So what happens to houses with the expansion?

[worldofneil]

Nothing will happen to existing houses. There might be more houses added, but I don't think that's confirmed yet.

There's two new servers being added to EU (one to Chaos, one to Light), so that's 8640 new houses right there (4,320 per new server).

The keynote on Friday said about Ishgard housing, but crafters/gatherers would have to make it happen first (that's all the details we got...)