I have just cut a new doorway in a cavity wall, through from the kitchen into the new extension. This was originally the external wall prior to building the extension. I have two questions: 1. Do I just use concrete lintels above the door on each skin, extending 6” either side? 2. How do I make good the base of the doorway, with respect to the dpc and the existing cavity? – I assume the cavity here needs to be filled in with concrete but would need its own dpc preparation first?