Undetectable Extension Charm

The Undetectable Extension Charm extends the internal dimensions of the target object without affecting the external dimensions.

References from the canon

• Hermione cast this on the beaded handbag that she took to the wedding; it is used to carry books, a tent when not in use, clothes, and all the other supplies of the journey (DH9). While the bag makes an unusual noise if dropped (DH8), it does not appear to weigh more than a normal bag of its size and can even be stuffed down a sock in an emergency (DH26).
• The spell could be misused to fit many wizards into a small space such as a restroom stall, so the Ministry has strict guidelines.  Mr. Weasley with his enlarged Ford Anglia (CS3) and Hermione with her large-capacity handbag were breaking the law, but no charges were ever filed (Pm).
• While the Ministry allows trunks to be enlarged inside (Pm), the Fake Moody (Barty Crouch Jr.) misused the Extension Charm by holding real Mad Eye Moody hostage in a pit-like dungeon inside his Seven-lock Trunk (GF35).
• Enlarged family tents are allowed by the Ministry (Pm), and thus the Weasleys were able to use Perkins old tent at the Quidditch World Cup campground. Looking like a "shabby two-man tent" on the outside, it was a roomy "old-fashioned three-room flat" on the inside, complete with bunk beds, a kitchen, and a bathroom (GF7).