Be aware that artificial turf can get very hot underfoot. You could lay a narrow old brick or limestone paver path to clothes line for easy access and plant ground covers or low profile herbs (eg.oregano and thyme) to use in cooking. Hardy herbs or flowers such as rosemary, lavender, geraniums against the edges of the space toward the back. Even add some large long troughs of flowers on the retaining wall to pretty up the area. It looks like it could get quite warm there but maybe a jasmine or similar climber to climb up a frame attached to the part of the large wall where there is no clothes line. Good luck, it will be a lovely space I'm sure once you get started.

Analysis first, then plan, as Tookoo recommends. Use native plants, as neotoma recommends. Get specific plant recommendations locally, as Terri & sgelade recommend. Here is my two cents worth, for the shortterm. a. Remove and stockpile the lawn edging and most of the mulch back against the fences. b. Take a can of spray paint, or a bundle of surveyor's flag, and mark the largest circular or curved form that will fit within the fenced areas, no matter where the trees stand (inside or outside of the line) to form the temporary edge of your lawn area. See if the entire family agrees on the alignment or just do what looks best to you as the Senior Gardener. c. Add the wood edging to the new edge, or, it will look a lot better if the edge is not visible, using a steel edge whose top edge is down just above the grass roots and no longer visible, and not an impediment to the lawn mower. (when you form the plant beds, you will dig down a couple inches at the edge, where it meets the steel, and then slope the bed up from there, and the mulch will be contained at the level of the lawn, beyond) My recommendation, after you call in your landscape advisor, is to fill the beds totally with ground cover, beneath the shrubs, so no mulch will be seen after the two years of plant growth, using only a finer textured mulch which will become part of the soil bed. (Maybe the bark mulch will smell good in your new fire pit, along with the wood edging.) d. Plant the entire area with grass seed or sod and enjoy the beauty. The curved edge will make the space appear and feel larger and will decrease the maintenance of the smaller bed areas. On the subject of tree placement: analyse first, then plan. Look at all the views beyond your fenceline from all windows of the house, the living/ dining areas, all the bedrooms, to enhance the good views and to block the big, the bad, and the ugly. The large trees on your list will block views from the upper windows, The small horizontally branched trees will be a piece of sculpture in your downstairs view throughout the year, while their flowering and fragrance yet another reason to be alive. For instance, if you have an attractive small tree that has attractive flowers and foliage, PLUS has attractive bark color and has a sculptural quality in the off-season, you won't have to visit the museum quite as often, with a sculpture garden right out your window.

Ask your family what they would like to see, not designwise, but plant materialwise - color, texture, fragrance. Even ask your mother-in-law. You have provided some architect's dream, with the bold geometric forms, the bold palms and cycads. How about something warm and fuzzy. Something to rub your hands on as you head to the door. Some color and fragrance beyond the white and palm trunks. How about vines to soften the hard edges of the planters, fill them with ground cover plants so you won't see the mulch in a couple years, add bulbs of all seasons to bring cheer, glossy leaves to brighten your day. Bring nature to your entrance area to warm up the arrival of your mother-in-law. How about saving room for a couple chairs and small table, to sit out and prove that people actually live in this piece of marvelous sculpture, and add the warmth of an outdoor room. Spend some time at the library with books about the architecture of Frank Lloyd Wright. He is the one who introduced the architectural design idea of the outdoor room. Come to think of it, I don't remember ever seeing a garage door in any of the photos. Guess I'll have to take another look.

I totally agree about removing the lawn and adding a path if it works into your budget. It will highlight the area as a thoroughfare and also remove the maintenance of mowing. Now if you were in Melbourne, I would recommend adding some focal colour using Grevilleas and then underplant with Correa and Dianella. But you are in Perth and I'm not sure about their suitability there! WA is known for it's great array of wild flowers (including Kangaroo Paws) so I am sure you can put together a really attractive blend of local natives. Advice from a local nursery would be beneficial. Good luck with it!