I have a solid wall full width rear extension and am looking to add a half width second story. The existing extension has a flat roof and solid brick walls. The current extension houses a kitchen which I would prefer not to interfere with as it's fairly nice. I can internally insulate during renovation in the future so do not want to knock this down.

The wall is ~215mm masonry with a render on the outside.

Should I build a cavity wall above this for the new second floor extension?

How do I overcome the wall width being larger than the wall below?

Current wall:

  • Render
  • 215mm Solid Brick
  • Plaster

Example thin cavity with internal insulation:

  • Render
  • Brick 102mm
  • Cavity 50mm
  • Concrete block 100mm
  • Internal insulation to meet building reg 0.28 u value

In the above example aligning the render means concrete wall is 63mm above the wall and 37mm overhang is this an issue?

Is timber frame an option?

What about SIPs?

  • Why would you allow the additional wall thickness to result in an overhang outside instead of just accommodating the extra thickness on the inside and keeping the outside dimensions the same?
    – brhans
    Jan 10 at 14:23
  • The above aligns the render so the overhang is on the inside. I don't know how a builder would approach building it given the plan would dictate an internal overhang. I assume there would need to be a beam of sorts
    – user128099
    Jan 10 at 19:40

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