It is an offence to mark graffiti on property that can be seen by the public unless the owner has given you permission.
Graffiti includes defacing, writing, scratching or drawing on or property so that the marks can’t be removed easily with a dry cloth. This includes stencil art and engraving.
There are very serious penalties for making or intending to make graffiti, including large fines and imprisonment.
It is a more serious offence if the graffiti would offend a reasonable person. Political comments are an exception to this, if they are reasonable.