Extension of an extension - Wall plate and subsequent roofing

[Lengsville]

Hi all,
I've taken much advice from this forum, first time poster, as I can't seem to find a post with a similar issue that I have now.
I am doing some research and planning into a ground floor extension. The semi-detached house has been extended in the past by roughly 1.6m (full width ~6m), but I wish to extend by a further 3.4m to make it 5m in total (therefore 5x6m).
Ideally, I will be able to leave the existing extension intact, have the new brickwork laid, then re-roof the new 5x6 structure. This is so that I can keep the kitchen in use whilst the project is ongoing as I plan to do most of the work myself to keep costs down.
The existing extension has a solid double skin wall with no cavity (215mm). I understand that to conform with building regs, I require 50-60mm Kingspan and 10mm gap within the cavity of the new walls.
My issue arises when planning the roof structure for the whole of the new extension, specifically the top plate which sits on the internal skin.

1. If the internal skins are built in line with each other, the top plate is not an issue, but the new outer skin will protrude on the external. Does this make the roof design complicated to terminate at two different points, or can the new external skin be approx. 1 course of brick lower than the original extension?
   
2. If the external skins are built inline with each other, the top plate cannot run in one length. Can an internal structure be built to support the top plate within the original extension?
   

Hopefully you're all still with me, and there is a practical workaround for this issue

Regards

 

[chappers]

I would say it depends what you want to do on the inside. if you are wanting kitchen units running along that wall then go with option 1 if not I would go with option 2 and adjust the wall plate up accordingly on the new work. If you expose the last rafter on the old work the height you need to build to should be pretty clear

 

[^woody^]

You design your wall to meet whatever requirements you want. A wall can be as thick as that existing one if you wanted it to be and it could meet current b/regs.

Roofs are never a problem

 

[Lengsville]

Ah, I think I see what you mean! For some reason I was under the impression that the wall plate needed to be just one single length, with all bird mouth cuts in the same place on every rafter, but of course, the height of the new internal skin can be adjusted to suit, to intersect with the same bird mouth cut! It makes no difference if the bird mouth cut is slightly 'higher' up the rafter on the new inner skin. Thanks for your input! I can stud the existing inner wall to mate up with the new for kitchen units.

 

[Lengsville]

Thanks for that, Woody. So I could add the insulation on the inside of the wall instead of building a cavity?

 

[^woody^]

Thanks for that, Woody. So I could add the insulation on the inside of the wall instead of building a cavity?

Yes. I suppose some of the garage conversion threads would have the detail.

Or if need be, a timber frame wall would keep the thickness down.