Part L regs and extensions off of existing property?

[abitmorethandiy]

Searched a lot on the internet, but the regs don't see overly detailed in explaining the answers to my queries.

I have a 1960s built 3 bed house which is on a generous corner plot, the hillside is wrapped around the house making the front a two storey home and the back more like a single storey build, the existing cavity size is 50mm.

I want to extend the side and go up into the loft space, having spent a couple of years working out what we'd like to do we're now at a stage where Part L will affect our drawings.

The question(s):

1. Given we can't move the rear wall, we probably need to rebuild both outer and inner skins anyway based on multiple windows / doors taking up most of the existing space. It makes sense to do this, but can you just use the existing foundations and bring the inner leaf in by the 150mm requirement with suitable batts filled inbetween?

2. Can I reduce the 150mm to 100mm if using high performing u-value rigid insulation as i've seen suggested on other places?

3. The font of the property cannot be extended the same way as the rear because it has a lower floor below and the wall cannot be rebuilt, how can you adhere to Part L when building above an existing inner leaf 50mm cavity wall? This will apply to several walls to the front of the property due to going over an existing extension which also has 50mm cavities.

Diagram of the rear suggestion using existing foundations, foundations are very deep as we're built into the hillside for both front and rear.

If more info required please ask, i'm just trying to gauge next steps and if this stops our project or complicates an already complicated build.

Thank you for your time

 

[cdbe]

Not sure I understand, you don't need to upgrade existing walls. If building on top of an existing wall (eg hip to gable loft conversion), match the cavity and add additional insulation internally or use a timber frame internally etc, plenty of options that aren't rebuilding existing walls.

 

[^woody^]

If you are extending, why do walls need to be rebuilt?

 

[abitmorethandiy]

Not sure I understand, you don't need to upgrade existing walls. If building on top of an existing wall (eg hip to gable loft conversion), match the cavity and add additional insulation internally or use a timber frame internally etc, plenty of options that aren't rebuilding existing walls.

Thanks for your reply, ok i was hoping to keep my questions more around a point than get too specific on detail and answer things as and when they come up. The extension i want to add means some walls of the existing house need to be extended (on the same plane), this means that going from the old 100mm blockwork + 50mm cavity to the new 100mm + 100/150mm cavity causes the wall to either jut out on the continued plane or due to the face the old wall has several openings within and patching and rebuilding to close up for windows / doors etc its probably in my opinion easier to just take that out since the roof will be off anyway. Sketch model of the existing house and problem being indicated below:

House rear:

House inner with main reason for questions above?

What am I hoping to achieve and why my questions are being asked around re-build / part L regs?

Hopefully this makes more now, quick google map of the area / plot as its built into the hillside:

 

[^woody^]

I can't make out what's going on.

If you are drawing the plans yourself you are better off either showing an existing and a proposed in orthographic view, or clearly showing new and existing on the sane plan using colour, or thicker lines or something. Not text.