You used it on a home extension with the justification that he wouldn't have to share a bedroom with your partners son! I am shocked , to me that was never your money to spend, ever. But you think it is.
But if you invest money in a bank or other fund, they don't keep it in a big locked box for you and only open it every year or so to add a bit more for the privilege of storing your box of money for you. If you entrust it to them, it's their money to spend and do what they like with, on the proviso that, at the agreed time, they will return it to you with interest as agreed/as per the terms of the policy or investment structure.
This isn't really dissimilar to what OP has done. The only real difference is that the bank/fund would use the money whilst it's in their charge to invest in projects with possibly questionable morals. OP has invested it in something that will directly benefit the owner of the money, without depleting it at all - in fact, she's tripled the investment with her own money, at no cost to him, for his benefit, and then he will still get the money back with interest at the agreed time.
Can you imagine a lovely kind bank borrowing £10k from you, paying £30k of rent for a house for you to live in in the meantime, and then giving you your £10K back, with interest, over a decade later? If you can, let me know, as I'm moving all of my accounts to them!