Building on a Sloping Site

ABOVE: The site of this contemporary self-build in Manchester, designed by Stephenson Bell, drops eight metres from front to back. At the front of the house, a footbridge allows access inside and a garage is raised above the living area. Read more about this self-build.

It’s really only those who buy plots in the Fens who can be sure of absolutely no slope and, for the most part, the plots that come onto the market for selfbuilders invariably have some gradient to them. In certain areas, a sloping site is the norm and in fact adds to the appeal for that region — as with slopes come views.

But they don’t come cheaply and the greater the slope on the land, the more it’s going to cost to build on it. Indeed, looking at a sloping site for the first time can be quite daunting, especially if you’re having to look down or up a steep escarpment. But always try to position yourself at the midway level so that you can appreciate the extent of the work involved; and look next door or down the road to understand how others have solved the problem. That may be by employing the tried and trusted regional vernacular or it may be by employing modern and innovative solutions to an age-old problem. In all cases, the opportunities are there and, if the price is right, the self-builder can take advantage. Here’s the main options.

Build Out of the Slope

This involves taking the highest point of the land and then building a level platform out and over the slope. In certain situations it is ideal but in others, particularly where the issue of ridge heights is exercising the minds of the planners, it simply won’t work. If the underbuilding is to be solid then although the flooring – assuming the use of a beam and block suspended floor – will cost the same as for a flat site, the underbuilding itself will add to the costs. On the other hand, there is little or no extra spoil to be disposed of. An alternative, and one that is often put forward by those with a green turn of mind, is to build out on stilts, leaving the ground beneath relatively undisturbed and minimising the property’s contact with the surface. In fact, unless the heights are quite extensive, much of the ground that is left beneath the building is useless for any purpose and is unlikely, being dry and shaded, to promote the growth of natural vegetation. But if carefully managed this can, nevertheless, be an attractive solution.

Build into the Slope

With this solution, the starting point is the part of the house that is at the lowest point of the slope with the ground floor cut into the hillside. The advantages are that the ridge heights and, therefore, the impact on the landscape, are minimised. In addition, as most of the floor will be cut down below the natural depth of vegetable soil, a solid concrete oversite can be employed which, whilst not necessarily cheaper, may be easier taking into account the slope of the ground.

A disadvantage is the fact that much, if not all of the spoil, will have to be removed from site, and at something like £200 for a 20-ton load, that can pretty soon build into quite a considerable outlay. Another problem is that, in most case,s the size of the plot will not allow the cut to be amortised over the distance between the house and the top of the slope — and that means having to build a retaining wall, or even a series of retaining walls. These will cost at least £100/m run when one metre tall, and close to three times as much if two metres tall.

Cut and Fill

Here one takes the midway point of the house relative to the slope and extends the floor level equally into and out of the slope. Therefore, at the top end of the property there may well be an element of building into the slope but, unlike the solution above, this is likely to be fairly minimal and there may in fact be no need for retaining walls, with the ground simply sloped up from the house oversite level or the surrounding pathways. Additionally, in many cases the resulting extra spoil can be utilised to make up the levels on the lower end of the house, thus doing away with the need to dispose of soil. Where the slope on the land in either direction is fairly minimal, this is often the ideal solution.

Build with the Slope

All of the above solutions involve an element of ‘fighting’ the land: seeking to alter it to accommodate this level intruder. Another solution is to design the property to ‘run with’ the land or follow its contours. How successful this is depends to a large degree on the nature of the slope. Most storey heights are fixed at around 2.5 metres and, if the land is sloping sufficiently it becomes relatively simple to tuck one storey on top of another in a series of interconnecting levels. However, each storey may not fully overlap the one beneath and the design may have to take into account the need for additional support. If the levels are right then carting extra spoil away may not be needed. If running with the contours of the land means that elements of the building have to be cut into the slope of the land, then this may mean that parts of the resulting accommodation will find themselves below ground level in whole or in part — and that will require ‘tanking’ or waterproofing in just the same way as a basement, plus soil away, as for the second option above.

ABOVE (LEFT): Steel and timber stilts enabled this house to dramatically project across its rocky, wooded hillside site in Perthshire (Read more about this self-build); (RIGHT) The significant slope to this Glaswegian Modernist house dictated a split-level design.
BELOW: A projecting porch gives prominence to the façade of this oak frame Cornish self-build (Read more about this self-build); BOTTOM: This new Scottish Baronial-style castle maintains its presence, though much of the accommodation is hidden away from the front approach (Read more about this self-build).

Design Issues

Undoubtedly one of the advantages of a sloping site is the opportunity to do something different in design terms — but it has be well thought through. Split-level design looks great but it’s not for everybody, with level changes and internal staircases within the various living zones as well as between distinct zones of usage.

If the slope is up from the road, then the upper storeys can tower over each other and the adjacent properties — which can present both aesthetic and planning problems. If the slope is down from the road, then the fact that the visible elevation is minimised might detract from kerb appeal. If the slope is from side to side, then there is perhaps the best advantage to be gained, although care needs to be taken to hide or minimise ugly underbuilding.

The garage is likely to be the biggest stumbling block in design terms. If the land slopes down from the road then getting the garage into what may be the top floor is easy in structural terms, but unless great care is taken, the doors could dominate the principle elevation of an already diminished façade. If the land slopes up from the road, then the house sitting on top of the garage may well become over dominant in the street scene. The solution, in both cases, if space permits, may be to detach the garage.

How Much Will it Cost?

You should expect to pay at least £1,500 extra for every degree of slope. Soil away is the greatest expense at £200-250 for a 20- ton load, plus extra labour and plant costs. The flooring element will remain the same but there is extra cost involved in having to step foundation concrete. Drainage may be unaffected, but if site levels are in opposition to the inert levels of drains or sewers, a pumped system is likely to cost £2,000. Landscaping may attract extra expense for steps and retaining walls.

More self-builds on sloping sites:

• A Stunning Self-build on a Sloping Site
• A Riverside Cottage on a Sloping Site
• An Eco Home on a Sloping Site

Further reading:

• Site-specific Solutions for Problem Plots
• How to Make the Most of an Awkward Plot
• How to Find a Building Plot