I am having PVC double glazing fitted at mo. The windows are brill! The back door brill! But the french doors leading to a conservatory has an additional sill (which I didn't ask for - but it covers a cavity where a wall was removed) and a high 4" threshold when I asked for a low threshold (the back door is perfect 2" and I wanted the french doors the same). My concern now is for the front door which will be fitted soon - this is an internal door (there is a porch on the external walls) and I asked for a low gold mobility threshold - expecting just a gold strip across the join between porch and hall, but now been informed there will be a white sill like the french doors. Can any one advise as to whether a pvc sill is necessary. Many thanks. K