Yes, I know where the story/book/series is going when I sit down to write it. The plots of both the Mortal Instruments series and the Infernal Devices series rest on a lot of carefully planned misdirection and foreshadowing — both of which are very difficult to do if you don’t already know where you’re going. (Yes, you can go back and rewrite a single book to insert foreshadowing, etc. — but you can’t rewrite books that have already been published). I also write better when I’m aware of structure — when I know not only what’s going to happen, but when it needs to happen, when clues need to be dropped, and where and when certain things need to be emphasized or de-emphasized. I outline not just the series, but individual books, and not just the individual books, but each chapter, scene by scene. I also outline each character’s arc — where they start out, what they want when they start out, where their chief moments of growth/discovery are, and how they end up.
Now, that doesn’t mean this is the right way to do things — it’s just what works for me. Some people sit down and just wing it; I can’t do that. It also doesn’t mean I never change anything if it isn’t working — I do, and then I adjust my outline accordingly.
For those interested, there’s advice about how to outline a book on my ‘writing resources’ page.