DH and I got married 18 months ago. He moved into my house with his DCs, 12 and 10, a year ago but is only just selling his flat. He will make 50k from the sale of his flat that he wants to invest in my house. I own the house in my sole name. The house is worth 1.5 million and the outstanding mortgage is £470,000. However, there is a fixed charge of 105k in favour of my ex father in law and a 7.7% floating charge in favour of my ex DH. DH and I have a pre-nup which basically says no sharing of assets and that he doesn't get an interest in my house unless he makes a capital contribution of at least 50k. So, the intention is that he will pay 50k to me (which I will use to reduce the change to my ex FIL) and get an equivalent interest in the house (approx 3.3%) and that he will start paying a 1/3 towards the monthly mortgage payment. So, the current monthly payment is £2,100 so he will pay around £700 each month. Question is, how would we quantify his interest? At the end of the mortgage term, if he has paid of 1/3rd of 470k, I guess his interest is worth 156k or the equitable equivalent at that time. What happens though if we break up after 5 years or 10 years? He would get his 3.3% share but what about his monthly mortgages payment? What would he get for those? I want to make sure he isn't paying 'dead money" but at the same time he has paid nothing towards my house as of yet and I want to safeguard my share for my children.