Confidence and self-esteem are your two biggest enemies here. I agree with openheart's suggestion about striking up a neutral conversation. But even that might be difficult if you don't know her.
Never underestimate the power of simply saying hello. Say hi to her next time you catch her eye. Just hi. No need to do a whole introduction thing. Just hello. Chances are she has noticed you as someone who works in the building as well. She will probably say hi back. Just smile and continue with your business. The next day, say hi again. Pretty soon, you will be greeting her every time you see her and she will be doing the same with you. After a while, when you say hi, you can say something like, "Hey, I see you and say hi every day and realized I don't even know your name. I'm _____." That will start up a conversation.
And then from there, it is all personality and compatibility.
Good luck!
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