Thanks.

The plan got somewhat smaller, but it still ranges from 0 to 6400 feet, and 456 to -1890 feet. I cannot see any objects scrolling around the border.

I attempted going to each level, selecting 'all', deselecting the house by holding shift & dragging across, and then deleting the remainder. Save & re-open. No change to the plan size.

I tried deleting the background image and saving / reopening. No change.

I tried 'select all on all levels', deleting everything, and save/reopen. Still the same oversized plan.