Filling hatched areas in image
As mentioned in another answer, image erosion is quite efficient to fill the gaps between the wall hatched lines. The key point is to choose a structure element that is big enough to allow connection to be done.

You should use a structure element with both horizontal and vertical dimensions of at least half the distance between hatched lines (try different values!). You may end with a final erosion with a small structure element to remove remaining noise (the thin and lonely lines in your case).

Then it is your responsibility to detect and refine the borders ;)

Here is attached the result I obtained with a square structure element of 19x19 pixels on a sample of the image you linked (logical OR one) assuming that the distance between lines is 19 pixels:



Hint: The output is smoother with a larger structure element but artefacts appear at the corners.