IHT is payable on estate valued over £ 325,000 (nil rate band). This allowance can be transferred to surviving spouse so a married couple can leave £ 650,000 free of IHT.
In addition, is a Residence Nil Rate Band (RNRB) which effectively increase the nil rate band by another £175,000* (2020/2021 tax year), which can also be passed to a surviving spouse.
This means the total IHT threshold for a married couple will be £ 1,000,000 form April 2020. This is 2 X £ 325K + 2 X £ 175K.
* RNRB is reduced if the overall value of the estate exceeds £2 million.