Squatting has become a criminal offence in most of Britain.
In England and Wales, those who take over someone's vacant property and live there for free, may now go to jail for up to six months.
Squatting had previously been just a civil matter.
The government says it wants to protect homeowners - some of whom have returned from holiday to find themselves locked out of their own homes.
But homeless charities say vulnerable citizens will suffer, while hundreds of thousands of properties lie empty.