@Jack4 yes your last sketch proposal is better, at least an acceptable proposal assuming the levels work! Who is submitting the Build Over/Near Agreement to the Water Company? You or your architect?If you look on your water company's website you should be able to find all the criteria you need to adhere to, it's usually pretty clear. I wouldn't waste time trying to submit anything to the WC for advice, they will want a fee before they look at anything.
That's what I suggested at post 3. It will work provided you have enough fall. I did something worse some years ago as we had turn the line out from the back of the house to clear an existing conservatory (built over the chamber) and then come back on ourselves as the new extension was on the side as well as replacing the connie. Oh and there were separate foul & SW just to add to the fun.
Coincidentally, along the way, my client spoke to one of Anglian Water's APPROVED contractors who suggested something similar to what your "architect" came up with
You need a proper drawing to submit to the Water Company with falls and dimensions
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local