One question is, in your area, would your new build (except for foundation) be considered a 2012 build, or a 1965 build (or whatever age the foundation is). In my area, if you keep the foundation, but build new, the house is considered the age of the foundation for all real estate purposes... So, someone looking for a house built in the last 10 years, wouldn't find yours on the MLS.

Is the foundation in good shape?

Thanks for the quick reply. I'm looking at a handful of lots, all with relatively old foundations (1920 - 1960). No idea if they are in good shape or not, since it was just a "hm what if?" idea, not well researched. :)

It also depends on how big a house, and what type of foundation you would install. You will want to get a structural engineering report on the foundation system before you proceed. They should be able to give detail on any major issues (such as excessive settlement and drainage problems). Depending on what you're building and how finnicky you are, even "typical" settlement for an older home may exceed the tolerances you and your builder are comfortable working with for new construction and there may need to be some repair.

My wife and I own a large side-split ranch house that we plan to mostly tear down later this summer. The construction vintage is late '60s and the existing foundation is pier and beam. It has held up pretty well but will need jacking in a couple of spots where it has settled more than our design will tolerate. We will also be doing push/pull in a few places. I asked the contractors how much extra to wipe the slate and start clean, they estimated at least $100k for the foundation so that was out of the question for us. (Construction is very expensive in our area.)

Another potential factor in using an existing foundation is if the town considers your new house on old foundation to be built in 2012 or the date of the old foundation?

In our town, being built before 1965 means the setback requirements for sheds and additions are much easier to work with. If our house was built 1 year earlier we could have placed a large shed within 5' of the property line, but as it is, we had to limit the shed to under 100 sq feet or put it 15' from the property line. We went with the smaller shed.

So, sometimes an older build year has advantages.

You may have to show the old foundation meets the present code to use it again.

That may be as simple as digging to expose the footers for inspection (often at a few places) or as bad as being required to upgrade it anyway if it is not adequate.

We've done it both ways. Tear down to use old foundation and build new.
We are amazed at the value of the cost of our new foundation for this new house. Compared to the cost of other items on the build, the excavation and pour of the foundation was CHEAP. And well done with fabulous finish to the concrete. I even decided at footer stage to add the cold room under the porch. 8X36 foot CHEAP SF
I would definately do new after building new this time around Retrofitting a house to existing is a chore and you are limited to design.
Foundations not the expensive cost for this build. The finish out will kill us ;) tile granite wood floors kitchen on and on. Its whats after drywall that piles it on.