250mm or 450mm inspection chamber / mh - Which size??

[100-1171570483]

Hi All

I'm adding first floor extension with en-suite, and I need to branch into the existing soil pipe under ground level @ 550mm deep (existing SVP is build inside the house so can't tee into above ground level) I've ready exposed the existing SVP under ground and is ready to install new inspection chamber....Which Size??

I would like to use 250mm IC because it cheaper / neater BUT... What I don't want to happen is; I install 250mm inspection chamber and Building control knock it back because I used the wrong size.

Thanks for your help

[flowjoe]

250mm is fine to an invert depth of 600mm