I think you can include a clause in the purchase contract that the house must be preserved, right? Whether that will happen, of course, I don't know. Since it should be old acquaintances, maybe a promise is enough for them. Or the buyer will think carefully again about what he is taking on and back out ;) The desire for preservation seems to play a role more often. We also once looked at another property with an old building where the seller so far couldn't come to terms with the idea that we want to demolish. Maybe in the future we shouldn't be so honest... But then in the end, there will be a clause and it won't have helped us after all...