This refers to the shape of the land; is it flat, does it slope or roll? Land-form has high aesthetic potential and determines the surface drainage patterns within the garden. Consider any changes to slopes that may be required in the garden and ensure that there are adequate measures in place to deal with any drainage issues that changes might cause.
If your site is flat then you can work with that. Emphasise the horizontal and try to achieve really level flat areas that you can use to contrast with any vertical or upright entities that you choose to include. A really flat lawn will offer a good visual base for a herbaceous border or for larger trees that grow in an upright or columnar habit. You can also model the ground to good effect, especially if you have some subsoil and topsoil arising from the excavations necessary to build your house. This can be used to create, for instance, a terrace leading to another flat area which will offset your larger lawn. You can try creating some rounded and sculpted earth forms also, but these are rarely successful unless they are based on real features that occur in the wider landscape.
Tip: If you are looking for a starting point to help you do some earth modeling in your garden, start by looking at Japanese gardens which echo natural landforms. This is not to say that you should attempt to create a Japanese garden, but simply be informed by what you see there.
If your garden is gently rolling, has any kind of slopes, valleys or other features then work with these features. Rather than trying to flatten the mountain as it were, accept that it is there and work with it. If you have a naturally occurring small valley which is wet at the bottom, then plant materials which will grow there. If you have sloping ground, use it to build in a small adventure playground for your children or use planting to draw attention to the slope, making it part of your design.
During the assessment phase keep in mind that you will, during the design phase, use your existing landforms to good aesthetic effect. Ensuring that features such as pathways and planning schemes follow existing contours or other landforms will help you to integrate the garden with your surroundings. Your enjoyment of the space will be enhanced as a result & your garden will help you to feel that you are part of the landscape area where you live.
Tip: Consider placing a pond at the base of slopes and let it fill and empty as the seasons change. This will provide aesthetic interest, and will have a practical application in terms of drainage. It will also be eco-friendly, providing a natural habitat for a range of plants and animals as the seasons change. It is important of course to take all the measures necessary to ensure that ponds or other water feature are safe for people to be around.