Utility kitchen waste pipe into external trap.

rowifi · Aug 16, 2025

In a new build extension. Is there an approved way to plumb the sink and utilities into the external trap without having any above ground pipework on the external wall. Ive used a back entry trap before, but whats the arrangement through the cavity and up through the floor slab.. or is this simply not done.

ShaunCorbs · Aug 16, 2025

Why not just bring it in untrapped and use appliance traps ?

rowifi · Aug 17, 2025

ShaunCorbs said:
Why not just bring it in untrapped and use appliance traps ?

I'm asking because i dont know what is legal requirement for attaching to sewer pipes from internal pipes. Is this a 'done thing' for kitchen and washing machine pipework?

Ric2013 · Aug 17, 2025

Is it a done thing, you ask. I'd say it is a done thing. After all, plenty of houses use internal SVPs. You'd be creating a stub-stack (which doesn't always need an air-admittance valve). Document H of the Building Regulations gives quite clear examples on how to detail this. I'm not sure if you would be permitted to use 40/50mm pipe all the way to the outside, but realistically in a new-build extension, I can't see the difficulty in coming into the house with 110mm.

rowifi · Aug 18, 2025

Ric2013 said:
Is it a done thing, you ask. I'd say it is a done thing. After all, plenty of houses use internal SVPs. You'd be creating a stub-stack (which doesn't always need an air-admittance valve). Document H of the Building Regulations gives quite clear examples on how to detail this. I'm not sure if you would be permitted to use 40/50mm pipe all the way to the outside, but realistically in a new-build extension, I can't see the difficulty in coming into the house with 110mm.

I'll check that out. Thanks.