Building further brick courses above existing solid 9" brick walls


I want to build an extension above my existing double garage which was built in the 1930s. The current building is made of 9" solid brick wall on all 4 sides, with a twin ridge roof with central valley. I want to remove the existing roof, build 6 to 7 feet of additional brick courses and then finish with a single ridge new roof (resuing the existing tiles where possible). My preferency would be to build the additional courses as a cavity wall but I've heard conflicting advice from two building surveyors, one saying that this is possible and the other saying that it would not pass building regulations.
Another option I have been given is to build the courses as solid 9" with the addition of vertically hung tiles on the outside. This option is not as aesthetically attractive because none of the existing main house has any vertical tiling, it is all plain brickwork. There is a 10 foot alley way between the existing house and existing double garage.
A third option is to build solid 9" courses and to then waterproof on the inside with a membrane and appropriate insulation board. However I am even less keen on this option and again one of the surveyors said that this would not pass building regulations.
I want to create a bedroom and an office in the proposed new storey above the garage.
Does anyone have any comments - particular about the option of building cavity wall coursework above the existing coursework?
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Are the foundations adequate to carry this large extra load?
Rob Graham
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I believe that the foundations will not be a problem but I am in the process of digging holes at two corners of the garage to confirm the depth and construction of the foundations. Encouragingly there has been no detectable movement of the brickwork over the 70+ years that the garage has stood. Assuming the foundations are adequate as they stand, what about opinions on whether a cavity wall can be built on top of the exisiting 9" wall?

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I can't see why a cavity wall would not do, but I don't know the building regs. Surely it would be simpler to ask the building control of your local authority (the horse's mouth) than discuss the opposite opinions of two surveyors.
Rob
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On Thu, 13 Oct 2005 22:08:03 GMT, a particular chimpanzee named
produced:

If the wall below is only 9" (225mm) thick, then a cavity wall will not be feasible. The cavity would need to be at least 50mm thick even with some insulation on the inside face, which would mean a corbel of at least 50mm. Far better would be to build a solid wall, and then either render or clad the outside and put insulation on the inside face.
Another option would be a timber framed structure.
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Hugo Nebula
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My experience is hardly the same as yours but my have some relevance:
My 1850's house has solid brick walls. I recently converted my integral garage, which included removing the old wooden doors and bricking up that gap, which was done with a solid brick wall in the same pattern. This new brickwork was then painted on the outside with liquid damp proofing before being painted white to match the existing house. On the inside the wall was dry-lined using duplex plasterboard, although I don't think this bit was a requirement of the building inspector, whereas the rest was.
I guess the liquid dampproofing I had to do was the best option for me as it could be painted white to match existing. Perhaps if my house had been brick then I would have been advised to tile it like you have told.
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