In the UK you have to tell your insurer if you do that and it can impact your insurance significantly in some cases. The best one I could find was with Aviva they allow 10mm variation in width of the tyre from the manufacturer spec, so if the original size was 225/45 you could buy 235/45 or 215/45, but anything beyond that would require you telling them and have an impact on your premium. Chaging the rims to a different size always requires telling the insurer, even if they are the manufacturer's original rims(which is incredibly dumb, I bought my car with factory 20" wheels, I bought an original set of wheels directly from the dealership in 19" size but I still have to tell my insurer because it's a "modification" to the factory spec of the vehicle).