If it is art, and fair enough, then it ought to be on a canvass. Unless the owner commissioned it and got permission from the council planning dept first.
i love his work, against some of the cretinous dross that passes for art, Tracy Emin for one, and i have seen some of her stuff, other than the unmade bed, her works can sell for hundreds of thousands, but his work is usually, though not always in a hideaway place, and not particularly intrusive.
I live on outskirts & near to Birmingham & when I travel through Perry Barr area I can see house gable ends with urban art painted on them & to me they are absolutely beautiful they help to transform an old area into something really nice,so to each his own I say.Ron.