Writing a car off does not relate to the damage as such but it is down to whether or not the insurance company deems it economical for repair. If the cost of the parts for repair exceeds more than about 60% of the market value of the car then the insurance company will write it off.
Two examples...
A guy I play cricket with had an old Vauxhall Vectra and another one of the team reversed into the side of it... very little damage... dents to front and rear doors on the passenger side. It was written off as even though the damage was slight because the car was worth nothing.
Years ago I was talking to Nick Mason (drummer with Pink Floyd with a vast collection of historic cars) at the Silverstone Historic meeting where he was racing his Maserati Tipo 61 Birdcage. I asked him if he was nervous racing it because it could be written off in an accident. He said no, because, the value of the car was so great that it would always be worth repairing even if he had to pay for the parts to be re-manufactured... He even added that racing it increased its value because it added to the car's history...
Having said all that, when an insurance company writes a car off it is required to categorise the damage A-D with A being the most severe and D the least... The guy with the Vectra got it back as a CAT D and drove it around un-repaired for another few months before be bought... a newer Vectra
