*Buildings built within DLC worlds count*
This will come over the course of the game but it will take a some time to achieve it. The 100 buildings must all exist at once over all 3 worlds, if you destroy one it will not count towards the 100 until rebuilt.
Once i finished the story i had built 84
buildings, the majority of which are required for progress in the game (i did the bare minimum to progress). All buildings count, from houses to castles.
If you're still short of the 100 once you have finished the story you can just build houses/huts (hearts are not required to finish the building
) till you get the 100.You can check the leaderboards from the start menu and open 'Total Buildings Built' to see how many you have built (the leaderboard can be a little slow to update)
also notes that you can build houses in a new world and have then count.