100% requires you to do several things. You can track these on the pause screen in the Leaky Cauldron.
1. Collect all 200 Gold Bricks
2. Find and purchase all 20 Red Bricks
3. Find and Purchase all 167 Character Tokens
4. Find 50 Students in Peril (24 Located in story levels, 26 around Hogwarts)
5. Get True Wizard in all 24 Levels.
6. Collect all House Crests across the 24 Levels.
Once you have all 200 Gold Bricks, head to Knockturn Alley and enter the bonus level where you will unlock the final piece, the Voldemort character token. Purchase this for 1,000,000 studs and you will get 100%
I had to wait a couple of minutes for it to pop, so just walk around for a bit to see what happens.
My personal tips:
Start with the Red Bricks as these will help you get each of the others by giving you a good headstart on studs and detectors.
Remember to build all 4 models below the Leaky Cauldron once you have gotten all House Crests.
Keep a note of which Students you rescue in Hogwarts, finding the last one will be a pain if you have to go through the list. There is no detector for this one.
Spells that you buy DO NOT COUNT.