What is IHT400?
IHT400 in plain terms
IHT400 is the main HMRC form used to report the value of an estate for inheritance tax purposes when the estate doesn't qualify for the shorter, simpler process. As the executor, you (or your solicitor, if you have one) are the person who submits it.
The 12-month deadline
The IHT400 inheritance tax return is due within 12 months from the end of the month in which the death occurred. Missing it doesn't just mean a late form — HMRC can treat it as enforcement territory, on top of any tax already accruing interest from the earlier 6-month mark.
What this page is (and isn't)
This is a plain-language explainer, not tax advice — it does not tell you whether your estate needs to file an IHT400, and it does not calculate what you owe. For estate-specific guidance, consult a solicitor or accountant.