In England and Wales, squatting usually refers to occupying an empty house in a city. The owner of the house must go through various legal proceedings before evicting squatters. The owner must prove that they have a right to live in the property and that the squatter does not, while the squatter has the opportunity to claim there isn't sufficient proof or that the proper notice hasn't been given.
In order to legally occupy a house you need to have exclusive access to that property, i.e., change the locks and keep the place secure like you would in a normal home. This is covered in a legal document known as Section 6, a copy of which is often displayed on the front door. There has to be no sign of a forced entry; meaning broken windows or door locks. This would prove that you have illegally broken into the property. The legal process of eviction can take a month or so, or even years, so the squatter has the time to find somewhere else to live. This is what happens when the property is owned by a council or a housing association. Private Landlords have been known to use various means of intimidation to convince a squatter to move out.
Local Council planners, who are facing rising court costs when evicting squatters (because their resources can not stretch to renovating their own properties) often resort to taking out the plumbing and toilets to deter squatters.
To show that the occupier of the squatted building is in fact in physical possession of the property, squatters often put up a section 6 notice or even multiple copies on different sides of the building. Doing so shows that there are people living there and they have a legal right to be there. This also makes clear that anyone - even the technical owner of the property - who tries to enter the building without permission is committing an offence.
Some properties are still occupied by squatters who have resisted eviction for 20 years. Squatters have a right to claim ownership of a dwelling after 12 years of having lived there if no one else claims it. St. Agnes Place in London had been squatted in for 30 years until the 29th of November, 2005, when the Lambeth Council evicted the entire street.