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I have a bit of a dispute going with my builder who’s dug footings, poured concrete, laid 3 courses of trench blocks & should be going on to top this off with B&B floor then dig drains & make a connection to the sewer. We’re building a 1.5 storey house, block inner & block outer walls with render. After a visit from BCO & SE, we needed to abandon our plan for 1m trenches with 600mm poured concrete, & dig deeper trenches (1.95 across about 25% of the trenches & 1.65 deep on the rest) & pour more concrete, to take the level up so there was no more than 650mm of “trench block/underbuild”, to quote the SE, below B&B. The builder said he’d not allowed for any trench blocks o in his original quote, however, his invoice for extras resulting from the BCO visits includes £3215 for trench blocks. That’s just for the blocks – he’s also charged £600 for extra sand+cement & £2400 (95m2 @ £25, yes rounded up by £25) to lay the 300 trench blocks & 500 regular size concrete blocks. The builder says the trench blocks cost £10.71 each. I said they should cost him less than £7 each (before VAT). He said they’re expensive because he’s used 7N trench blocks. They’re 440 long x 350 wide & I think they’re 220 high, but I may have the height wrong – some of these are 215 high. My first question is: how can tell if what he’s used are 7N trench blocks? My understanding is, it’s unusual for this type of block to be used in this situation. Are 7N & the usual 3.6N trench blocks different densities? How much should one of these weigh? He didn’t allow for any trench blocks in his original quote – he was going to build a cavity wall straight off the concrete at the bottom of the 1m trenches. He hasn’t yet explained the benefit of the trench blocks over using regular blocks + cavity.