The 3-D model we provide to our clients is a "read only" version. It can be rotated around (viewed from all sides). A cross section can be cut anywhere they like. Additionally, any component of the model can be removed for a better view of the rest of the model. For instance, similar to how the roof on dollhouses can be removed for a better view of the inside, likewise the client can remove the roof (or any other component) of their virtual 3-D model. They can do all of this on their own, without our assistance, using a free software package. They can also print any view they want.
Again, though, let's keep in mind what they have cannot be modified by them. Once they're done "playing around" with the model, there's no way to save the changes.
The work product they are paying us for is the plans and calculations. The read / write version of the model remains our property and is only an "instrument of service" while the read only version is theirs to keep. I suppose if they wanted to pay us enough, we may release the read / write version . . . but not until we had a complete waiver of any liability associated with the model, signed by them. That being said, up to this point we have not released the modifiable version to any of our clients. We also already have a fairly enforceable limitation of liability agreement in effect with every one of our projects.
We do not typically command a larger fee for modeling in 3-D. As previously mentioned, what it has done is allow us to be substantially more competitive, though. We are working in a more efficient / coordinated fashion, keeping budgets better in check. Clients will often select us over the competition due to the provision of the 3-D model. We may not have otherwise gotten these projects.
Having the 3-D model is also beneficial once the project gets to the engineering stage. All of the structures we do (at this point) are four stories or less, usually light commercial and custom residential (concrete, wood and steel mostly) performed for private sector clients (we don't currently work for, nor have the desire to work for, government clients). Typically, these types of structures are not highly repetitive (like a parking structure or highrise) and can be very difficult to visualize in 2-D (at times). Vertical offsets in floor diaphragms, clerestories, multiple ceiling heights on the same story, complicated stick-framed roofs (think turrets, curved roofs, dormers, attic trusses, etc.), hillside projects with stepped foundations and / or retaining walls, etc. are all features that can be difficult for the engineer visualize / design for.
With an inhouse 3-D model, the engineer can (in about five seconds) cut and view a scale cross section on any part of the structure. The engineer can also (in about the same amount of time) have an interior elevation of any portion of the structure. Little details that may otherwise get missed by the engineer and may affect their detailing / calculations, get caught.
We are currently moving into exporting the 3-D model into structural analysis software packages (RAM or RISA, for instance) for analysis. Though the bugs are still being worked out, if one exports the model into RAM, a very thorough structural analysis (with calculations) can already occur. If changes need to be made to the 3-D model (say changing an assumed beam size or footing size), it can be changed in the analysis package and synchronized / updated in the architectural model.