Hate to be a bother, but Bravil doesn't really need a new sewer system, since they have the canal in the town(according to a loading screen, that is their sewer system). Though if the new tunnels drained into the canal, then I guess there wouldn't be a problem, though I'd guess that either only the castle has such a system(since it's slightly better then the shacks), or everyone takes their bucket of filth and tosses it in the canal(kings can have servants do that, though).
Besides, the houses are built upwards, one on top of the ground house at the least(except for guilds, stores and inns).