I attach some thoughts from Pinterest about small garden spaces. The key is the three dimensional nature of the designs...a clue to success. A clue to garden design is to itemise/list nice to haves, needs and wants, taking into account views, overlooking, climate and seasons, indeed not dissimilar to house layout, and of course changes in fashion, plant growth etc. Another must is to observe sun angles (OK that's climate and seasons) but it's easy to make assumptions. Obviously think about family changes, as you would for house layout.
https://www.pinterest.co.uk/pin/216454325818739013/?utm_campaign=popular_pins&e_t=ecee4d7b9e454551878dfe4f94fa37c3&utm_content=216454325818739013&utm_source=31&utm_term=6&utm_medium=2012