What the plan lacks that would bug me is plantings of height. There is no greenery that isn't on the ground or that could produce some shade. As a solution, I'm including trellises (that can't be seen) at each side and an arbor that spans them. Or, it could be two small tree forms instead -- something whose height and spread is rigidly controlled -- at each side of the patio. Whatever it is, it provides a little more sense of separation between the seating areas, without actually separating them ... like they're in two adjoining rooms with a wide doorway between. I would relocate the hose reel so it was less inconvenient to use and widen the steps as well. I moved the BBQ grill to get more space for veggies. I'm showing it in front of the hose reel, but the reel would probably be better attached to the steps instead. Needs more exploration to see how these little things could be more finely tuned.

Do an internet search for brownstone small garden plans for a great set of ideas you can view, take the elements you favor and puzzle them together into what you want in your new space. Then enjoy your haven behind the house. Brownstones are architectural gems, have fun with your project.