Is this a criminal offence in the UK? 10PTS short?

[darik2]

My Dad says this isnt a criminal offence but i'm not so sure

If you leave you door open and somebody walks into your house, but doesn't steal anything, is it a criminal offence?

My Dad says that its not a criminal offence unless they take something, or they actually break into your house by smashing a window orso, and at most you can get them for trespassing which is not a criminal offence its a civil offence so they can only be sued and will get no jail.
All you can do in this situation is ask them to leave and use "reasonable force" if they refuse

So what i am wondering is all this true? is there any laws which are a criminal offence to walk into somebodys house in the UK? Or is my Dad right about everything?

Also can you explain in as much detail as you can please?
Thanks

[gilleabart]

It depends on the intention of the person; section 9(1)(a) of the Theft Act 1968 (Burglary) makes it an offence to knowingly (Collins) enter a part of building as a trespasser with the intention to steal, cause GBH or criminal damage.

9(1)(b) creates a separate offence of Burglary; this is committed by attempting to steal, cause GBH or criminal damage after already knowing entered a part of building as a trespasser.

To make the distinction clearer :

A 9(1)(a) burglar intends to steal, cause GBH or criminal damage before entry,

A 9(1)(b) burglar enters as a trespasser then sometime thereafter steals or attempts to steal, causes or attempts to cause GBH or criminal damage.