Buying a home, adding value, making it fair

Arch87

Sorry for the long post, trying to provide as much detail as possible.

I got into quite a lot of debt when I was with my ex partner, due to a number of loans being taken out under are joint bank account, and ex then declaring himself bankrupt when we split. Over the last 5 years I've managed this debt well and all being well it will be paid off in Nov 2019.

3 years ago I used a large sum of money from savings and inheritance to put down a deposit on my own home. My parents took out the mortgage, due to my poor credit rating and therefore legally own 'my' property. I have paid the mortgage payments every since. The agreement was I would pay off the mortgage and the property would be transferred into my sole name (6/7 years left on mortgage).

My partner owns his own property, and has a small mortgage left to pay.

My property is located in my home town, whilst my partners is located in a small village just outside of his home town. (30miles apart).

My partners granddad has passed away, and we are looking at purchasing the property from the beneficiary. (Partners parents / uncles / aunties). This would be for the market value of the property, but the lowest valuations (they differ my 50k). This property is located in my partners home town.

We have applied for a mortgage, and due to my poor credit and my partners self employment we can only get a mortgage for just over half the value of the property. The mortgage will be joint 50/50 split. The remaining balance is then being split 50/50, so we are equal.

My Partners share of the remaining balance will come from the sale of his property. As his property is close by, he feels its better if he sells. If the property sells for the market value, my partner will have enough to pay off his remaining mortgage, have his share of cash needed and have some funds left over. (He is also going to cover the 2nd home stamp duty charges if we purchase prior to his sale)

My share of the remaining balance is coming from my parents. They are going to loan me the amount. However I am retaining my existing property and renting this out, as if (hopefully not) anything happens, I will still have a property in my home town. The mortgage payment is approximately 1/3rd of the rental value, and therefore this will pay the mortgage. The other 2/3 of the rent will be used to pay off the amount I've borrowed. This effectively will allow the property to be transferred into my ownership in a few years.

The property we are purchasing requires a lot of work, to make this a family home. This includes full re-modelling and refurbishment internally and a large rear / side extension to create family rooms, garage, additional bedrooms. I can't afford to pay for the work, but I can contribute to things from my monthly disposable income. My partner however is in the 'trade' and is going to complete most of the work himself. We have been informed that the work we plan to complete will significantly increase the value of the property.

My question is; if anything happened between me and my partner within the next 5 years and the house had to be sold, is there anything we could put into place to reflect the funds he has used / additional value he has put on the property?

I suppose I'm suggesting;
0-5years I get my 50% share of the purchase price. Plus 20% of the increase in value. My partner would get a 80% share of the increase in value.
5-10 years increase the % share for me
10 year plus: 50/50 share

I know I'm thinking worse case scenario, but as this is a 'family' home to him I don't want him to think he could loose out, and I'm just setting these things out now, so if something did happen it's less hurt and arguments. I also am trying to consider the fact that I will also own another property, in a few years.