Splitting house value after 2nd marriage

[noshinyshitstogive]

Have namechanged for this as I don't want it linked to my other threads. I know we need to see a solicitor regarding this but I wanted to get the views of you wise ones before we do. The situation is this:

Bought my house in 2010 before I met my (now) DH. Bought for £225K with a £150K deposit
Met DH in 2012 but he was in a rented house. He didn't move in with me.
Married in 2018. House was worth around £320K at that point with a £70K mortgage (had some extra borrowing for home improvements).
Extended house to create space to blend families just before we got married. Since then DH has paid half of all mortgage and bills etc but house still in my name.

He agrees that we need to draw up something which states how much of the property we each own. This is important as he has two kids and I have one, and I would only want to leave my assets to my own child.

Thoughts?

The equity existing in the property when he started paying half the mortgage needs to be ringfenced. Then because you're paying 50/50 any equity over and above what existed when he started contributing should be split 50/50.

About only wanting to leave your part to the kids - you don't know when if that will ever happen as kids may lose touch with you/etc, you may gamble everything away be sued, defrauded, just trying to say what we want and what we get are two different things.

If ex is prepared to be fair see what he comes up with then do the clacs and at that point consult a solicitor/etc

Btw, how much he owns you mean you will buy him out if poss?

Sorry, to be clear, we're not divorcing. We just wanted something documented which states how much of the property we each own should anything happen.

@LunaAndHerMoonDragons that's along the same lines as what I was thinking - thanks.

You might have to state it as percentages though, to take into account any big increase (or decrease) in value.

Good point, but I feel like a percentage is really really difficult to pin down especially since the extension added a lot of value. The house is probably worth £500K now.