Getting bottom curves of a door opening in a wall

I need to make part of a floor under the door as seen in picture. For that i need the curves. I tried alot using different methods.

  1. I got the geometry of door which is solid and then exploded the geometry and took the bottom surface. But problem is that it gives door geometry of just the door and not the whole width of wall.
  2. Now that i have location point of doors. I am thinking of somehow creating all 4 corner points, then create rectangle of these points. Then take curves out of this rectangle.

Any help or idea would be appreciated.

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