Are there actual building regs that state front and back external doors to a house should have Ground Level flush with interior floor level? I have seen some references that indicate this, but it would seem at odds with building regs pertaining to keeping 150 between dpc and ground level. In particular, where patio doors can be several metres long i imagine the regs would expect that 150mm not to be breached. Is it ok to have a step down from house to GL?