How Do Mirror Wills Affect Inheritance Tax?

Mirror wills and inheritance tax interact in a specific way: leaving everything to your spouse through a mirror will does not remove the tax, it postpones it. The spousal exemption carries the whole estate across the first death tax-free, and the bill lands on the second death instead. Structured correctly, a couple can pass up to £1 million to their children before any tax is charged, because both partners’ allowances are available on that second death. The catch is that the £325,000 nil-rate band and the £175,000 residence nil-rate band are frozen until at least April 2030,¹ so property price growth eats into the shelter every year.

Why the Tax Bill Waits Until the Second Death

Mirror wills are two near-identical wills, one for each partner, each naming the other as the main beneficiary and the children (or other chosen people) as the beneficiaries after both partners have died.² Because the first partner’s estate goes to a surviving spouse, Section 18 of the Inheritance Tax Act 1984 applies. Transfers between married spouses and civil partners are exempt from inheritance tax with no upper limit.³

No tax is paid on the first death, however large the estate. The full liability crystallises when the surviving spouse dies and the estate passes to the next generation. That is the point at which allowances are measured, the estate is valued, and 40% is charged on anything above the threshold.⁴

Two boundaries are worth flagging before going further. The spousal exemption applies to legally recognised marriages and civil partnerships only. Long-term partners who have not formalised the relationship do not get it. And where one spouse is a long-term UK resident and the other is not, the exemption is capped rather than unlimited, which can create a tax charge on the first death for couples with mixed residency status.³

The Transferable Nil-Rate Band

Every person has a nil-rate band of £325,000, frozen at that level since April 2009 and set to stay there until at least April 2030.¹ When a mirror will leaves everything to the surviving spouse, the spousal exemption covers the whole transfer, so the first partner’s £325,000 allowance is not needed and sits unused.

Section 8A of the Inheritance Tax Act 1984 allows that unused portion to transfer to the surviving spouse’s estate.⁵ The transfer is a percentage, not a fixed sum: if the first spouse used none of their allowance, 100% moves to the survivor and the second estate has a threshold of £650,000. Executors have to claim this. They need documentation of the first death and how that estate was distributed. If nobody claims it, the extra allowance is lost.

The Residence Nil-Rate Band and the £1 Million Figure

A second allowance of £175,000 per person, the residence nil-rate band, applies when a home is left to direct descendants: children, grandchildren, or stepchildren.¹ It is also frozen until April 2030. In a typical mirror will arrangement, the home passes first to the surviving spouse under the spousal exemption, then to the couple’s children on the second death.

Like the standard nil-rate band, this allowance is transferable. When the first spouse’s residence nil-rate band goes unused, the survivor’s estate can claim both, giving up to £350,000 of extra shelter on the family home.⁶ Add the two nil-rate bands (£650,000) to the two residence nil-rate bands (£350,000) and you reach the £1 million figure often quoted as the combined threshold for a couple.

This only works if the property ultimately reaches direct descendants. Leaving the home to a sibling, a niece, or a friend disqualifies the residence nil-rate band entirely, and the couple’s shelter drops back to £650,000.

The £2 Million Taper

The £1 million ceiling is not available to every couple. Estates worth more than £2 million lose the residence nil-rate band at a rate of £1 for every £2 above the threshold.⁵ A surviving spouse whose estate contains the couple’s combined £350,000 in residence nil-rate bands starts losing that shelter once the estate crosses £2 million, and can lose it entirely at higher values. For couples in that bracket, the mirror will still defers tax to the second death, but the £1 million figure is not what they should be planning around.

Working the Numbers on the Second Death

Take an estate worth £1.2 million on the second death, where the couple were married, the first spouse used none of their allowances, the home passes to the couple’s children, and the total estate is below the £2 million taper. Both sets of allowances are available.⁵

Combined nil-rate bands: £325,000 + £325,000 = £650,000
Combined residence nil-rate bands: £175,000 + £175,000 = £350,000
Total tax-free threshold: £1,000,000
Taxable amount: £1,200,000 − £1,000,000 = £200,000
Tax at 40%: £80,000

That £80,000 comes out of the estate before the children see anything. Push the estate above £2 million and the arithmetic gets worse quickly, because the residence nil-rate band taper shrinks the tax-free portion at the same time as the taxable portion is growing.

Bringing the Bill Down

Mirror wills only decide what happens on death. The size of the estate at that point is shaped by what the couple does during their lives, and a few well-established reliefs sit alongside the will.

Annual Gifts and the Seven-Year Rule

Each person has an annual gift exemption of £3,000. Gifts up to that amount in a tax year fall outside the estate. Any unused portion of the annual exemption can be carried forward one year, and no further.⁷

Larger gifts are potentially exempt transfers. Survive seven years after making the gift and it leaves the estate entirely. Die within seven years and the gift is added back for tax purposes, though taper relief reduces the rate on the gift itself after the first three years:⁸

0 to 3 years before death: 40% (full rate)
3 to 4 years: 32%
4 to 5 years: 24%
5 to 6 years: 16%
6 to 7 years: 8%

Taper relief applies only to tax on the gift, not to tax on the rest of the estate. For couples close to the £2 million taper on the residence nil-rate band, giving away assets earlier rather than later can be worth serious thought, because it may keep the survivor’s estate below the point where allowances start disappearing.

The 36% Charity Rate

If at least 10% of the net estate is left to charity, the inheritance tax rate on the rest of the estate drops from 40% to 36%.⁹ On a large taxable estate the four-point cut can save the beneficiaries more than the charitable gift costs them. An estate with £500,000 above the nil-rate band saves £20,000 in tax by switching from 40% to 36%, so a £50,000 charitable legacy costs the family £30,000 in real terms while sending £50,000 to the charity.

Getting the 10% test right on the second death, when the calculation actually happens, is not straightforward. The net estate for this purpose is the value after debts and allowances but before the charitable deduction itself. Drafting help is worth having.

One Thing Mirror Wills Do Not Do

Mirror wills carry no promise between the partners. After the first death, the surviving spouse can rewrite their will completely: disinherit the children, leave everything to a new partner, redirect the estate to anyone at all. Nothing in a mirror will stops this.² Couples who want the ultimate beneficiaries protected by law, rather than by trust, need mutual wills or a trust arrangement instead. That is a separate question from tax, but it changes whether the tax plan actually reaches the people it was designed for.

1
GOV.UK. Inheritance Tax Thresholds and Interest Rates
2
Cornell Law Institute. Mirror Wills
3
Legislation.gov.uk. Inheritance Tax Act 1984 – Section 18
4
GOV.UK. How Inheritance Tax Works: Thresholds, Rules and Allowances
5
GOV.UK. Inheritance Tax Nil-Rate Band, Residence Nil-Rate Band From 6 April 2028
6
GOV.UK. Work Out and Apply the Residence Nil Rate Band for Inheritance Tax
7
GOV.UK. How Inheritance Tax Works: Thresholds, Rules and Allowances – Gifts
8
Legislation.gov.uk. Inheritance Tax Act 1984
9
GOV.UK. Inheritance Tax Reduced Rate Calculator