Revisions log for Answer #1046

There is a special window sill mortar for granite or stone window sills.

Wood is glued or screwed.

2 and 3. Let's take a 120 cm wide window for simplicity, then you also get a 120 cm wide window sill. Then plastered on each side depending on the width of the window frame 1 to 3 cm beige plaster, then you just get this wall sill. It should be clear in advance how far the plaster will go.

With drywall, the sill is simply sandwiched between the drywall boards, so you don't have an overhang. Implementing this overhang in drywall would be too expensive.