Total price of property = �220.000. At this time, before b/f contributes anything, your �128,000 entitles you to 100% of net proceeds. �92,000 mortgage = say 40% to make it easy in round figures and = say 2% pa repayment over 20 years, again in round figures for easy illustration. Therefore if the proposed arrangement lasts the whole 20 year mortgage term, and if you hold the property as tenants in common and with the b/f paying half mortgage repayment, for the first year the percentage of shareholding would be 100% you b/f nil, at the end of the first year this would change to 99% you b/f 1%, and then for each following year 98/2, 97/3, 96/4 and so on until you reach 80/20. This is in round figures, and there may not be much point in mucking about with fractions of percents. To be totally effective you should Register the revised percentage holdings annually at the Land Registry. Your contract must also say what happens if the b/f defaults upon any payment(s) or abandons the arrangement