get a price 1st edenjohnny, i used to get engineers sign off on extension builds when I was in that game. if he's decent and looking for work, he won't do you and you should be okay with 3 visits, pre-during and post concrete pour visits. You might get him at 2-350 per visit, hopefully not much more. What you're paying for mainly is his indemnity insurance. As a pro engineer, if you or any future owner has an issue, he'll have certified all is well, so if claims ensued, you'd have some grounds to pass this on or get sorted. You'll need some steel in the founds too, i think its called 303, go into a local builders providers and they'll tell you what you need. Won't cost much but you'll have a much better job.
the other posters are 100%, you must have a pro engineer sign off if you ever sell on. What is also important is that if you ever look to release equity either, you'll need it then too. Banks won't loan money on a property if there's any question (merited or otherwise) about the build, particularly something as fundamantal and impossible to examine as the founds. best of luck, sounds nice where you are!