Having 21 panels requires a rectangle 17ft by 24ft (each panel is 5.4ft by 3.25ft). That's far from an estate: if it's a two story home with a flat roof you're talking 816sqft compared to a US median of 2,300sqft.
https://www.infiniteenergy.com.au/wp-content/uploads/2016/06...
There are roof designs where even on a large house 21 panels won't fit, but looking around my neigborhood most houses could easily do that many.