[MartinJF]

Mods can increase the value, make a car more attractive, reduce value, increase performance, decrease performance…

From my point of view cosmetic mods seem to hold onto some of their value, e.g., wheels seem to be worth about 50% of retail on the used market. Other mods like Clear bras seem to make a used car a little easier to sell, minor cosmetic mods like pedals do not seem to make any difference. Body kits, harnesses, roll bars and the like make a car harder to sell and people seem to swap back to stock prior to selling and sell the parts, this makes the car easier to sell and generates more $. I would not pay much for a body kit unless I planned to do that mod myself, but I tend to keep stock bodywork on my cars (just my preference).

Performance mods are another issue since people seem to worry about abuse, warrantee… I have lightly modified a number of my cars for performance and did not have problems selling them or getting a little cash out of the mods, I seemed to get 15% to 20% the cost of the mod back, I could probably have got 50% if I removed them. My experience is limited to suspension (shock & spings) intakes, exhaust, headers, cats and software (except that you cannot remove and resell software!). I have never done a turbo or supercharger mod, but I tend to think that the market for that kind of mod is very limited. If I did that I would remove it before I sold it.