Scenario 1. Mother died many years ago and 50% of residential property passed under terms of Will to children. Other 50% retained by father who continues to live there. The 50% of the property owned by children is effectively in Trust. If father dies after 5 April will his estate receive the full new £425,000 NRB ?
I assume you are referring to the newly introduced residence nil rate band (RNRB).
On father's death he will be entitled in principle to the RNRB for the tax year of death multiplied by two as mother did not make use of her RNRB (as it hadn't been introduced at that time). EG death in tax year 17/18 gives father a RNRB of £200K.
Scenario 2: Father died some years ago. A Trust was set up prior to death meaning up to the then current NRB went into trust (seems some discretion as to what assets are in the trust, but includes half the residential property). Mother continues to live in property and children will inherit on her death.
Mother, as for father above, will be entitled to twice the RNRB for the tax year of death.
The issue concerning trusts refers to the fact that on death if the deceased left their interest in the residence on a
discretionary trust then as such a trust does not qualify as a lineal descendant of the deceased there is no entitlement to the RNRB.