How to Fill In Form IHT400: A Box-by-Box Worked Example for a £600,000 Estate

To fill in form IHT400 you work through it in order: enter the deceased's details (boxes 1–28), then list each type of asset in the "estate in the UK" section — but you don't write valuations straight onto IHT400. You attach a supplementary schedule for each asset class (IHT405 for the house, IHT406 for bank accounts, IHT407 for possessions), total each schedule, and carry those totals back to the matching box on IHT400. The form then sums everything, and from box 109 onward the calculation applies the £325,000 nil rate band (and, if you claim it on IHT435, the £175,000 residence nil rate band) to work out the tax.

Form IHT400 is the full Inheritance Tax account. You complete it as part of the probate process when there is Inheritance Tax to pay, or when the estate does not qualify as an "excepted estate" (gov.uk: IHT400 and schedules). It looks intimidating because it's long, but the structure is logical: identify the person → list the assets via schedules → carry the totals across → apply the allowances → calculate the tax. Below I walk a single estate all the way through so you can see exactly which number goes in which box.

Step 1 — Deceased's details (boxes 1–28)

Pages 1 to 4 of IHT400 are administrative. There are no valuations here — you're simply telling HMRC who died and which boxes/schedules apply. Work down the page; you only complete boxes that are relevant.

Boxes 1–17 — about the person who died

Boxes 18–28 — which schedules and circumstances apply

This block is a checklist. Each "Yes" tells HMRC that a supplementary schedule (an IHT4xx form) is coming with the account. For Margaret you would tick Yes to the questions that map to her assets and No to the rest:

Step 2 — Complete the schedules first, not IHT400

This is the single most important sequencing point, and where most DIY executors go wrong. You value each asset on its own schedule, total the schedule, and only then copy that total onto IHT400. Trying to fill IHT400's estate boxes before the schedules means you'll be erasing and re-entering figures all afternoon.

IHT405 — the house (£400,000)

On IHT405 you describe the property, give its open-market value at the date of death, and note who is dealing with the sale. HMRC expects a defensible valuation — an estate agent's written appraisal is fine for most estates; a RICS "red book" valuation is wiser where the figure is near a tax threshold. Margaret's house totals £400,000, which carries to the houses/land box in the estate section of IHT400.

IHT406 — bank and building society accounts (£150,000)

On IHT406 you list each account: provider, account number, the balance at the date of death, plus interest accrued but not yet credited up to the date of death (a commonly missed item). Margaret's two current/savings accounts and her cash ISA total £150,000. That figure carries to the "money" / cash box on IHT400.

IHT407 — household and personal goods (£50,000)

On IHT407 you value the car, furniture, jewellery and other chattels at their open-market (second-hand) value — what they'd realistically fetch, not the insured or replacement value. Margaret's car and possessions total £50,000, carrying to the household-goods box on IHT400.

Step 3 — Carry the totals into the "estate in the UK" boxes

Now IHT400 fills itself in. Each schedule total drops onto its matching estate-section box, the form adds them into a gross figure, you deduct debts, and you reach the net estate.

Step 4 — Where the £325,000 nil rate band is applied (box 109 onward)

The simple calculation begins around box 109 on IHT400, where you bring forward the net value of the estate chargeable to Inheritance Tax. From there the form walks you through subtracting the available allowances before applying the 40% rate.

Every estate has a nil rate band of £325,000 — the slice taxed at 0%. This threshold is frozen at £325,000 until 5 April 2031 (gov.uk: IHT thresholds and interest rates). The standard rate above the band is 40% (gov.uk: Inheritance Tax).

If we stopped at the basic nil rate band only

That's the figure most people fear — but for an estate like Margaret's, it is almost always too high, because two further allowances usually apply.

Step 5 — Claim the Residence Nil Rate Band (IHT435) — the most-forgotten relief

Because Margaret left her home to her direct descendants (her children), the estate qualifies for the Residence Nil Rate Band (RNRB) of £175,000, frozen at that level until 5 April 2030 (gov.uk thresholds). The RNRB only applies where a residence passes to children, grandchildren or other lineal descendants — nieces and nephews don't count — and it tapers away by £1 for every £2 the estate exceeds £2 million (gov.uk: RNRB guidance). Margaret's £600,000 estate is well under £2m, so she gets the full £175,000.

But it is not automatic. You must positively claim it by completing supplementary form IHT435 and attaching it. Skip IHT435 and HMRC will not add the relief for you — this is the single most expensive omission on a DIY IHT400.

The corrected calculation with the RNRB

Claiming the RNRB cut Margaret's bill from £109,760 to £39,760 — a £70,000 difference from completing one extra page. And if her late husband's unused allowances are transferred, her combined £325,000 + £175,000 + £325,000 + £175,000 = £1,000,000 of allowances would wipe the Inheritance Tax out entirely. That is exactly why you never skip the RNRB and transfer schedules.