, not 81.1% of the old £275,000. The percentage is what the form asks for.
No time limit on the first death
HMRC’s manual says the transfer may be claimed where the surviving spouse died on or after 9 October 2007. For married couples the first death can have occurred at any time, including under the old estate duty or capital transfer tax, as long as there was unused nil rate band (HMRC Inheritance Tax Manual, IHTM43001, checked 1 November 2026).
For civil partners the first death must be on or after 5 December 2005, when civil partnership became law. A first death in 1980 still counts for a married couple; the executors need the marriage certificate, the first death certificate and, if there was one, the grant and the tax paperwork from then.
The home allowance transfers from before it existed
The residence nil rate band began on 6 April 2017, and a spouse who died before then could not have used it. GOV.UK says that where the first of the couple died before 6 April 2017, 100% of the allowance is available for transfer unless their estate was worth more than £2 million (GOV.UK, Transferring unused residence nil rate band for Inheritance Tax, checked 1 November 2026).
The first spouse did not need to own a home. GOV.UK’s form IHT436 is for an estate claiming the residence allowance where the deceased’s spouse died before 6 April 2017, or on or after it without using all of theirs (GOV.UK, Claim transferable residence nil rate band (IHT436), checked 1 November 2026). The survivor’s own home, left to direct descendants, is what the allowance attaches to.
The claim: forms and the two-year clock
GOV.UK says to use form IHT402 with the IHT400 account to transfer any unused threshold from the previously deceased spouse or civil partner (GOV.UK, Inheritance Tax: claim to transfer unused nil rate band (IHT402), checked 13 September 2026). The residence allowance is claimed on IHT435 with IHT436 for the transferred part.
HMRC’s manual says the claim must be made by the personal representatives within 24 months after the end of the month in which the second deceased died, or, if later, three months from when they first acted (HMRC Inheritance Tax Manual, IHTM43007, checked 1 November 2026). Its example is a death on 10 October 2014 with a deadline of 31 October 2016. A late claim can be admitted at an officer’s discretion, for reasons such as serious illness or events beyond the claimant’s control (IHTM43009, checked 1 November 2026).
Smaller estates: no full account needed
An estate using a transferred allowance does not always need the IHT400. GOV.UK says an estate is usually an excepted estate if it is worth £650,000 or less and any unused threshold is being transferred from a spouse or civil partner who died first (GOV.UK, checked 1 November 2026).
That covers two full £325,000 bands. An estate above £650,000 that relies on the home allowances as well sends the full account with IHT402, IHT435 and IHT436. Our probate forms page covers which account applies.
More than one marriage
A survivor who was widowed twice can claim from both, up to a ceiling. GOV.UK says the percentage of transferred residence nil rate band is limited to 100%, so that a claim from each late spouse cannot total more than 100% of the maximum available amount (GOV.UK, Transferring unused residence nil rate band for Inheritance Tax, checked 1 November 2026).
The Act caps the £325,000 band the same way: where the transferred percentage would exceed 100, the increase is limited to one nil rate band maximum (Inheritance Tax Act 1984, section 8A, checked 1 November 2026). So two full allowances, £650,000 plus £350,000 for the home, is the most any single estate can have. A divorce ends the entitlement; only a marriage that ended in death leaves an allowance to pass.
Before the second death
The transfer needs paperwork that is easier to find now than in twenty years. The first death certificate, the marriage or civil partnership certificate, the will if there was one, and any grant or Inheritance Tax forms from the first estate are what the executors will need to show what was used.
If the will leaves the home to the children, the residence allowance attaches; if it leaves everything to a niece, it does not, and our hub page sets out who counts as a direct descendant. The gifts page covers gifts made in the seven years before death, which use up the threshold first.