Sloping Sites Solved

ABOVE: Sandra Metcalfe and her late husband Dick built a new oak framed house on a perilously steep riverside site using reclaimed materials and bags of ingenuity. Read more.

A sloping site is both a problem and a gift. The problem: it adds complexity and, therefore, cost to the project. The gift: when you’ve finished, you’ll have a much more interesting house — possibly with a great view.

Of course, it all depends on the slope. It may be that you don’t even realise you have a slope until you get the laser levels out and start surveying; on the other hand, if you go and buy a cliff, it will not come as a great surprise to find out that at least half your build budget will be eaten up by the ensuing civil engineering works.

But slopes have a nasty habit of causing cost overruns. The last house I built was on a gently sloping site in Cambridgeshire. It’s a flat county; we don’t have slopes here, or so I thought. In fact, the rise was about three metres from front to back, over the length of a 50-metre plot. When I purchased the plot, I paid little attention to the slope, thinking it largely immaterial. However, my oversight came back to bite me.

First, the planners were insistent on a design for the new house with a ridge height more or less level with the neighbouring house, which was slightly lower down the slope. This in turn caused me to have to excavate a huge hole on which to place the house; in fact, around 40 muck-away lorry loads went out of the site, blowing my contingency sum before we’d even laid the foundations. It was a formative moment in my putative career as a self-build author and came to form the basis of Brinkley’s Slope Law, which I coined as a result of my experience. It stated that every degree of slope adds an extra £1,000 to the project costs. It has since been revised as a result of building cost inflation and currently stands somewhere between £1,500 and £2,000; but the principle remains the same: slopes cost.

In truth, the purpose of inventing such a ‘law’ wasn’t to use it to make accurate cost assessments of the excavations required, but merely to flag it up as a large cost that you might easily overlook. But, typically, you might expect a 1 in 10 slope – about 6° – on a modest building plot to add around £10,000 to overall costs.

Of course, the slope may well fall away from the road frontage, in which case you have a very different engineering challenge facing you. In fact, I recently looked at a pair of plots located in the bottom of a disused quarry; in order to make the project viable, you would have needed to ship in about 50 lorry loads of hardcore to make up the ground to a sensible level. Whilst the engineering problem is totally different, the cost implications are remarkably similar. Essentially, you have to build off a level base, and whether you are excavating into a slope or making up ground, you have to shift a lot of earth.

An alternative approach is to go for split level. Whilst this may save some money on the groundworks, it invariably adds to costs in the house because you are introducing a series of height breaks into the design. This will include features like extra staircases, different floor heights, more complex service runs and, critically, a much more complicated roof shape. Again, it’s difficult to be precise with the overall cost impact of going for a split-level design, but it’s likely to cost rather more than simply excavating to a level surface.

On the other hand, split-level designs, if done well, can add considerably to the appeal of a house. You instantly avoid the rectangular boxy feel that can be a feature of many new homes and it’s possible to get lots of interesting sight lines through windows placed at heights you wouldn’t normally anticipate.

Another feature – and cost – of sloping sites is the garden. The chances are that you will have to incorporate garden steps and very possibly retaining walls at the edge of your excavations. A split-level garden may look interesting but they are hard work to maintain and they can be really difficult if you are watching out for toddlers. However, older kids come to appreciate the slope: you have a winter toboggan run and a summer water slide!

And for the man who cuts the grass? Welcome to the world of extreme lawnmowing.

Self-builds on Sloping Sites:

• A Stunning Contemporary Self-build on a Sloping Site
• An Oak Frame Home on a Sloping Site
• A Traditional Self-build on a Sloping Site

Further reading:

• How to Make the Most of an Awkward Plot
• Design Solutions for Sloping Sites
• Site Specific Solutions for Problem Plots