I've installed 3rd row jump seats (occasional seats, per Land Rover) in my Discovery 2004 SE. Before installing, I read up on the forums and found some uncertainty and paranoia regarding the legality and possible insurance complications that might arise from aftermarket 3rd row seat installations. So, not to settle the matter for everyone, everywhere, but to simply give my specific example, here are two "data points," if you will, for what happened with me and having a huge wreck, with 3rd row jump seats partially installed but not insured, and with actually getting my insurance to cover the conversion of a 5-seater into a 7-seater.

Also, for reference, my insurance is Farmer's, and I'm in Arkansas.

Wrecked With Aftermarket 3rd Row Seats—Well, to be clear, I only had 1 jump seat installed, as I had only just begun installing them when the huge wreck happened. The jump seat was not in use, and I was the only person in my Land Rover, and to further complicate things, the jump seat wasn't properly installed; it was in progress, so not all the bolts were even into the side. Nevertheless, in a paranoid state, I tried to unbolt and remove the jump seat at the scene of the wreck to avoid such crazy scenarios as I had read about on the forums, where insurance would deny my entire claim for an aftermarket seat customization. I was also "at fault." I had all my tools in the back right there and actually disassembled it pretty fast, but the darned thing was completely locked into the floor, and I couldn't get it out, so I gave up and hoped for the best, leaving the jump seat semi-attached. This was all after I had dealt with the police and the other party, and was waiting on the tow truck.
Resolution: Insurance didn't even mention the additional seat.

Insuring a 5-seater-turned-7-seater Discovery—Several months later now, we've fixed the family Disco and I've finished installing the 3rd row jump seats. They work great! I contacted my Farmer's insurance agent by email, and within an hour, he emailed over paperwork for "customization," for $600 extra in protection, as that's how much I told him I paid for the parts. He also changed the vehicle to a 7-seater on the policy, so it is officially insured as a 7-seater. I double checked that liability would cover 7 passengers, and he confirmed it would. The added monthly premium will be about $1/month more. One of the easiest things I've dealt with regarding insurance.

So there you have it. Hope this helps some people in the future with deciding whether or not to upgrade to a 7-seater. As always, check with your insurance, and your state laws will provide vary as well. Our insurance is amazing, and our agents always work with us.