God no, he's not getting that much! The value of the house has sky rocketted! I just need to work out how much it would be worth if I'd bunged it in a Junior ISA or something lame!
I was with you right up until this.
You invested his money in your property. That’s capital that means your interest payments have been lower over that time. You’re being dishonest if you don’t release the increase in value of his investment. So the value before it was invested, how much 10k is as a % of that. Then the value now, and the % of your house that his investment represents is what you owe him if you’re being honest with yourself and him.
If you’d invested it in stocks and shares that skyrocketed due to lucky investments, would you take the remainder of his interest that was over a basic shitty junior isa rate?
You invested his money in bricks and mortar. You benefited financially from this investment - in terms of not having to borrow and pay interest on that 10k.
You will also have benefited financially from the increase in value on your house from having an extra bedroom which is generally the best way to increase a property’s value.
Your ‘but he got a cool new bedroom!’ is neither what the money was given for nor is it usually how a parent thinks about their child’s living space - you pay for that, not him.
If you’re not willing to treat this as an honest investment, At the very least figure out what interest you would have paid on the mortgage over that period for the 10k - so how much you saved. You owe him at LEAST that much.