Undetectable Extension Charm

The Undetectable Extension Charm, or simply Extension Charms (Capacious extremis ) is a charm that extends the internal dimensions of the target object without affecting the external dimensions.

History of use
Undetectable Extension Charms are advanced spells, but are subject to strict Ministry of Magic controle, because of its potential to create breaches of the International Statute of Wizarding Secrecy. There is a law in place (which is strictly enforced) that Extension Charms are not to be cast for private use, but only for the production of objects (such as school trunks and family tents), which have been individually approved for manufacture by the relevant Ministry Department. Hogwarts school trunks, like the majority of wizarding luggage, are issued with these charms.

The Extension Charm can be used on cars. Arthur Weasley used this spell, among others, on his Flying Ford Anglia. It was able to fit all of the Weasleys living at the Burrow at that time, Harry Potter, and all of their luggage comfortably. Molly did not recognise it for what it was, stating "Muggles do know more than we give them credit for, don't they?" Mundungus Fletcher cast this spell on an automobile he stole on Christmas of 1995 to transport the Weasleys and company to St. Mungo's Hospital for Magical Maladies and Injuries. It managed to fit all of the above minus Arthur and Percy, as well as William Weasley, Hermione Granger, Alastor Moody, Remus Lupin, and Mundungus himself. 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 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.

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.

Etymology
The incantation Capacious extremis comes from a corruption of the English words "capacity" and "spacious", and the Latin extrēmus, meaning "extreme".

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 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).
 * In the American versions of Harry Potter and the Order of the Phoenix, the spell used on cars to increase their capacity is identified as the Enlarging Spell, though the spell goes unidentifed in the original British text. Pottermore, however, clarifies that the spell used on the Weasley's Ford Anglia was the Extension Charm, thus making this embellishment non-canon.

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
 * Fantastic Beasts and Where to Find Them (film trilogy)
 * Pottermore

Notes and references
Sortilège d'Extension indétectable Capacious extremis