Want to enclose the area, except for an opening where you climb up.

Your post title and the question do not match. Which do you want to do; enclose or extend, maybe both? This would require a mermit in most areas and rightfully so since a structural engineer would need to calculate the new load on the loft. Something is carrying the weight of the loft, possibly a header and it may not be big enough. Ask your local building inspector what he will require before he'd OK your permit

Extending infers a cantilever, enclosing infers a wall. What are you trying to do.

Thanks for the reply. Sorry for the incorrect terminology but I know nothing when it comes to construction.

Well, tell us what you want to do and we'll try to help you reach that goal. Enclosing is usually fairly simple, extending is another story.