When I told my insurance I was going to change my standard back box to a Cobra Race box they said it makes no difference to my insurance an there would be no charge just thanked me for letting them know.
For the record, it was Admiral (Multi Car), however, it does appear that they are well within thier rights to charge an admin fee for change of details.
She did say that it was regarded as a modification because it’s not a genuine part.
This does make me wonder then, how you would be covered using a pattern part ???
Either way you put it is a modification from the original specification, the get out from the above is the replacement must be OEM or OEM equivalent (her saying “genuine part” is a little misguided), a cobalt exhaust is designed in some way to be nicer looking and therefore a cosmetic change and therefore does involved your insurance. I would not classify it is an informational only change either, because it is not, ultimately you have changed the conditions on which the original policy was offered by Admiral to you and in this case they have been willing to accept the change to the vehicle and still cover you.
Worst case could have been they would charge you for the cost of the modification and the admin charge.
I happen to know how much mine would charge for changing the exhaust, but because it is classified as “change of exhaust system” naturally the price is far higher to £65 for the insurance (excluding admin charge - however as I have been with the same insurer for a few years I do not charged). This sometimes works for you, for example Suspension and Steering changes - lowered suspension and ARB’s covered under the same class, flipside is the exhaust situation which is more frustrating as you pay a standard amount as though you have changed the whole system. In someway it is easier than every little item change.
I also own a kit car which I built myself. There are only 60 of these cars and all of them are different because some “factory” parts were never available. I had to have the exhaust system made for it because this was one of the parts never supplied. I’m always asked about modifications from standard and there is no truthful option other than to say “No” in that respect.