If I ever wanted to add a room onto this house, it'd be a sunroom.
Here's my mood board:
I would add the sun room to either off the dining room (where the white french doors are), or off the master bedroom (where the sliding glass doors are).
It'd be a simple black steel design blending in with the house's roof line. I'd replace the siding with cedar plank boards for a warm inviting feel. Grey Slate tiles for a natural look, and forest green curtains that can be extended down inside the glass for shading if needed.
It'll be a simple layout. 2 chairs and a coffee table, and a large comfortable corner chair and floor lamp to curl up in with a good book.
Well that's my idea for adding a room, what do y'all think? Arhaus.com has a lot of great furniture and decorating ideas to check out for your own ideas.