IHTM05045 | Completing the account: signing the IHT400 (Scotland)
From HM Revenue & Customs · Inheritance Tax Manual
Although only one personal representative (IHTM05012) (PR) signs the form C1 inventory (IHTM05025), IHTA84/S216 requires all personal representatives (PRs) to deliver an account (IHTM05032) of the estate for Inheritance Tax purposes. A PR who has not signed the IHT400 cannot be said to have delivered an account. This may create difficulties later if the issue of penalties (IHTM36001) or notices (IHTM37001) arises.
The joint and several liability (IHTM30011) of PRs in Scotland is no different to that of PRs in the rest of the UK, so all PRs should sign the account. This applies to both the IHT400 and the IHT 205. Any case where the need for all PRs to sign is challenged should be referred to Technical.