Steel goalpost frame understanding

[GasManDan]

Hi all, 

 

we are planning to have a rear pitched roof extension with centre vaulted ceiling & bifold doors. 
 

as I’ve never done this before, I’m trying to understand the steelworks; we already have 2 steels in the original house as a smaller flat roof extension has been added (which will be demolished and replaced by above) so our structural engineer has put the onus back on the builder to check they are adequate. 
At what point does the steel goalpost frame get built!? If we/the builder opts to do the traditional strip & fill footings, do the steels have to be set in these footings or is it a case of once they go off, build the steels off of them and then basically brick up around them?  
 

ive attaches a pic of the structural drawing as you can see a bit more detail of what we plan to do!