All else being equal, their property taxes go down, and their wealth goes up, because their land is now worth more. https://twitter.com/Rozzybumm/status/1132575895355453442 …
2/ if you do an addition and your house doubles in value, yes, YOUR taxes double. ...but if every house in town doubles in value, taxes do not automatically double. (tho richer people prob live there now, who want more ammenities, and so vote for higher taxes, and thus...)