Thanks for the response. I hadn't considered the drain to much, id presumed we could build over this but from reading about it I guess it's not always that simple.
At the very end of our driveway there is a manhole cover. It's the same for our neighbours property. This is about 10 metres away from the house. Would that suggest it's a private drain. Attached is a photo of what it looks like. Yes correct the arrow includes our land we've account for about 800mm gap between the extension and the boundary to maintain access to the rear. Haven't considered too much about the drainage and of course will be a discussion to have with the architecht. Still very early days at the moment just trying to gauge how much we would need to budget for.
I think once we hit 3k per Square metre this doesn't become worthwhile for us, I figured going for relatively minor internal adjustments, it would save money but seems costs may still be too high. I've approached a local builder to advise anyway but appreciate the feedback