What the result means
The envelope is the largest rectangle left after your chosen working gaps and door clearance. Shop below it, because the actual roof, gutters and assembly method may need more room.
What to measure separately
Check the complete delivery route, including gates, narrow alleys, corners, steps and overhead cables. Ask the maker for the largest panel or component.
Fit is not permission
The tool does not apply planning limits. A building can fit physically and still need a different position, lower height or a planning application. Run the planning checker before settling on a model.