Yes you should tell your insurance company but vosa don't need to get involved. Least that's what I was lead to believe when I was digging around a while back. Also...you could have the body removed and a new custom one made and as long as you don't alter the chassis or axle etc then again, no need to involve vosa. Same goes for converting a drop side to a tipper
Let me explain whats happened to me. Back in may i ordered a nissan cabstar double cab tipper. But i did not want the tipper body that nissan were going to supply, i wanted the vfs three way tipper. Vfs use a scattolini tipper, and in fact all nissan single cab dual wheel tippers are fitted with it, but one way only. The single wheel tippers are tipmaster, but for some reason, the double cabs are fitted with something else.
Now, because i am having my own tipper body fitted, albeit a standard off the shelf, fitted to all other single cab cabstars, the vehicle cannot leave the vfs premises and be road registered until it has a certificate of conformity. This has to be done by vosa, who then issue it. Mine was done on 19th december, and i am still waiting, although it may arrive monday.
I happened to query this with a customer who works for vosa, or dsa as they are now called? Who said it applies to any body modification that alters carrying capacity and load bearing, as well as alterations that can alter the structural rigidty of the vehicle.
His comment was thus, most wont bother, but in the event of an accident, or a routine stop check, it will then be flagged up and dealt with depending on the nature of the modification and its intended purpose.
Drilling holes in exisiting factory fitted body to accomodate extension sides would come under this remit,
Converting a dropside to a tipper would also require re calculations as to payloads which need to be indicated on the vinplate, and this can only be done by authorised and approved bodybuilders with input from vosa, in order to achieve a coc!