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.
There are many different material options for constructing retaining walls, the most common is concrete blocks. Concrete blocks are relatively cheap and create robust, stable and long-lasting retaining walls. A blockwork wall can be rendered or faced with a wide variety of materials including brick, stone and even stainless steel sheets. I like facing retaining walls with slate cladding and pebble mosaic sheets to give textural contrast to other hard landscaping materials. Other options for wall construction include brick, stone, gabion cages, poured concrete, railway sleepers, timber and even compacted earth.
The choice of building materials depends on budget, the style of the garden and what the wall is to support. Railway sleepers are cheap and quick to erect, but they are not as robust or long lasting as blockwork and are not recommended for higher retaining walls that must support large volumes of soil. Although they often seem like a cheap option, gabion cages can be time consuming to erect and more expensive than a stone-faced concrete blockwork wall.
Getting rid of excavated soil is another important consideration. Excavating the levels and digging footings will create a surprisingly large pile of soil. Some, if not all, of this soil can be used to back fill the new terraced levels. But there is usually some excess and this needs to be factored into the project costs. Grab loaders and skips are expensive and it’s surprising how many people overlook this element when estimating project costs. A couple of sections through the various garden levels will enable some idea of how much soil will be left over after ‘cut and fill’.
Plans for any garden construction project should always include a construction detail and setting out drawing. These are particularly important for facilitating the construction of a sloping garden. One or both of these drawings should include multiple sections through the garden showing existing garden levels and new finished levels of each garden terrace as well as the positions and finished heights of each retaining wall. This allows for accurate quotations as well as ensuring the build goes smoothly.
Once all the terraces and heights of the retaining walls needed to create them have been established the next thing to do is decide how to access the various garden levels. Steps are a logical and obvious choice for getting from one level to another, but a winding ramp will create a lovely informal journey around the garden and break the harshness of the hard landscaping. If the garden includes a children’s play area some fun ways to navigate this area include a helter skelter, rope ladder and a fireman’s pole.
If steps are being used to link the levels then creating walls with a finished height that is a multiple of the ideal step riser height of 150mm will make construction much easier. Ramps should have a slope of no more than 1:12 so this will also dictate the height of retaining walls.