Out House/Wet Room

Morning All.

I intend to utilise a small area on the back of our house to build a small wooden out house come wet room. It's basically somewhere to chuck the boots and coats and the wet dog after walkies. It'll only be about 1500mm x 2000mm floor space by 2500mm to the top of the single pitch roof. Nothing too fancy, timber clad, tiles roof and a stable door. What I'd like to know is:

The timber frame, what is the best timber to use? A specific grade?

Should I cover the outside of the frame with marine ply before cladding with horizontal laps?

Can I tile straight onto normal ply/plasterboard on the inside of the frame?

What insulation is needed, if any?

Thanks in advance, hope this makes sense without me sounding like a wally!

Without pre-build professional advice on site you might be building in a number of faulty, damp causing, details when attempting to construct a lean to against your house.

Why not google any number of diagrams and pics of porches - it will give you an idea.

FWIW: marine ply is a sales con for most dry land construction needs.Just use exterior ply.

Thanks for the reply, Google wasn't really giving me the answers I wanted if I'm honest but will bear your thoughts in mind.
Thanks for the tip on the ply!