Building regulations plans for domestic extension

I have planning permission for a 67 m2 single storey extension and I now need a designer to make plans for building regs approval.
As I don't want to have to employ a structural engineer at extra cost, you would need to be able to specify the materials - with particular guidance needed on the roof timbers as the span is over 6m. I have guide sheet of my ideas for this, which I attach with a couple of the designer's plans. I will be building this extension myself asap.