Self build extension. Drain,blocks+wall constr. questions

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Hi I have a semi with attached side garage. I drew up some plans which passed for a basic ground floor box behind house and garage and poured the footings this last fortnight under a building notice. So no professional or builder input to this point. It is bank holiday weekend and I cant speak to a BCO so wondered if the wise here could just give me some pointers.

1) Does the internal cavity wall have to a particular density or type block below DPC ie can it be 7.3nm concrete below DPC and lightweight above or doesnt it matter?

2) I am utilising more than 1m of the rear of the garage as an en suite for the extension which will need a new wall constructing to separate the garage from the extension. I originally showed the new wall on my plans as a traditional cavity but suspect I may be able to build a single skin stud wall. Could anyone detail the typical construction for this type of wall to pass BC? From the web I suspect 4'x2' studwork on top of a couple of courses of brick with DPC then kingspan foam in between and 12.5mm fireproof plasterboard both sides. Does it need a vapour barrier or does some boards have that integrated?

3) I am connecting the new en suite to the existing drain. Will it need to be vented to outside or can some kind of auto internal vent next to toilet be used nowadays? It is not going to be a straight run as shown in the plans. I will need to kink it slightly around the corner of the house as a I have a baywindow in the way which is not going to be removed until the extension is watertight. Are joints sold in shallower angles than 45degrees or can you get flexi joints as used in kitchen sink waste etc?

Many thanks for any help and I realise some of this is pretty basic stuff but I would rather ask now than make a mistake I have to correct later.

 
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I will answer my own question for the benefit of future readers

3) An air admittance valve can be fitted to the side of the toilet above the waste pipe so negating an external vent. A mean to rod should be available too in event of blockage.

These black couplings can be used to create very slight bends in the pipework underground where necessary

 

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