We're putting an extension on the house. Which means we'll probably have to put in a new soakaway (adding another £1000 to the cost). The current soak away isn't 5 metres from the house but how do I work out its size and construction ? I understand slightly older properties might just have an area of rumble serving as a soakaway but if we start digging where we think it is what else might we find ? (It's a 1950s house).