Yes, you need to tell your insurer about any and all modifications. That includes changes to the engine and vehicle mechanics, plus cosmetic changes. If you don’t, you risk invalidating your cover. So if you need to make a claim, your insurer could refuse to pay out.
Even things like fitting towbars, roof racks or locking wheel nuts should be declared, though they’ll probably not make a difference to premiums. If you’re in any doubt as to whether or not something counts as a ‘modification’, it’s best to tell your insurer about it to be on the safe side.
Remember that some modifications – like fitting an immobiliser or parking sensors – could even bring your premium down.