" . . . the weight of the siding plus the paint will be too heavy resting on that narrow of a band."

Did his nose grow a few inches at that point??

What do the drawings show? Who does this "builder" work for? Post and elevation here so people can offer help. But as you have realized and has been pointed out, this is a decorative element and doesn't have anything to do with the siding & paint weight. Did he actually say this with a straight face? If so you need to sit down with him and get on the same page as one of you has not described what you want properly as this really makes no sense as written.

If you want a clean look, then just use one type of siding all around, top to bottom. Either the smooth plank or the board and batten would look very nice. But you definitely should post an elevation for better responses.

I cannot make this up!

We spoke again today about it and he said things will start to sag. When I showed him photos of other homes with the same look I like he said "no way that looks sturdy enough for all that weight". He also said, homes that have that must be on concrete not framing.

Here is my elevation plan. Note: That I did not have the designer draw the exact exterior details to a tee. It was my understanding the exterior details can be changed unless it something is structural. It never occurred to me to call out the detail of removing the corner board and calling out the size of the transition board.

I then asked him what is the most narrow he would go...he said he won't agree to anything less than 10".

Thank you for the replies. I have yet to find one professional that agrees with him.

Ask for Z flashing on an Azek AZM 197 drip cap on top of a 5/4 x 4 (or 6) Azek board screwed through the sheathing into blocking between the studs (or the band joist of the floor). Since it can only be as strong as the screws, the height of the band will not matter.

Or you could fire him.

I hate to say this but I believe the size of the band is the least of your architectural problems with this house. Do you have a 3D model of it? As it is currently designed the use of two cladding materials is not helping.

"We spoke again today about it and he said things will start to sag."

I only read that far but all I have to say is...........WHAT?

Well...after all of that. The one thing we did agree on is that with all the different roof lines of the home, 10 foot ceilings, large floor trusses, etc, board and batten wasn't going to look the best anyway. We are going with 9-1/2 plank on the bottom and 6-1/4 plank on the bottom with mitered corners. Thank you all for your advice.