Professionally designed gardens have extremely wide flower borders. Usually a minimum of one metre wide, but often far more. For normal sized suburban gardens this is not realistic. However, when I walk around my neighbourhood most gardens I see have borders about 20cm wide which is just far too narrow to plant anything at all. A great border can be achieved with a depth of 0.5 metres, but be as generous as you can be. Think about whether you need to keep the grass at all. I never understand why some front gardens have a small postage stamp of lawn, it would drive me crazy to pull out the mower just for that. I would definitely put down a weeding fabric, cover in pea shingle and plant to my heart's content.
So many gardens have this top layout. A thin border of soil with a large rectangle of grass. This means the plants have to be lined up in a single row like little soldiers. Boring, and also leaves big patches of soil in between them - hello weeds!
By rounding the corners of the lawn and removing a thin strip from each side creates larger, wider borders while still retaining a meaningful sized lawn. Plants can be staggered in groups, reducing space for weeds to crop up and giving you the opportunity to plant a huge amount of seasonal variety and interest.