Thanks for your question. I will try to help.
Firstly, if they hold the house jointly (as joint tenants) then each person's share would pass to the other upon death regardless of any direction made in any Will. If this is not what your son wants then you should sever the joint tenancy by using Form SEV from the Land Registry (you will have to send it to them and if you have any questions about completing the form you should call their customer service number - they are very helpful):-
I can tell you how your son can check whether he holds as (1) joint tenants or (2) tenants in common if you wish
they will then hold their interests as tenants in common, meaning that their respective shares will pass according to their wills or under the intestacy rules. His partner need not sign the form provided you follow the instructions.
In the absence of any express agreement there is presumption that the proceeds of sale are split equally, so his partner is not acting unreasonably at this stage in demanding 50% of the equity in the property in order for her to transfer her interest to your son.
A joint owner can attempt to claim more than 50% of the equity in circumstances where they have paid for material improvements to the property which have materially increased its value and where it was intended that the owners wish to keep their finances separate. However, the issue with this is that he would have to litigate in order to make this claim by instructing a solicitor and applying to court and it’s very expensive and uncertain. For example, the amount of legal fees that it would take would probably not be worth it if he only paid £4000 for the kitchen, so he would probably be advised by a solicitor to not make a claim for this reason.
So, essentially your son is left in the position where he should try a negotiate a buy-out price with her that he can stomach and then apply for a new mortgage so that the property can be mortgaged and transferred to his sole name with her agreement. Obviously he would have to instruct a solicitor to do this
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