So, the other day I learned how to create modular kits for static meshes (Buildings in my case), how to use overlapping UVs, create tiling textures, and make masks to convey multiple material types on a single material. But now comes the normal map. As a test, I used the nVidia Photoshop plug-in, and created a normal map from the diffuse map. Obviously, crap results were gotten. So, just to be clear, in order to make convincing and clean normal maps, I will need to make a high-poly version of every individual piece in my modular kit and bake them, correct? Just slap the finished diffuse map on the mesh, and use it as a reference as I sculpt each and every brick and cranny?
well baking normal map is an option. needs a little training cause it can be a bit tricky 
but you can also use alternate systems like quixel suite or substance painter , or even zbrush .
I had trouble with this, too. But I found a great way to do it from your diffuse is to generate a heightmap first. The heightmap will get stuff wrong but you can edit it in photoshop or a 3D editing program. Once you are satisfied with your heightmap, then convert it to a normal map.