When we had our new 'upstairs' put on the bungalow we had to pay for an engineer to calculate the strength of the RSJs that would be needed to hold the floor etc in place. Effectively our new floor is suspended from three RSJs, one in the apex of the property, two just above the 'new floor level'. Our work was done by Stephen Astin and he did a superb job but unfortunately he died of a heart attack a few years ago. He did play merry hell about the size of two of the RSJs, he reckoned that the top one needed to be larger than the two bottom ones and that the engineer just calclated the top one and said all three to be the same. Still better to be over engineered than under
As for costs, as ours wasn't just a loft conversion it won't give you much of an idea one thing to remember is that it does cost a lot more than just the builders quote though when you add in carpets, curtains, poles, paint, trimmings, furniture etc.