We closed on the house and are now immersed in choosing a fridge and tiles for the kitchen and figuring out what work we can do on it with a very limited budget. All of which got me sidetracked thinking about building for the ages.
Suppose you wanted to build a house, in Southern California, that would last for 500 years. How would you do it? You can't use wood--the termites would eat it. Metal's likely to corrode. The materials that seem to stick around for 500 years are things like stone and maybe brick. (Nijo Castle is a brick structure that's around 500 years old.) But SoCal is an earthquake-prone area. Over a 500 year span a serious earthquake will occur, and stone and brick seem rather brittle. I wonder how durable flexible concrete is.
Also, how do you handle the mechanicals? For example, what do you use for plumbing? Clay pipes, maybe? And what about power? The grid might not even exist any more in 500 years. You could use solar, but then you're faced with power storage problems so the lights stay on at night, and batteries wear out pretty quickly.
Also, what about heat? Passive solar? Or fireplaces (and buy a whole lot of wood to keep yourself warm)? And for sewer, would it be septic, municipal, or some combination of both?
One approach might be to put most or all of the mechanical systems in a foundation and then build the rest of the house out of more flexible materials like wood. There are pagodas in Japan that have been around for 500 years, but they get rebuilt every 20 years to the same plans. Putting the complex stuff in the foundation might make the rest of the structure much less expensive to replace, to the point where you (and future generations) could afford to do it periodically.
Maybe the most successful approach would be to be famous enough that for the next 500 years people are interested in keeping your house around regardless of how you built it. That's kind of cheating, though, since if it's a museum it's no longer being used as a house.
Overall, a 500 year house is an interesting thought experiment.