- "Undetectable Extension Charm. Tricky, but I think I've done it okay; anyway, I managed to fit everything we need in here."
- —Hermione Granger[src]
Undetectable Extension Charm is a charm used to extend and expand undetectably the inside of objects.
The charm has two effects: one, to extend and expand rapidly the inside of an object. Eg: make the inside of a wardrobe bigger. The second effect is to make the charm undetectable except to those who cast the charm or were told about it.
 Known Uses
- Hermione Granger used on a small beaded bag in order to fit all that the trio would need on their search for horcruxes.