At one point we were planning on doing what you describe. The Timber Frame end of the job is a pretty simple task as long as you design and cut for it in advance. Be sure to get the addition on your site plan when you go to zoning and planning the first time, it should smooth the process when you go to add it down the road especially if you 'start' to add it before Z&P approval expires.
The foundation is where you have to be a little creative, I'm assuming you are going with concrete and a basement. You can either pour for the entire future structure or just the main house.
Pouring the entire foundation invites problems; water leaking in and just plain ugly. I'd only do this if zoning and planning seemed a bit iffy on the addition, around here Z&P approval only lasts 5yrs, meaning the project must be started within that time. By having the additions foundation poured you've already begun and won't have to go through Z&P a second time.
I think the best approach is to locate a Bilco door to the basement near the middle of the foundation wall the addition is to attach to. When you go to add the addition the Bilco's opening (and any windows)creates a passage between the old and new portions of the basement and the Bilco can be relocated.
As far as the curtain drains go, the best approach is to make the side with the future addition the "high side" of the system if at all possible. This makes tying in and extending a simple task.
Hope this helps.
