So, we're looking to do a single storey extension at the back of our terraced house - this is complicated a bit by the fact that a build over agreement will be needed to deal with a shared sewer for neighbouring properties (which 'terminates' here by turning, at a manhole, and travelling under our house to the main sewer).
I've done a good bit of research around this, but I've not found any examples of work like this where there has also been a lateral drain joining at the manhole (that is: our neighbour's drain doesn't join the shared sewer before it comes to the manhole - instead the drain comes direct to the manhole).
Here's my rough diagram as explainer (including my first version of a possible approach to dealing with it):
(The angles are approximate, and the left and right edges are the boundaries with neighbours. I'm pretty sure of the connections though).
There's a concrete patio covering this area currently, and we're on a tight budget so we'd really like to pin down whether we're going to be able to get Thames Water agreement before we get as far as opening up to lay down foundations. So far our initial chats with possible architects have not been very useful on this aspect of the project.
Clearly the current manhole will need to be removed, and Thames Water have been able to go so far as to say a Y junction to get a replacement in would be the way.
...but is that going to suffice if the drains for both ours and our neighbours also join at that point? (as they do in the image proposal)
....if not, will they perhaps be OK with the neighbours lateral drain being swept up to instead join at the new inspection chamber (with ours also being taken out to join there instead presumably)?
Architects and Thames Water have all sounded fairly positive so far - but also very vague and none of them have actually been in a position to confirm how the sewers could actually be dealt with!
Perhaps we're just going to have to bite the bullet and pay up for a Build Over application before we even proceed to get architects involved, and without precise details (beyond the manhole invert being 1.5m)? If not it seems likely we'd end up £2-3k spent before we even know if the drainage is actually doable to Thames Water's liking!
Thanks in advance!
