Seems to go by a variety of names and some posts here, but, blimey, it's expensive and difficult to get for a self build.
Well we got a party wall agreement between ourselves and our neighbour. They were very good. We have all our PL and House Insurance updated and Self Build Insurance. However, a couple of weeks ago we found out that PL only covers negligence on the part of the contractor, if employed, or the client if the client is negligent. If everyone does there job correctly, but a major, unforeseeable, fault still arises, you, as the client, will not be covered. You need specific Non Negligence Insurance, sometimes called Party Wall insurance, JCT 21.2.1 or JCT 6.5.1 Insurance.
So for example, if a waterpipe is damaged through trenching and is not evident during works, but later undermines your neighbours foundations and their back wall falls down, you as the client are liable. We thought, great, get some of that to protect our neighbours.
Minimum cover of £500,000 will cost us £1300 + VAT and broker commission £200. Combined with Self Build Insurance it was over £2000. That's a lot of money for doing everything right. Plus they want to know how deep the foundation trench will be. Does anyone really know this until they start digging? Problem is it is not available once the work has started. What a screwed up industry.