AIBU to feel uneasy about my partner's inheritance plan?

I don’t understand why you haven’t sought legal advice for yourself already - that seems a basic and it sounds as if his wishes have become somewhat entrenched.
You have three choices:
Don’t buy with him.
Upon the first death, the house is sold and assets divided within say 12 mi ths or a stipulated period.
You agree to his plan.
I’d be doing 1 or 2.

So you buy them out, they get their money and you get to own the whole of the house.

I think it might be better if you both stay in your own homes.
Of course he wants to leave his half of the house to his children. You just have to read on here about the many cases where children have been left with nothing because the second/ husband/ wife has taken it all even though the parent wanted their children to eventually inherit. I'm not saying you would leave the children with nothing but no one can predict the future.
He is right to ensure his children 's inheritance is safeguarded

If I was you, I would discuss with him why its the only option.

The more I think about it the more having two houses sounds ideal to me

I'm sure a solicitor would be able to iron this out OP, put something in that you have to maintain to a reasonable standard (flesh out what this means eg free from damp, waterproof Windows and doors, operational heating system etc) so that they're protected from it being left to rot, but also add that they have no rights to enter the house or any say in when or how any works are done as long as the house is maintained to a reasonable standard etc. Or effectively write it so that the house is in a trust for them but they can't access the trust until you die, which may be another solution.

If he is insisting that no it's half theirs and they have equal say in all house related decisions eg what new kitchen you get, then I wouldn't buy with him because that's clearly completely impractical for people that aren't co-habiting. For instance what if you wanted adaptations for disablement in old age but they refused because it would lower the value of the house

It’s very different having not receiving inheritance because a parent is alive omo.
If they’ve lost one parent and then the other dies, it’d be unfair for them to have to wait that long because of the partner their living parent chooses. It wouldn’t sound like he’s put them first in that situation. That’s the way I see it.
He shouldn’t leave his partner without a home though.

I am not a lawyer. The basic arrangement of leaving his share of the property to his children with your right to remain seems reasonable and fairly common.

The further stipulations are what I see as unreasonable. It reads as though his children could order you to, say, install new bathrooms and a kitchen at your expense merely because they want it, even the existing ones are fine. This seems very wrong. Generally speaking, I think you'd be expected to keep the property in a good state of repair, fair enough, but handing over this control to them would be a deal breaker for me.

I understand this but equally, I have heard of arrangements like this going wrong because the person still living there allowed the house to go to wreck and ruin thus reducing its value significantly.

Obviously not saying you would do this but I understand why they would want assurances it will be looked after since its partly their asset too.

Why not keep both your properties, live in one and rent the other out for extra income and share the profits on that? that way neither of you will need to worry - you can still live together but each own separate houses. I dont understand why you MUST own a house together

I agree that having someone else having an interest in your home is not optimal.

But I can also see why he’d want to guarantee his money went to his kids.

Read all OP's updates, this is EXACTLY what he thinks.(specifically update at 10.41am).

He's outlined this exact scenario.

They can tell her to put in a new kitchen/bathroom/ whatever, "so that it's easier for his DC to sell when she dies".

And OP has to pay for it!

Shocking.

To be fair that is also life. Lots of people lose one parent first and then the next potentially even decades later. If my ex was died, I don't think myself or my daughter would ever begrudge his widow living in their house even if he did leave half to her eventually.

Sorry, I misread, OP's post on this. I agree that this point is unreasonable

Surely you can understand why a person wouldn't want other people telling her when she has to replace a kitchen or bathroom? To make it easier for them to sell when she dies?!! Oh, and she has to pay for it all!!

That's ridiculous, and extremely unfair.

I wouldn't agree to this.

I'm not sure there is any suggestion that he does think that, I think it is just OP's fear/worst case scenario.

Its a normal and fair approach..

if you dont like it, consider alternative ways to own the house, like buying a cheaper property so you can buy them out.

Say you have 100k and he has 100k.
Rather than buy a 200k house, buy a 100k or 150k house. Then you have money leftover to buy them out.

Or you could outright buy the house and he can live there. Or he could outright buy and you live there. Then you have the money for a different house

I meant to say I don't think you should completely scrap the 50% trust plan, but have an exit plan covered.

It’s obvious you don’t have children. If you did, you would think this is fair and just. And he’s agreeing you have the right to live there for the rest of your life if you wish. If you sell or cohabit, his children would then get their half. As he would be able to of you went first. It’s standard. And why wouldn’t you want to maintain the house? This is what my dh and I have done to ensure our children benefit.

Wow, he really thinks that's fair, that his DC can tell you when you have to put in a new bathroom or kitchen, AND you have to pay for it?!!

No way would I agree to that!

Essentially, if he dies, his DC become joint owners of the property with you. So surely essential maintenance costs should also be borne by them?
If you decided to sell, any maintenance/upkeep/ improvement costs need to be split 50%, and that amount given to you out of the sale price, THEN split 50:50 with his DCs.

He is being very unreasonable, OP.