Capacious Extremis

The Undetectable Extension Charm, or simply Extension Charms(Capacious extremis[1]) is a charm that extends the internal dimensions of the target object without affecting the external dimensions. This charm is advanced magic and heavily controlled by the Ministry.[1]

History
Undetectable Extension Charms are advanced spells, but are subject to strict Ministry of Magic control. This is because of its potential to create breaches of the International Statute of Wizarding Secrecy. There is a law in place that Extension Charms are not to be cast for private use, but only for the production of objects. These objects have been individually approved for manufacture by the relevant Ministry Department.[1] Hogwarts school trunks (like the majority of wizarding luggage) are issued with these charms.[1] Family tents are also infused with this charm, in order to make them more inhabitable.
 * "Undetectable Extension Charm. Tricky, but I think I've done it okay; anyway, I managed to fit everything we need in here."
 * —Hermione Granger explaining her use of the charm[src]

Newt Scamander used the Extension Charm on a case during the 1920s. He used the enlarged space to house the many magical creatures he collected during his trip around the world, while gathering information for his upcoming book Fantastic Beasts and Where to Find Them.

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.[2]

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.[3]Eventually, Hermione also placed the portrait of Phineas Nigellus Black that had previously been in 12 Grimmauld Place in her bag as well[4] to prevent Severus Snape from spying on them.

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.

Theoretically, a hundred wizards could take up residence in a toilet cubicle if they were sufficiently adept at these spells.[1]