For general information of anyone concerned with this issue.
This subject was first raised a number of years ago (in my case via my owners club magazine), and has caused a lot of arguement since.
Rather than fall foul of an invalid insurance in the event of a claim, I approached my firm (face to face) and asked them just what does a modification consist of.
The reply given was basically, any change in the construction of the vehicle or the equipment fitted.
Fair enough, tuned engines etc. you expect to get hit for. The list I was given went a whole lot further, tyres (fair comment), car audio, sat-nav etc.
Without compiling a list of anything you could fit (if you went down that road), the simple answer is "anything that is non-standard at the original build spec. time".
I asked if, for example, after-market steering wheel (not allowed), non-standard paintwork (not allowed) any modifications to the lighting, heating or cooling systems (not allowed), replacement exhaust system (dodgy ground there).
Luckily the guy I spoke with had a practical view of the car world. He came up with a very easy solution to my problem. As some may know from previous threads on this forum, I own (apart from my euro-box daily drive) a thirty plus year old car. Having been through a number of owners, each of them appear to have done slight mods in their time, there are a number of "non-standard" items fitted. Go through the car carefully and write down ANY changes that have been made in its life that you are aware of.
This I did carefully and went back with list of mods.which he then submitted to the company. His logic was this, if you inform them of any changes (in writing) when you apply for insurance, they have the right to accept the car as declared by you or refuse to insure you on those terms. But having accepted your statement, they cannot then renage on the insurance.
When I look at my list I sometimes wonder just which world I live in. An uprated electric cooling fan (must be worth at least 2MPH extra), non-standard steering wheel (since removed and original refitted as I hated the thing), wing mirror on passengers door (good for safety?) and a slightly modified water system for cooling the engine (sensible on these cars). A different brand of tyres had been fitted (still the correct speed range) and the final touch, a new stainless steel exhaust system (two pipes for the V6).
None of these changes was questioned except for the exhaust. Is it a genuine manufacturers system? No. Why not? Because the company folded many years ago and you simply cannot buy them over the counter. Is it a big bore / noisy boy racer thing? No, its a nice quiet and damn good construction.
Not a complaint from them since - even at renewal time, simple question from them "any changes?" None. Insurance certificate issued.
Sorry to be lengthy over this, take my advice and notify your company of anything and everything that is non-standard. They can then only accept or refuse you!