This helps too! 
So! If you want to achieve the best result as possible (IMO!!) you should have the best uv for each mesh…
So to get rid of those blocky shadows at the corners you have two options:
- Cut you mesh up to the visible parts and keep only those visible parts on the uv space (SIDE)
- Cut your mesh’s surface into the visible parts and keep just the visible parts on the uv space (TOP)
- And always snap walls at their corners to each other!
I made a little explanation pic!
So what’s happening is that your lightmap has a certain resolution. If you have meshes blocking out parts (in your case the middle column touching) and they don’t align perfectly to the lightmap texels (lightmap pixel/TOP?) it can’t visualize a perfect lighting information… it’s between two “pixels”… so this is why you get those zip lines!
As far as I know you can stretch lightmap uvs to a certain amount. So for a perfect result follow 1 or 2 + you should also snap corner points to grid/pixel (have a let’s say 512 x 512 black and white checker image in your uv editor)
I would try to stretch your uvs to a rectangle with all side edges aligning to pixel/grid…