The instinct is that a half share in a jointly owned house is worth roughly half the property's value, maybe a bit less to reflect that you can't sell it without the other owner's cooperation. For most co-owners that instinct is right, and HMRC's own valuation practice usually gives some credit for it. For married couples and civil partners, it's wrong — and the rule that makes it wrong catches a lot of estates that never saw it coming.
The rule: Section 161 IHTA 1984
The related property rules exist to stop an estate being undervalued simply because an asset has been split between two connected people. Section 161 says that where property in someone's estate is “related” to property held elsewhere — most commonly, held by their spouse or civil partner — the share in the estate isn't valued on its own. Instead, you value the whole combined holding as if it belonged to one person, and then apportion that value back in proportion to what each person actually owns.
The practical effect is that any discount you'd expect for owning only part of something, rather than all of it, disappears. If a house is worth £700,000 whole and a couple own it 50/50, the related property rule says each half is valued at £350,000 — not at whatever a buyer would actually pay for an undivided half-share bought from a stranger, which is usually less than half the whole.
Why an ordinary co-ownership discount doesn't apply here
Where two unrelated people jointly own a property — siblings who've inherited a house together, unmarried partners, friends who bought as an investment, business partners in a joint venture — HMRC's Shares and Assets Valuation practice generally accepts that an undivided share is worth less than its arithmetical proportion of the whole. A discount of around 10% is typical for a straightforward residential co-ownership, reflecting that a buyer of a half-share can't sell the whole property, can't guarantee vacant possession, and is buying into a relationship with a co-owner they didn't choose.
That discount is exactly what Section 161 removes when the other owner is a spouse or civil partner. The logic is that a married couple's combined interest behaves, economically, much more like outright ownership by one household than like two strangers thrown together by an accident of inheritance — so the legislation values it that way, whether or not that reflects what either spouse's share would fetch sold separately on the open market.
It isn't limited to a shared roof
The rule is often assumed to apply only where a couple jointly own the same house. It's wider than that. “Related property” covers anything comprised in a spouse's or civil partner's estate, of a similar description, where valuing it together with the deceased's property would produce a different figure than valuing each in isolation. Two buy-to-let flats in the same block, one held by each spouse, can be caught in the same way as an undivided share in a single title — and so, outside property entirely, can matching shareholdings in the same private company, where combining a couple's holdings might tip either of them across a control threshold that changes the per-share value significantly.
The rule also extends, on a five-year look-back, to property that has passed as an exempt gift to a charity, a political party, or certain national heritage or housing bodies. That branch matters far less often for a typical property-owning family, but it's part of the same section and worth knowing exists.
Severing the joint tenancy doesn't fix it
Between unrelated co-owners, how the title is held can matter to valuation and administration. Between spouses, it doesn't help here. Converting a joint tenancy into a tenancy in common, or restructuring how a share is documented, changes nothing about Section 161 — the rule is triggered by the relationship between the owners, not by the legal form of the co-ownership. As long as the surviving spouse or civil partner holds the related interest, the aggregated valuation applies regardless of the paperwork.
Where this actually bites
The rule matters most where it isn't expected. A couple who structured a portfolio so that each spouse personally holds a different property, on the reasonable assumption that keeping assets separate keeps their estates separate too, can find on the first death that the surviving spouse's matching or comparable property is treated as related and the aggregated value applied anyway. It also catches couples who've held a family home as tenants in common for straightforward inheritance tax planning reasons — splitting ownership doesn't achieve the fragmentation discount they may have been advised, informally, to expect.
None of this changes the total wealth that passes through the two estates over both deaths. What it changes is the valuation used on the first death, and therefore how much of the nil-rate bands and reliefs available at that point get used up.
The relief if related property is sold at a loss
Sections 176 to 178 IHTA 1984 provide some protection where the aggregated valuation turns out to be optimistic. If qualifying land that was valued under the related property rule is sold within three years of death for less than that valuation, the sale proceeds can generally be substituted for the higher figure, reducing the inheritance tax charged to reflect what the estate could actually realise. It has to be claimed, and the conditions mirror the separate loss-on-sale-of-land relief in Section 191, so it's worth flagging to whoever handles the estate administration at the point a sale is being considered, not after it's completed.
What to check before probate values go in
- Whether any asset in the estate is co-owned with a surviving spouse or civil partner, in the same or a comparable asset
- Whether the valuer preparing the IHT400/IHT405 figures has applied the related property aggregation, rather than a standalone co-ownership discount
- Whether unrelated co-owners (siblings, business partners, unmarried co-owners) have had a discount applied where one is genuinely available
- Whether a sale within three years of death might trigger the Section 176 relief, and whether the claim has actually been made
Common questions
What are the IHT related property rules?
Section 161 of the Inheritance Tax Act 1984 requires that where a share of an asset in someone's estate is “related” to property held by their spouse or civil partner (broadly, the rest of the same asset, or a similar asset), the share is valued as a proportion of the combined value of both holdings taken together, rather than valued on its own. This generally produces a higher figure than an independent, standalone valuation of the share would give.
Does this rule affect an unmarried couple who jointly own a rental property?
No. The related property rule for co-owned assets is specifically triggered by a spouse or civil partner holding the related interest. Unmarried co-owners, siblings, friends or business partners who jointly own a property fall outside Section 161(2)(a) entirely, and a co-ownership discount can normally still be claimed on their share.
Can I still get a discount for jointly owning property with someone other than my spouse?
Generally yes. Where the co-owners aren't spouses or civil partners, HMRC's Shares and Assets Valuation practice usually accepts a discount, often around 10%, to reflect that an undivided share in land is harder to sell and doesn't carry control of the whole. That discount is what the related property rule specifically removes when the other owner is a spouse or civil partner.
What if the related property is sold for less shortly after death?
Relief under Sections 176 to 178 IHTA 1984 allows the sale proceeds to be substituted for the higher related property valuation where qualifying land valued under the related property rule is sold within three years of death for less than that value, broadly mirroring the separate loss-on-sale-of-land relief. It has to be claimed; it isn't automatic.
Where can I get inheritance tax advice on jointly owned property in the North West?
Valuing co-owned property correctly for probate and estate planning is part of the inheritance tax work we do for property investors and their families across Liverpool, Manchester, Cheshire and the wider North West. Get in touch and we'll talk it through.
Kieran Holsgrove is a Director and Co-Founder of Grafene Accounting, the property tax specialist firm based in Liverpool. He advises property developers, investors and landlords across Merseyside, Greater Manchester, Lancashire and Cheshire on tax structuring, developer VAT, SDLT and the long-view decisions that compound over the life of a portfolio.
This article is general information, not personal tax advice, and tax rules change. Your own position depends on facts we cannot see from here — please take advice before acting on anything above.