Thanks for the replies,
The extension in my head at the moment would be more that 40sqm, and so would require planning.
I am waiting on some engineer to get back to me - hard men to get - all seem very busy.
My own naive hope would be that because I am extending, I might get away with the planning laws that were there at the time in relation to the septic tank and percolation area. I simly don't have room / distance to fit one and it would mean that I would have to approach a farmer for some land behind my house, which is currently kind of part of his mother garden (unused veg patch) and while I may get it, it would could drive costs out of budget.
This whole "while you're at it" is a bit worring, but understandable I suppose.