-
If it's a standard job, you can just use an online calculator. All the engineer will do is put the measurements into a similar calculator, they don't actually break out their pencilcase.
We did it online and it was accepted by building control, and the house didn't fall down. Builder was happy to double check measurements etc for free.
We used this one:
https://www.beamcalculation.co.uk/
Ok cool, I'm happy to pay it just don't really have a point of reference for how much it usually is.