Leeds University,
Woodhouse Lane, LS2 9JT
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It's walkable from the city centre but if you are not that way inclined there are buses running from the top of town near the post office, just inside the inner ring road. Almost any bus from outside Bar Risa (opposite the post office) will take you past the University (e.g. no. 1, 28, 95, 96...). Look out for a big white building with a tower (the Parkinson Building) and get off near there.
If you are walking onto the campus...
Go under the arch on the left hand side of the Parkinson building and head up the steps. Take the first left, cross a courtyard and take the ramp up to the right towards a set of metal gates. Once you have passed through these gates the Union building is on your left. There is a map to help to get around the campus.
More info on Leeds University Union here.
If you are driving onto the campus...
This will only be possible on the Saturday and Sunday and after 17.00 on the Friday. Before the Parkinson building (big white one with lots of steps) take a left and pass through the (hopefully open) barrier. Follow the road as it winds around until you reach a car park under trees. You can park here out of office hours. Now re-trace your steps to the big white building and follow the walking instructions. :)