Extension floor crossing original foundations - how is this done?

[djaychela]

Hi.
I'm nearly at the point in my extension where the old ground floor wall is going to come out, and then I need to finish the floor. The new extension oversite is to have 120mm of celotex and then 65mm of screed on it to bring it up to the floor level of the existing floor. However, I don't know how you 'cross' the original foundation/wall and get the right support, etc. I've got a rough cross-section of what's there at the moment (see image), and I'm wondering:

1) What actually crosses these foundations and the current ~75mm gap that's between the original inner and outer leaf
2) The level of the original DPC is shown on the drawing - do I lap the DPM that's under the oversite into something and then bring that up to the floor level
3) Without digging up the original 1970s floor, it's difficult to tell what's under the existing concrete - would there be a DPM under that, and how is that connected into?

I'm sure these are pretty mundane things that happen on every extension build, but I can't find any concrete info on it, so any info is appreciated. The builder who did some of the oversite work and blockwork with me is long gone, and having found some of his errors since his departure, I'm not sure I'd want him back anyway!
I'm planning to get someone in to do the screeding, but want to be sure everything is right beforehand.

 

[SpecialK]

I used slates over the gap on mine. The PIR and screed alone would bridge the gap fine without.

Tape some DPM to the new floors DPM under the PIR.

I found liquid screed to be a similar price to classic screed. Also drys faster and is better with UFH and is laid in about 1 hour. No good for matching in to wonky existing floors though...

 

[^woody^]

Knock two course off, take the DPM and insulation across along with the concrete.