There are a number of possibilities. Anyone offering advice should know where you live so they can assess your climate for suitable plants. As MUSA said, a professional in your area is recommended as will be able to help you with layout and plants. If you don't wish to go down that path, there a many entries in HOUZZ that deal with narrow spaces and gardens between the house and side boundary.

Hi Aravind, Personally I'm a fan of minimising impervious surfaces, and would try to attain different levels of landscaping with native vegetation, lawn & permeable paving. This doesn't sound like what you are looking to do, but if you do go ahead with concreting a large portion of your backyard, keep in mind that water which would have gone into the ground will be redirected to somewhere new. Definitely make sure that the concrete slopes away from your house to avoid damaging your footings/house (instead of the potential termites doing it). Regarding the kitchen, it can often be worth chatting to an architect/building designer if you are likely to do more renovations down the track. They will be able to help you come up with (and draw up) an overall concept so that you know that your kitchen will work with any future renovations and don't end up knocking down or redoing anything, which can save you down the track. An example would be if your kitchen was not in the best location to make use of your living spaces or faced the wrong way. If you were gutting the whole thing, you could be better off moving it now to increase the livability/value later. A joiner or cabinet-maker will be able to go into detail with you and get it built.

Gum trees are provide a stunning canopy to any garden, but as with any large tree close to your home, you need to be mindful of its health and possible future limb failure. Sudden limb failure in Eucalypts is the result of periods of dry weather, impacts on a tree's root zone and subsequent moisture stress. I recommend creating a beautiful native garden in this area with groundcovers and grasses that are suited to the sunlight available. Make sure the area is properly mulched (organic mulch is best) and that you water the area during periods of hot weather. It's also worthwhile having an arborist look at the tree every 12 months of so to make sure it is still healthy.

"Macyjean where do you recommend the vertical feature go...? Yes trying to make sure trees aren’t too large they cause damage to the property via root system and/or branches or fruit/nectar falling on the roof/gutters/paving." Somewhere on the left, but start with hardscaping, driveway, draw in underground services, think about what sort of footpath you want. I agree that it might be well worthwhile to get a landscaper to draw up a plan. "Thinking bay leaf could be an option...?" I remember going on a garden tour of a property with a park-like garden featuring large trees and the guide pointed out the bay tree and said people don't realise how big those get. There might be dwarf varieties these days though? "With the secret garden if I go teddy bear magnolia how do I layer plants to give varying height and colour while spacing them out for future growth? Is there a rule of thumb with number of levels/layers?" Just my opinion but looking at your photo and for ease of maintenance I think the magnolia with an underplanting of lomandras or something like that would be plenty. I can also picture three layers with flowering shrubs in between but I think they'd need regular pruning to look good. Lomandras don't need much attention, it's recommended to cut them right back every few years to rejuvenate them but it isn't crucial, whereas if you're going for a certain look with shrubs and don't stay on top of the pruning they can look quite scrappy and if you prune too hard they can take a while to recover. "What’s the rule of thumb for spacing of plants?" I think there might be some kind of general rule of thumb based on mature width? But I'd base it on the plants you choose. The label sometimes recommends the spacing, or if not the growers' websites have more information.