sorry, i don't know the proper legal answer. However, common sense tells my you cant make 100k of improvements with 10k outlay - either the market has gone up since the original valuation OR the original valuation was wrong and too low. From what you say, you actually want to keep the 100k extra? you might well negotiate for your 10k back, but i seriously doubt you could successfully argue for 100k! (after all, it's not like you actually tested the market last year - you might have got much more than the estimate)