Anytime I'm designing a recognizable area (any place people could recognize), I always look for reference shots. I have a good 10GB of pictures copied from the internet and pictures from my camera about places I've thought about mapping, or just looking for inspiration, -or if I stumbled upon a cool pic I think I would use later. It ALWAYS helps me, and I highly recommend it.
Based upon books I've read and my own experience, gameplay (a heated topic) is normally better off if planned ahead
before aesthetics. You're building a level. The player knows they are playing a game. It's a level first, and the next best piece of game art (err. Valve quality environment) second. The world looks and
is the way it is because it's based on the layout, or the progression of the story. If you think: "hmm, I want the player to be able to get out onto the street!" -You would have just made a gameplay decision. You didn't just start to map out a brush or two and think: "hey, this could be a street!" -no; there was
some forethought (if little) that went into that decision. -My proposal is: -why stop there? Go ahead and flesh out (on paper, or on some other form of fast prototyping) as much of the idea as you have patience for and that time will allow! It's 10 times easier to scratch on paper 10 buildings than it is to build them out in Hammer, only to find you don't like the way they fit, and decide to delete them.
You've probably heard this before, but a good level has all aspects covered equally. A devotion to just one or two of them will deliver a sub-par experiences. Gameplay is just as important as visuals, visuals are just as important as sound, and so on... You're not just building a level, you're building an experience that needs to be developed all around. ESPECIALLY single player! SP FPS is meant to tell a story, otherwise, what's the point in playing by yourself. -You can assume the role of a character that the progression of the world centers around. By this fact alone, a player won't believe in the quality of the world, no matter how good it looks if they're not having fun in it. -That's where gameplay arrives knocking at your door asking for a place to sleep... You need to invite it in, and let it have a seat by the fire. -Give it some cookies and nurture it next to all your other best mapping buds (visuals and optimization).
-Aggh! I just made some weird allusion I'm gonna regret later

I better stop now; I must be getting tired.
-In short: Think about gameplay first as it helps structure your visuals (very rarely does it work well the other way around). And, give each aspect of level design the same amount of attention!