There is not really a problem with DIYing the design as long as you know what to do.
The problems come when you design your supports, layout, etc and then the engineer looks at it, (charges his fee) and then if you need to alter it - and then its another fee. Or he designs some expensive beams/supports which cost a lot more than a better or cheaper alternative
Or the engineer comes out, tells you to do something one way, but that is not the design you want.
Or if planners want changes to a dormer, which alters loadings
So there are potential problems, and you need to keep your eye on costs.
Obviously, it is is just two beams from wall to wall then that is fine
So ideally you want someone (who could be you) to design the conversion, knowing what will work structurally and be acceptable to planners and building control, and knowing that the beams or supports you want the engineer to calculate are he most efficient and cost effective so that it is only done once, is easy to build and does not cost more than it should
