When fitting a cavity wall for an extension to an existing wall of a house, it is normal practice to use a form of masonary support (furfix) with a vertical damp proof course inserted in the outer skin of the existing wall by cutting a 4mm slot. I believe that this would not be compliant with the latest Building regs because the outer brick of the existing wall will be of a type (high density brick or block) which would form a cold bridge from the inside of the extension to the outside via the outer leaf of the existing wall. If you therefore extend the cavity into the existing wall and insert cavity wall insulation you remove the cold bridge and also don't require a vertical dpc. However, if the extension is single storey, this cavity has to be closed below the extension roof to support the brick work in the existing wall above the new extension roof. If the extension wall is 85 to 100mm you cannot bridge this gap with a standard brick and when the cavity is bridged, you have to introduce a vertical dpc to meet up with the cavity tray which you need to install above the roof. Has anyone got a better idea on how to achieve compliance with the building regs without having to put a block in the outer skin of the existing wall to close the cavity and then edge a vertical dpc between the courses of bricks up to the cavity tray without breaking the bond pattern. I can't see any other way of doing it and it is not exactly very neat!