Burmy will evolve into Trash Cloak Wormadam at Level 20 if it is Female and it is levelled up inside a building.
The best place to evolve Burmy would probably be in a Gym but any building will do.
Source: Experience and Completing the Pokédex
Hope I helped :)
You will need to evolve a Female Burmy in a building to get a Trash Cloak Wormadam. It evolves at Level 20.
Source: Here on the DB