A sun lounge with proper walls, roof and foundations will likely be
more costly as it will probably need building regs approval - it may be
classed as an extension, not a conservatory.

Any open doorway will mean that the new lounge/conservatory will
require building regs approval. A door will mean the new
lounge/conservatory reamains separate and not coming under building
regs - unless it requires approval anyway by virtue of what it is.

Have look at, and a word with the technical departments of
www.igltd.co.uk or www.catnic.com or www.keystone-lintels.co.uk

dg