"I have been told that because the lifetime mortgage is already in place it's not possible to do a Trust Will."
Can I ask who said that? That doesn't sound right at all - especially as you already own the property as tenants in common.
My own parents are in a similar situation to you. They took out a lifetime mortgage with Legal & General in 2021.
I am aware that some lenders want people to go back to being joint tenants rather than tenants in common before they give the loan. But since you are tenants in common and you have a loan, then clearly your lender is not one of those companies that insist on that.
Each lender varies as to what happens where a couple owns a property as tenants in common and one of them dies. For example, Legal & General say that they may stop any further drawdowns after that point (so you can't increase the mortgage after that point).
But, other than that, there is no difference between people who own as joint tenants or those who own as tenants in common.
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"...it's not possible to do a Trust Will"
There are different types of trust that can be used for different purposes. It may be that the person you spoke to was thinking of another type of trust. But there is nothing to stop you writing your will in the way that you said that you want.
It very much is possible to have a will that leaves your 50% of the house to your children. As tenants in common you can leave your 50% to whoever you like.
Having a lifetime mortgage makes no difference to that.
You don't even have to leave it in trust, you can just leave it directly to your children and, assuming that they're over the age of 18 when you die, then your share of the house passes directly to them in your will.
OK, so where does the trust come in? You may be asking.
Well, if you leave your share of the house to your children, this means that they own 50% of the house and your surviving DH owns the other 50%.
They can then get a court order to force the sale of the house and turn your DH out (or vice versa if you're the surviving spouse).
The reason for putting the house in trust is that the trust stops this from happening. It says that the surviving spouse has the right to continue living in the house until his death (or other event mentioned in the will eg remarriage). This stops them being kicked out of the house by the children.
The trust doesn't get set up now, it comes into existence on your death. It is generally referred to as an 'Immediate Post-Death Interest Trust'. The terms of the trust are written in your will and this only comes into effect once you are dead.
So, you certainly can leave your share of the house to your children, with or without a trust. The trust is just there to protect the surviving spouse from being kicked out.