It is a great Q - there should be a video on this....I have built several beloved 12 strings and this is what I do:

Top Back and Sides are the same - as are all the braces except for the EXTRA Lower Face Brace.

Neck Width:

The easiest way to get the spacing at the nut is to buy one premade. At the bridge end mark out your preferred string spacing then add another set spaced 1/8" offset towards the treble side...check google images if not sure....

Neck Thickness:

I think most 12 stringers overcompensate but I do add 1-2mm...aim for 22mm@1st fret to 24mm@10th fret.

All tis would depend on the spec defined by the customer - if super heavy strings I would probable make it a tad chunkier all round (except the back and sides) ...

You don't need two truss rods.

My best advise is to work through the design process as described on the course - this should answer most of your Q's - then take a pic of you completed drawing and we'll check it over for you...

just pls don't ask me to restring it! 😛

If I'm not mistaken, and I'd like to hear the actual answer because I would like to put a 12 string on my list of to do's, the sides, back, and top are close to the same thickness as a 6 string guitar. The difference would be an extra lower face brace to stiffen it up for the extra pull. The only other things I can think of (after only one cup of tea) 🙂 is the headstock would be larger to accommodate the extra tuners, neck would be slightly wider to fit all the strings (that would show in your drawing), and whatever would have to happen to the bridge to fit all the pins. 

I would wonder if the neck should be a little thicker to help with the string pull. 

Cool question @koendb.  Maybe @markbailey or anyone that has built one can shed some light. 

Posted by: @tyreman

what about a 12 string neck? does it have to be slightly thicker to take the extra tension? I'm wanting to make an electric 12 string, and does that apply to acoustic 12 strings too? or is it just the same.

That is what Mark explains:

Posted by: @markbailey

I think most 12 stringers overcompensate but I do add 1-2mm...aim for 22mm@1st fret to 24mm@10th fret.

Posted by: @tyreman

I'm wanting to make an electric 12 string, and does that apply to acoustic 12 strings too? or is it just the same.

well it is the same if we are talking about steel string guitars. But it also depends on string gauge I recon.

I have absolutely no idea about 12 string guitars, acoustic or electric. I am intrigued by it though and would like to know the answer. I always feel like all those tuning pegs on a much bigger headstock would need a bigger volute at least (not sure if that matters or not though, that could just be in my head). Some guitars have those carbon fibre rods installed alongside the truss rod to give the neck extra stability. Maybe doing that would mean you could still have a thin neck, so it wouldn’t necessarily need to be any thicker for 12 strings. 

The only way to find out for sure is to ask the guru @markbailey 

Really good tips ... helpful for me...

@bpower, @Rocknroller912, @Boo and @markbailey thanks for your answers!

So as mark and bpower already mentioned, no change in thickness of body woods, only extra bracing.

I figured, indeed,  that I would need a slightly wider neck.
I do have the plans for a dreadnought 12 string already , so most dimensions, I do have, but these do not include thickness of the wood itself. For the rest I am planning to work my way through this build, following Marks lessons. Although I might have a go at a dove tail joint . I do have the templates to route these. Just need to make the jig for it.
Nut, saddle and bridge will be cut by myself from blanks.

Thanks guys!

Posted by: @koendb

I might have a go at a dove tail joint . I do have the templates to route these. Just need to make the jig for it.

Practice on scrap!

Posted by: @markbailey

Practice on scrap!

I will 🙂