Again, I agree with geraint!
There are some theoretical reasons why you may like to load the foundation strip centrally, but in practical terms for an extension, there will be no detriment to offset walls on the foundation strip, or even walls built on the edge
TBH, the BCO will check the foundation depth and trench width. By the time the substructure walls are built up to DPC, and then the banks of the trench knocked in, then BCO wont notice any off-centre loading and he won't even care
There are some theoretical reasons why you may like to load the foundation strip centrally, but in practical terms for an extension, there will be no detriment to offset walls on the foundation strip, or even walls built on the edge
TBH, the BCO will check the foundation depth and trench width. By the time the substructure walls are built up to DPC, and then the banks of the trench knocked in, then BCO wont notice any off-centre loading and he won't even care