Builders are 2 weeks away from starting the foundations for my extension. On Saturday I got a letter from Thames Water saying that they thought I was about to build within 3m of a minor public sewer and should submit plans to get their agreement. Me, my builder and architect have looked and there are no sewers at the back of the house where we're going to build - they all go out the front. What there is is a storm drain (with manhole cover marked 'Storm') which we'll be within a 1m of. Have tried to contact Thames Water and they are so busy they can't even speak to me for 10 days to discuss whether they've made a mistake or know something no else does. Have contacted local BCO who has checked sewer plans and said nothing showing at back of house. Reason for the post is that she then said that she believed that recent changes to regulations now meant that if a storm drain is either built over or within 3m, it gets re-classified as a 'public sewer' and needs permission from Thames Water. Is that true? Sounds odd to me. Can anyone help please?