Some years ago, JK Rowling sketched a rough map of Hogwarts, which has been posted on the Harry Potter Lexicon. The image on the Lexicon is very low-resolution - looks like it might have been made with a camera-'phone - so here is a cleaned-up version, so you can actually read the labels. [If your screen is set at a low resolution you may still not be able to read them: in which case, paste a copy into any graphics program and enlarge it.]
Unfortunately, this cannot be more than a very rough guide; it cannot possibly be anything like accurate. To begin with, we know the proportions are wildly out, because we are told in Quidditch Through the Ages that the length of a Quidditch pitch is 500ft, and if JKR's drawing was anything like in proportion, and her pitch was 500ft long, that would mean that the "lane" leading to Hogsmeade was also 500ft long - and 150ft wide!
Then, the position of the lake cannot be correct, since we are told in the books that they walk past the lake en route between the castle and the front gates, and between the castle and Hagrid's cabin; and also that it is possible, while standing on the lawn just in front of the castle, to see the centre of the lake. We know that the lake is entirely enclosed within the perimeter wall, not abutting it as it does in this sketch, because students are able to walk right round the lake without leaving the Hogwarts grounds. We are also told, with a high degree of certainty, that the Forbidden Forest is west of the castle, and with a strong degree of probability that the lake is south of it, and in JKR's drawing if the Forest is west of the castle the lake is north of it.
In order to draw up a map of the castle which is actually compatible with the books, therefore, I have used JKR's own map as a rough guide but have adapted it considerably. As a general rule, here and elsewhere, I treat what is in the books as the primary source. Statements made by JK Rowling outwith the books are treated as secondary to the books, used to clarify points which the books leave open, but disregarded if they clash directly with the books. Evidence from the films and other semi-canonical sources are refered to only if the books and JKR between them have failed to clarify a point.
I have tried, wherever possible, to avoid explaining away anomalies with the excuse "It's magic - it moves about." There are some instances, especially to do with the position of the Fat Lady inside the castle, where you just have to conclude that something does move but in general things clearly have a fixed or at least a usual position, otherwise nobody would ever find their way about, so I have tried wherever possible to find non-magical explanations for apparent anomalies.
