Can you describe the setting a little more? I think that is first and foremost when you’re trying to design a horror level. I am actually designing one based on a fairly enclosed and claustrophobic space as well that uses that small space for about 30-45 minutes of gameplay.
Right now you seem to be straddling the line between a cabin in the woods horror satire and a horror environment set in some sort of warehouse complex.
Now if you are just going with a normal type of unfinished basement, think of words that come to mind: Dusty, dark, musty, wet, dirty, uncomfortable. Detail wise you should see lots of structural elements like ceiling beams, the foundation, wiring, piles of junk/ stored debris and the foundation.
As far as lighting goes, you should not be able to clearly see completely around the level. It makes it harder to draw the player’s eye when everything is uniformly lit, and the darkness adds to the sense to closeness and uncertainty, provides ample opportunity to trick their eyesight and naturally begets players to be more reliant on their other sense: hearing. These are your two main tools in conveying other aspects about the environment that the player cannot experience; smell and touch, as well as providing ample opportunity to build atmosphere for a scare rather than just having some gay zombie or dead body fall out of a doorway.
Take footsteps and darkness for example. Imagine the player must find a car battery among junk or move a piece of furniture to get to a hard to reach spot. While they are walking to that setpiece set up a trigger or a timer to play footsteps just out of sync with the player’s own footsteps. Starting a step before and ending a step later from when they stopped and started moving. Have things move or fall or make sound in the peripheral of the player’s vision when they are trying to accomplish some task. Distraction and disorientation are good tools.
But as far as broader sweeping aesthetic choices you should make for your basement; you can have a combination of finished/unfinished elements in your basement, you just really need to convey your style so it doesn’t confuse the player rather than providing a context for the action. You really, really should use some sort of fog effect in combination with both custom particle effects/ dust motes.
And as far as off kilter lighting goes, I would stick to the orange-yellow-brown color pallete with maybe some highlights of blue and green here and there. It adds ‘weight’ to the atmosphere and conveys a sense of dust and age.