A good design can transform a sloping site into a spectacular garden. I have designed many sloping gardens, I love doing them but know how tricky they can be to get right. You need a thorough understanding of site topography and a detailed plan to address the level changes in order to avoid the many pitfalls of creating gardens on slopes.
The best way to design gardens on sloping sites is to create terraces using retaining walls to provide flat areas that can be used as functional spaces within the garden. The level changes in a sloping garden dictate naturally the creation of outdoor rooms at different heights each with a unique atmosphere. The spaces can be enclosed and/or sunken for privacy and intimacy or opened up creating a sunny terrace with a fabulous view. The steps and ramps needed to navigate and access the levels can provide different journeys around the garden as well as being sculptural design features.
The garden must be structured to create useable spaces that work with the level changes and meet the clients brief. In the UK, retaining walls can be no more than 1m high adjoining the roadway or 1.8m high elsewhere in the garden without needing planning permission. Always check house deeds as some have covenants that relate to maximum permissible heights of garden walls.
It is rarely feasible or desirable to create one large retaining wall to reduce the garden to a single level. Other than on a very small slope this would be overbearing, make some parts of the garden hard to access, and almost certainly raise the garden so that it infringes on neighbors privacy. Avoid very high retaining walls, and split the garden into fewer, shallower levels.
The new garden levels must not be raised so high that they are overlooking neighboring gardens. It may not be possible to construct a large terrace adjacent to the house if it sits on top of a steep bank. This would create a platform from which people could see directly into neighboring gardens and houses and this is neither desirable nor legal. In the UK, deck may not be raised more than 30cm from the existing ground level without planning permission. The solution is to create a seating area in a lower part of the garden – not quite so convenient, but more private for everyone concerned.
The height differences between various parts of the garden dictate how high any retaining wall needs to be for any given distance away from the house. The further any terrace extends out over or into a slope the higher the retaining wall required.