s9 Theft Act 1968: "A person is guilty or burglary if they enter any building as a trespasser and with intent to commit an offence mentioned in s9(2) stealing, GBH, criminal damage"
There are three elements for burglary:
Entry
Building/Part of Building
Trespass
Entry is there when it has been "effective". "Effective depends on whether enough of the defendant is in the property".
Collins
Brown
s9(3) Building: "building shall apply also to an inhabited vechile or vessel"
Stevens v Gourley: "A structure of considerable size and intended to be permanent or at least endure for a considerable time"
Trespassing is someone going beyond the consent they have to enter a building.
Jones and Smith
Mens rea for burglary is intent to be reckless that they are a trespasser. They must also commit one of the ulterior offences (theft, GBH, criminal damage)