It's a long time since I've had to deal with slopes, although coming from Dunedin, I developed a healthy dislike of steep sections early on.
We lived on St Clair Hill and getting home from school meant walking up the (from memory) 284 steps of Jacob's Ladder, which led to our house.
More steep sections followed as I chased the warm weather north and finally, in Kerikeri, I got a flat section. A mate with a digger made a few very gentle contours to create a driveway and parking area, and to disguise the remains of the shelter belt I had cut down. I can walk straight out of the house on to the terrace, then into the lawn, and then into the orchard, without any discernable change of level. I am one happy person.
So I have sympathy for people who ask me how to design a garden for a hilly site. I stifle the urge to tell them to move because the advantage with a hill site is, of course, that you get a view. There are another advantages, too. Changes of level present an opportunity to alter the style or atmosphere of the garden without having to create the whole vista from scratch. And you can plant trees and shrubs of differing sizes in such a way that each is shown to its best advantage. With a flat section, you're always whacking the top of something.
Gardening on a hill is no problem provided you don't want flat bits. Just plant the whole thing in small trees and shrubs and ignore it. But if you want design elements and landscaping like steps, paths, walls and terraces, then you have a bit more of a mission.
I've seen dozens of steep gardens "landscaped" with poles and half rounds of timber holding up rough terraces of dry, crumbly soil in which tatty ground covers cling precariously to life. If that's not the look you're after, then be prepared for planning.
Assuming the house is at the top of the slope, you need to organise landscaping and planting so you look down over a series of areas descending the slope. They can be casual or defined, but try to leave "breathing spaces" such as paving, seating areas or lawns, or your garden may look like a jungle from above.
And check out the growth patterns of what you're going to plant. If you have a penchant for big trees, be aware that you won't see much beyond them, so they'll need to go at the bottom.
The steep bits between the flat spots will be a challenge to plant.
Slopes don't hold water very well, and it's common to find the plants at the bottom where the water gathers doing very well, while plants further up the bank are struggling. It explains the popularity of the Grevillea Bronze Rambler, which has long been a common solution to that particular problem.
Retaining walls and steps will be a necessity but they don't have to dominate the design. Position them at an angle to the house or whatever building they're closest to, and soften them by planting something alongside. Grasses and groundcovers will spill over the edges and disguise any rough bits, and make them look as if they've been there for a while. My current favourite combination for this purpose is Lomandra Seascape with star jasmine. I like them, and fortunately they seem to like each other.
If you're building a fairly utilitarian retaining wall, something that grows over it will give you camouflage, but if it's a lovely dry stone wall, or one made of railway sleepers, you'll probably want to leave these exposed, so choose plants that don't creep.
Rocks, if they suit your site and if you can lay your hands on them, are very useful when it comes to slopes.
They hold up the soil, provide passive solar heating, and add design elements to spaces where it's difficult to position a three-tier fountain or a reflection pond.
And little areas of lawn will also enhance your design.
The trouble with slopes covered in plants and not much else is that they can look rather uninviting.
But if you can see a set of steps leading down to a small lawn with a garden seat, you'll be far more tempted to take your tea or your wine down there, and enjoy your garden at close range.