Can you help me estimate this beam deflection?

I'm having some building work done and it involves filling in an open stair well to make some extra floor space upstairs and adding a stud wall. Joist B is already there, Joist A will be newly fitted.



The intention of the builder is to double up Joist A, and (if not already doubled) Joist B as well, then build the stud wall on top of that. I asked if we needed SE calculations and was told it was really up to me if I wanted to spend the money.

I'd like to be a bit more certain than this, so can anyone tell me how I calculate this, or is it really an SE job? I've seen stud wall placed on doubled up (47x225mm) joist and it worked well for that kind of span, but that was in the middle of a room, not on the edge of a floor, and it wasn't hung off another joist.

Thanks!
S.

 

What size joists are they?

 

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