Have you thought about creating a 'island'
bed in the center of the lawn? It can be planted with a variety of
low-maintenance shrubs and perennial plants. This would be something enjoyable
to look at from the terrace, bench or swing. Because the lawn is partly shaded
by a tree, you could choose plant material that thrives in semi-shade.

Your neat box hedge is a really nice
feature! You could include a few box shrubs in the bed, this would visually
link the new bed to your existing garden, for a more 'finished' look. There are
a wide variety of box shrubs, some grow only a few feet high and require no
clipping.

Other shrubs to consider are: Azalea (bright
flowers in the spring), spirea (many different varieties with interesting
foliage and flowers) and weigela (bright flowers attract butterflies). You can
also include some low-growing holly bushes for additional evergreen texture
(similar to the box hedge, they retain leaves throughout the year).

Perennial plants are plants that come back
every year so they are great value for money. Shade-tolerant perennials include
hostas (beautiful foliage), day lilies with large, colorful flowers, astilbe
(delicate feathery blooms) and japanese painted fern (silver-grey leaves).The bed can include a few stepping stones
so you can get in there and add mulch or fertilizer easily. Another helpful
item is 'landscape edging' that runs around the edge of the bed. This prevents the lawn
and weeds from growing into the bed and saves you time weeding. You can choose
a color and style that suits your terrace, for example this item looks like a
pale-colored stone. Because it is flexible you can use it for curved edges too.

One more thing, how about including a small
fountain in the center of the bed - this would look really pretty!

thank you very much for your ideas. I completely forgot about this forum as nobody replied for a long time. We finished this project and moved to another one. We've build a wooden veranda with double glass and UV filter. It makes a huge difference during hot summer. We imported red bricks from Netherlands to create paths around raised beds. We make our garden also optically larger but have a look at the photos,