When adding an extension, consider external openings in the existing cavity wall and where the new extension floor joins the original property floor.
Cavity Trays Ltd manufactures a range of Level Threshold Trays (LTT), for extension and remedial applications.
These encapsulate and separate damp ground level substrate from dry.
They protect against damp ingress at an often-overlooked construction link, and versions are available to suit all dimensions of cavity walls and solid walls.