We are building a small rear extension and have used an architect reccomended by a friend. He came round before Christmas to discuss the build and take measurements. After back and forward with a few small tweaks to internal doors we then got building warrant drawings from the structural engineer used by the architect. It all went to LA planning and was approved with no issues.
Our builder came by yesterday to mark out the foundations in the back garden with a view to starting soon and discovered that the plans only left a 40cm gap down the side of the extension to access the back garden. It turns out that instead of going by his visit measurements anx observations he has used the buildings registry map of our plot boundaries to calculate the distance from our boundary with our neighbours to the new extension wall, but 50cm of that space is actually a very thick block retaining wall which stops our garden which is higher from collapsing into the neighbours garden which is about 4 foot lower down. We can't remove it or put a pathway over the top of it. The best solution is to move the extension over slightly so it now sits right against the boundary with the other neighbour which leaves a bigger gap for garden access.
We contacted him with the problem and he has straight away amended the plans but wants £150 + Vat. If the building warrant structural drawings also need amended we are looking at another £600. Am I being unreasonable in expecting he should be rectifying the mistake at no cost to us? Surely that is something he should have picked up on?