It seems to depend on the insurance company. Some count as "modification" as anything that you do to the car after it's left the factory. Others count it as anything other than standard specification.
I suspect that the second category is small but growing rapidly.
With one insurer a few years ago (I forget which one), I had to declare anything other than standard - not an easy thing to do when the car isn't new and you're not the first owner by the way!
I advised that the car had rear parking sensors and was told that the insurance would increase (only by a few pounds) because of this.
When I queried why a device that decreases the likelihood of damage to the rear bumper would increase the price I was told that if there was an accident, the bumper would cost more to replace (with sensors) than it would if it was just a plain bumper.
It does make sense (to a degree), but I strongly suspect that it's just another money grabbing scheme by the insurance companies.
The best advice seems to be to tell the insurance company that the car has not been modified but does have some factory fitted optional extras. It's then up to the insurance company if they want to ask and investigate further, but if they do ask, you must tell them otherwise they'll simply refuse to pay in the event of a claim on the basis that the information you provided wasn't accurate.
If they do ask and do increase the premium as a result, personally, I'd try a different insurer which doesn't class options as modifications although I suspect that as time goes on, those will become harder and harder to find.
