B. multiplex might provide a little better screening...but it's often almost as wide as it is high, so, if you have a tight space, it might not be a good choice. A Phyllostachys will be a much more erect plant for a narrow planting area.

Here is a link that might be useful: B. multiplex picture

Hi. I'm in LA too. Can the branches be wider than 12" ? Do you need coverage low or just high for the upper level windows? If the plant can only be 12" wide its really too narrow of a space for total coverage because you will only have the stalks (called culms). If you only need coverage higher up and you can let the branches grow out then you have a better chance of coverage.
I don't know of any bamboo that will stay only 12" wide at the base. If there is a deep barrier (1' for a clumper or 2' for a runner) then the bamboo will stay where you want it. Or you can put in bamboo barrier and not have to worry about it. Otherwise I would suggest a fence and a trellis/lattice above it, possibly with a vine.
For narrow, very upright, shorter bamboos Check out Thamnocalamus, Fargesia nigra, Alphonse Karr, Chimonobambusa marmorea (runner) or maybe Tootsik (runner). Just Google them and you will see lots of photos and info.

Hi. I'm in LA too. Can the branches be wider than 12" ? Do you need coverage low or just high for the upper level windows? If the plant can only be 12" wide its really too narrow of a space for total coverage because you will only have the stalks (called culms). If you only need coverage higher up and you can let the branches grow out then you have a better chance of coverage.
I don't know of any bamboo that will stay only 12" wide at the base. If there is a deep barrier (1' for a clumper or 2' for a runner) then the bamboo will stay where you want it. Or you can put in bamboo barrier and not have to worry about it. Otherwise I would suggest a fence and a trellis/lattice above it, possibly with a vine.
For narrow, very upright, shorter bamboos Check out Thamnocalamus, Fargesia nigra, Alphonse Karr, Chimonobambusa marmorea (runner) or maybe Tootsik (runner). Just Google them and you will see lots of photos and info.