laying foundations for a small conservatory

[neilstone]

I want to lay foundations for a conservatory and know that the footings need to be 500mm deep,however when i started to explore by digging up the ground there is a soakaway pipe that runs through my garden and into my neighbours garden (i live in a terrace house),this is approx125mm from where my foundatins will be.the problem that i have come across is that when i started to dig down i got 350mm deep and came across water i bailed it out and it came back but only 250mm deep, i have been told this is the water table but im not so suer it is.????