Building an av from scratch!

I'd like to build my own avatar from the ground up but I haven't been able to find any tuts/FAQs on doing so, only tips on how to apply textures and use the in-game tools to mod my av.

First of all, is it even possible, or are we restricted to just modding a few stock avs provided by Linden?

Secondly, any resources at all would be very helpful - I've found out how to apply textures, do sculpted prims, make custom animations and so forth, but I can't seem to find out how to start from scratch with an av model, kinematics and whatever else is required to make the body work in SL! Thanks for your help!

(Apologies if this is the wrong forum but these seems closer to building than just av animation.)

That's because there's no way to create one.
You can only modify what you have.

You cannot make an avatar from scratch. Everything starts with the pre-existing basic humanoid model. You can attach things to it, texture it, adjust the parameters of its shape via the appearance sliders, twist in all sorts of ways with animations, but you cannot replace it. Every single avatar you see in SL is just a modification of the same avatar you were given when you started.

Well that's very helpful! What about some of these avs I see which are animals and so forth, like the ponies etc.? I assume they're just some preset av that you can't change into something with 6 legs for example?

Anything that looks and moves in ways that normal humans don't, like a horse, is most certainly the result of an animation applied to the avatar and body parts made of prims attached to the body. Creative residents can even simulate extra appendages by cleverly attaching them in key areas and using scripts to animate the prims themselves.

Wow, that's fantastic, looks like I'm going to have to learn animation to get the av I want! I'll start reading up, and thanks for the tip!

That's exactly what I made avatar toolbox for. There's a bunch of freebies for it too.
http://avatartoolbox.googlepages.com/AvatarToolbox.html

Actually...you kind of CAN build one from scratch. You have to use a clever method of alpha layering. Basically, you hide your default AV with these alpha channels...then you create attachments for the places you want all new content (sculptie arm, hand, head). So your constructing your bits and parts, and attaching them to where they belong...

Or you can use both custom and default AV bits--my HAV unit is a perfect example of this.

The only downside is when you stand in front of an alpha channel (window, plant, etc) they will see the hidden geometry--it will be transparent, but noticeable by color shift.

You can make your AV whatever you want if you're good enough. As long as you hide your original shape with alpha work.

By "clever method of alpha layering", you mean invisiprims, right Michael?

In any case, while your suggestion is certainly valuable, I wouldn't quite define it as "building an avatar from scratch" in the sense that the OP seemed to be meaning it. I got the impression she was asking about creating her own character model from the ground up, complete with its own skeleton and rigging system. That's not possible (unfortunately).

What you're talking about is attaching things to the existing humanoid model, which is pretty much what everyone else was talking about. While that is the right thing to do (since it's all that can be done), it doesn't really fit this particular definition of "from scratch".

Naturally.