Sunny fallows

It’s not an extension, it’s a porch, which is permitted development if under 3m2. my question is where how the 3m2 is measured

Hi everyone I’m planning g on building a porch on my terraced house. I know it is permitted development if it’s under 3m2, which the building itself will be. However I want the roof to extend about 2m over the adjacent window, does this put it over the permitted development size limit? It says 3m2 measured externally, is that the building itself or the whole area under the roof?