Undetectable Extension Charm

The Undetectable Extension Charm (incantation unknown) is a charm that undetectably makes things bigger on the inside only.

Effect
This sophisticated charm has two main effects: to expand an object's internal space while retaining its external dimensions. The charm also magically disguises itself so that no one but the caster (and those the caster confides about the charm in) can realise that this spell has been cast over the object, even if they are experiencing it.

History of use
Hermione Granger used this charm on her small beaded handbag in 1997 so that she was able to pack all the supplies she believed she would need while hunting down Lord Voldemort's Horcruxes with Harry Potter and Ron Weasley. This included books, changes of clothes, money, Harry's Cloak of Invisibility, and healing supplies. Eventually, Hermione also placed the portrait of Phineas Nigellus Black that had previously been in 12 Grimmauld Place in her bag as well to prevent Severus Snape from spying on them.

Possible uses

 * The same or a similar spell was used to make tents at the Quidditch World Cup and Alastor Moody's trunk larger on the inside.
 * This was possibly used also with the Cast-a-Spell Kit because it holds a training room.

Behind the scenes

 * The effects of this spell are similar to the effects of the TARDIS in Doctor Who.
 * The effects of the spell are also very similar to Mary Poppins's carpet bag.
 * This spell appears to be advanced magic; if it were more simple then one might expect Hogwarts students to be using it on their trunks to fit all their supplies in (however, they do not). Hermione did comment on her own usage of the charm, this being something for a witch of her intellect.
 * This charm may be used by wizards in the aversion of Muggle attention.
 * This charm appears to have a number of interesting side-effects when cast over an object. For example, when Hermione drops the bag she has cast this charm over in Harry Potter and the Deathly Hallows it makes a suspiciously loud thump (making it seem heavy), yet she carries the bag with ease (despite the number of objects inside). Of course, she may have also cast another enchantment to make the bag easier to carry, too.
 * This charm may be related to the Fidelius Charm, given its ability to prevent everyone but the caster and their confidants from learning of the charm's effect over an object.

Appearances

 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1