In response to Mr. Hume's post, I would caution anyone from looking at the engineering as a mere formality. In many states, an engineer as a matter of law cannot simply review and stamp plans. Many do, but that doesn't change the fact that they shouldn't. Work stamped by a Professional Engineer must be work done under their direct supervision. In most cases the plans were not created under their supervision. Some states explicitly prohibit "review stamping" of plans. Too many people see the engineers stamp as the thing they are buying and not the value that a good engineer brings to the design team.
When should you bring an engineer in on the process? Well, I think that answer varies by project. In the case you described, it sounds as though at least having an engineer taking a look at the plans between your first and second visits with the owner would be a good idea. A good engineer on a moderate sized house will be able to develop a gut feel for the design without crunching any numbers in a short amount of time. This engineer may be able to provide some valuable information just in that initial look. The real design could take place a little later.
To me, the most important thing is to involve the engineer early and ask the engineer when would be a good time to really start designing.
