Thanks for your question. I will try to help.
If he is putting in more money than his girlfriend to finance the purchase of the property then they should opt to hold their interests in the property as tenants in common. This means that their respective interests pass according to their wills (i.e. not automatically passing to the surviving owner) and that they can own unequal shares in the property.
They can then make a declaration of trust, which is a deed sworn by both of them in which they specify how the proceeds of sale of the property (i.e. when it is eventually sold) are to be divided. This could take account of your son’s deposit by either specifying that you should receive this amount first and dividing the remaining equity equally (or as a percentage proportion) or it could simply state that he receives x% and your son’s girlfriend receives y%.
In order for the declaration to be as binding as it could possibly be they should both take legal advice on it before executing.
However, please note that the courts have a wide discretion to overrule such declaration in the event that they marry and then divorce, or if they have a child and then separate. In the case of proposed marriage they should certainly execute a pre-nup before they actually marry, but for time being it is not really appropriate given that marriage is not pending and therefore a declaration of trust would be more appropriate in the circumstances.
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