"What happens to his half/ her life interest if she sells?"
It is common in wills written like this for there to be provision for the person with a life interest to sell the property and buy another one. Here is an example of how it may have been worded in his will:
The Property Trustees may at any time during the Trust Period as to the whole or any part of the Trust Fund in which the Life Tenant has for the time being an interest in possession pay out any proceeds of sale of this property or any substituted property (purchased as a result of this clause) to purchase a freehold or leasehold property which will be held for the benefit of the Life Tenant on the same trusts to which this clause refers.
So, if there is a clause like this in your dad's will then the Trustees can choose to use some or all of his share of the property to buy a new place for Kim and the Trust will still own whatever the relevant percentage of that new home is.
If she is downsizing then there will likely be money left over. You don't get to keep that money. It needs to be still held on trust until Kim dies.
It's probably easier to understand with an example:
Let's say that the house is worth £200k so Kim's 50% share is £100k and your dad's share which is now in trust is also £100k (round numbers to keep things simple).
Whatever happens she cannot touch your dad's £100k that is in trust.
Suppose she wants to downsize to a property worth £100k. This would release £100k as cash.
Now you (the Trustees) may choose to go along with this and agree for the Trust to purchase a 50% share of the new house.
This will mean that both Kim and the Trust each own 50% of the new house and also each has £50k in cash as well.
Or, as Trustees, you may say to Kim that you do not want to contribute at all to the purchase of the new house. In this case, Kim will own the new house 100% but have no cash and the Trust will have £100k in cash but no share of the new house.
Or you could do any percentage that you like. Maybe you only want to put in 25%?
In that case Kim would own 75% of the new house and have £25k in cash and the Trust would own 25% of the new house and have £75k in cash.
It's totally up to you as Trustees as to what you do.
So, let's say that you have decided to go in 50/50 with Kim. So the Trust now has a 50% share of the new house and £50k cash.
Great, we can just divvy up the cash between the children (if you have any siblings) or just have it for yourself, right?
No!
That money is still an asset of the Trust - you don't get your hands on it until Kim passes away or if there is some other condition in the will that is met (eg sometimes remarriage or cohabiting can be written into a will as a condition to end the Trust).
Up until then it is normally written that Kim would be entitled to any income from the Trust but, of course, she can't touch the capital.
So, with the £50k cash you now have in the Trust it would probably be best to invest it; whether this is in unit trusts, investment trusts, individual shares, government bonds etc.
I suggest that you should speak to a qualified advisor about this.
Any capital growth is retained by the Trust, but any income (such as interest or dividends) is paid to Kim.
For example, you put the £50k in a savings account. According to Moneysavingexpert, the top rates at the moment are around 5%. So £50k in a savings account could earn £2,500 per year in interest and that must all be paid to Kim.
That's great for Kim but not so great for you.
Alternatively you could invest in things that go for capital growth and don't provide any dividends at all. Great for you, not so great for Kim.
However, as Trustees, you are required to take the interests of both parties into account (unless the will says otherwise) when deciding what to invest in.
This would normally be done by selecting a mix of investments, some of which concentrate on capital growth (good for you) and some which concentrate on providing income (good for Kim).
I really would recommend that you speak to a qualified advisor about this.
Then when Kim finally passes away the assets of the Trust can then be passed out according to the will.
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"...there was a failed gift of some money. This IHT will be around £16k and the estate can afford to pay it."
If the failed gift (or total value of all gifts if there was more than one) was more than £325k then the person who received the gift is primarily responsible for paying the IHT. (assuming that your dad divorced and was not a widower)
This is due to Section 204(8), Inheritance Tax Act 1984.The estate also becomes responsible if the tax is unpaid after 12 months.
In this situation, the executors of your dad's will should seek to recover any IHT due from the person who received the gift.
But if the total value of failed gifts is less than £325k then there is no IHT for the person who received the gift to pay. It simply means that there is less of the tax free threshold available to the estate.