This week I've designed and built a small timber framed screen porch which will attach to a barn that I built a few years ago.
The design took a few hours and the off-site cutting and fitting has taken a few days. Again I'll mention that I believe that designing in a digital modelling environment
is far preferable to designing something in a person's brain and then representing it with lines and text. 
In designing in a 3D modelling environment, a design is compiled from 3D digital objects, much as a building is a collection of 3D objects. 
The resulting views are actual views of an actual digital building, as opposed to lines and symbols designed to give an impression of a building that only exists in someone's imagination.





Here are a couple of pics of the porch in it's final resting place.

One situation where I find 3D modelling really beneficial to the design process is; in constructing stairs in a difficult or confined space.
It's relatively easy to construct the existing situation in 3D digital space, and solve the design problem in as close to real time as you can get on a computer screen.
This is also a situation where I feel that an integrated design-build process really shines in efficiency, as opposed to the designer and the builder being seperate.
The following photos should demonstrate what I'm talking about. Designing the stairs took a couple of hours and building them at my home took about 2 days.
I will bring them to the barn that they were built for tomorrow morning (dis-assemble and re-assemble).

Here are the finished stairs in a barn that I built a few years ago.