New Ground Floor Toilet/Shower

1 Feb 2021
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United Kingdom
Hi All, I'm planning a new extension and having viewed a good number of forums I've not been able to assure myself I know the best approach to my issue so i'm putting it out you guys for your advice please.

Planning a wrap-around, ground floor extension which will cover the driveway up to the neighbours boundary.

Below is a simple schematic of the existing drain layout, sink feeds to a gully by the wall which feeds into the inspection chamber for the main sewer. The first floor bathroom is above the kitchen and the sink and bath waste feed down an external pipe into the same gulley. Round the corner is the soil stack which the toilet feeds into from above.


New extension shown below, new bathroom highlighted in cyan, possible drain routing shown for reference. We'll place a new inspection chamber beyond the extension to route the dishwasher and sink feeds from the new kitchen back to the main sewer. The current kitchen will remain and become the utility room so my plan is to re-route the pipework to exit the wall adjacent to the soil stack for direct connection to the stack. I will box in the soil stack with insulation (as it will be internal with the extension) for sound proofing and install a rodding eye to the stack with an access panel. It will be out of view so I don't mind the access panel, may even just plaster it and break through if ever necessary. Any considerations here?


For the new downstairs bathroom, we will have no external access or space for a soil pipe externally so need clarification on what is the best option:

1) Could the toilet, sink and shower waste connect to the existing soil stack under the slab level? I understand if possible it would need to be >450mm above the drain invert? Would I need a stub stack/AAV for the toilet waste within a box section which the sink (above ground) and shower (below slab) would connect to?
2) If we can connect to the existing soil stack will the stub stack need venting with the use of an AAV or similar product or is the connection to the open soil stack sufficient? Distance is <3m.
3) What is the best solution for routing the shower waste under the slab as I don't want to have to raise the entire level of the bathroom to run a single pipe? Could this connect to either the stub stack or the soil stack under the slab?
4) If using a stub stack with AAV would a direct connection from the bathroom to the main sewer be a preferred option to connecting to the existing stack?
5) Do I need to consider any one-way valves for either the stub stack or the ground installed shower waste system?
6) Are AAV's a reliable solution or do they have their faults?

I've not involved a plumber yet as don't want to waste anyone's time as i'm still designing the layout which i'll send for approval, this is just one of a couple of areas i'm not sure about which could impact the design. I know i'll need a build over agreement for the sewer.

Above the bathroom will be a flat roof and the garage is adjoining so we can create some extra space for a stub stack by moving a wall back if necessary.

Thank you for your help in advance. Welcome your questions.


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Can I assume as you want to build over your driveway you are planning to do front/side wraparound extension?

If so you have diddly squat chance of getting planning permission for that
Thanks for the reply though there are several other houses in the area which have done the same as what we are planning
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I'd be speaking to the Water Company first and see what they say, if they want the sewer diverting then anything else at this point is purely academic.

If they do permit a Build Over, then you will not be allowed to make blind connections to the sewer, all sections of the system need to be accessible for rodding/jetting should the need arise. This is not something I'd want to do from inside the property unless an absolute last resort, and I'd be very surprised if the Building Inspector would allow it.

Given your proposal, should the Water Company permit you to build over their sewer, then I would be contacting Building Control for their views on what they will/wont allow before going any further, your proposals in the drawings looks a to me like a non starter I'm afraid.

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