"My parents’ house is old styled and the living room is 6m X 3m and height 3m. The patio door is 1.35 m wide. The home furnishings are 25 year old. There is a double rail curtain track behind the crown molding 3m long. Could I hang window treatment that extends the full length of the wall instead of just the window? Which solution is aesthetically correct? "
Not only you do not need to extend the curtains along the wall but we consider you should avoid it.
We highly recommend that a house should not be overcrowded with furnishings. So we suggest that the curtains should extend the width of the patio door plus 10cm to 25cm on both sides. You can choose curtains hanging ceiling to floor, as the higher the curtain is, the more beautiful it will look.
When the narrow wall in an oblong room is entirely covered with curtains a sense of narrowness arises, not allowing the rest of the house to be highlighted. Moreover there is space enough to place a chair or another piece of furniture, without having fabrics one in front of another!
If the narrow wall in a long narrow room is covered with windows, we should choose lightly patterned curtains avoiding too many pleats. In the photo below, for example, you can see the living room in an old house with a ceiling height of 3,20m. A rail curtain track behind the crown molding is running along the window walls, but the windows are only1.45 wide! Although the room is quite spacious we chose to hang roman panels and drapes that extend beyond the width of the windows. However, we did not cover the entire wall.