Laminate Installation Beginner

I'm replacing carpet with laminate flooring.

My subfloor is close to 20mm lower than the door, maybe a little more.
Is underlay cheaper than attached underlay?

I don't know if it is a problem to have the flooring lower than the front door.
It would also be lower than the kitchen area too if I did that.

 

On a ground floor you need to use a sheet dpm first then a underlay.

 

Ok. the height difference will still be low can I add another layer of plywood to fix this? by 6mm lower.

 

Thank you for the help.