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?