How Hard Is It To Build On A Slope? 

Building on the side of a hill has always been an attractive prospect for builders, architects and homeowners alike. Why? Aside from the spectacular views, hillside builds represent the ultimate challenge. For builders and architects, building on a slope brings a unique opportunity to excel at their craft and be creative in a way that standard flat site builds don’t allow for. And for homeowners, the chance to tailor the build in ways that better express their individual personalities.

Understandably building on a slope is not quite as straightforward as building on a standard flat site. The build process is far more complicated, and each site faces its own unique challenges. But, being aware of how hard it is to build on a slope will ensure the rewards are worth the extra effort!

How Do You Build On A Sloped Section?

In New Zealand, several construction methods are commonly used when building a house on a sloped section. These are discussed below.

  • Cut And Fill. This option requires levelling out the section before beginning the build process. This is done by adding soil or removing it and usually involves the use of retaining walls.
  • Pole Houses. Designing a home that has minimal effect on the landscape is a common choice for hillside builds. It involves the use of long poles or ‘stilts’ as a way to hold the house or part of it up above the slope, effectively creating a level building that doesn’t necessarily follow the slope of the ground.
  • Buried In The Slope. Designing a building that partially disappears into the slope can be a great solution for some sites, especially where blending in with the surrounding scenery is desired.
  • Split Level. Homes built across multiple levels are a great way to accommodate the changes in height of a sloping section. Split level homes are connected by short flights of stairs or transitional areas allowing you to take advantage of the natural slope of the section and maximise views.
  • Cantilevering. Where part of the house juts out above the slope of the land is a striking way of producing a distinctive build while leaving the land beneath relatively untouched.

4 Benefits Of Building On A Slope/Hill

Building on a slope also provides a unique setting for the most innovative design. Housing is tailored to the landscape, and every home is different – there are no cookie-cutter designs on the hill! This is why hillside suburbs are often some of the most sought after places to build or buy a house. Check out some more of the benefits of building on a slope below.

  1. Epic Views. Whether you are hovering over a steep grass-covered hillside with blue sky for miles and miles, staring out at the deep blue ocean waves or taking in the sweeping sprawl of the city from above, hillside views are unique, relaxing, often breath-taking – the highlight of many homes.
  2. Interesting Landscaping. Hillside sections are never boring! There are always unique features, landscaping potential that will only add to the aesthetics and appeal of your home.
  3. Natural Light. When designing your hillside home, making the most of the views will also mean making the most of natural light and sun. While free warmth and great lighting are good, there are also other benefits such as better sleep and higher productivity.
  4. Improved Privacy. In general, homes built on a slope are shielded from the prying eyes of neighbours or passers-by, giving an added feeling of seclusion.

What Are The Challenges Of Building On A Hill?

There are two types of hillside builds – an upslope where the house rises up away from the road, and a downslope where the house follows the slope downward away from the road. Both of these offer slightly different challenges, however, overall, it comes down to the gradient of the slope. Less than 10% incline is considered the easiest to build on. Anywhere between 10-20% is considered a moderate slope, with anything over 20% considered steep. As the gradient increases, so do the associated costs. Some of the additional challenges associated with hillside builds are:

Increased Time And Cost Of Foundation Work

The cost of the complex foundation systems will always add to the price of hillside houses. This is because they typically require more materials like concrete and reinforcing and timber, more research, more excavation, more labour, and more time.

More Complicated Drainage And Sewage

Proper attention to drainage for both surface and subsurface water is one of the most important factors to consider when building on a slope. Runoff from rainfall needs to be collected to avoid runoff to neighbouring properties. In many locations this will require use of a stormwater storage tank so that older council infrastructure isn’t overwhelmed by rainwater collected by the increasing number of houses being built around the Port Hills. Retaining walls also need adequate drainage to prevent water from pooling and undermining their structural integrity. On the other hand, sewage may need to be pumped upward or have its flow restricted downward depending on the building location in relation to the sewerage connections.

Limited Access To The Site

Getting building materials, trucks and contractors on site is also an essential aspect of hillside builds. Even flat sections can become muddy and difficult to access after the slightest bit of rain; sloped sections are much more difficult to access even in drier weather. Additional site works may be required just to provide the necessary access for cranes, concrete trucks, infill trucks, subcontractors etc. which may then need to be reversed or corrected once the build is complete.

Ground Formation Issues

The type of soil you are building on can affect any build situation in New Zealand, but it is even more important for a sloped property. Christchurch’s Port Hills have been analysed very closely since the earthquakes, and there are many factors that will influence how easy a site is to build on. It is important to find out where rock is located, as rock excavation is time-consuming and expensive. The ideal situation is where most excavation is through loess and other easily-excavated materials, with foundations able to extend down to solid bearing on rock. Of course some of the most spectacular sites are in areas with shallow or exposed rock, so if you’re considering a site like this then you should allow a bit more in your budget for rock excavation.

Landscaping The Construction Site

While the challenge of landscaping your sloped section might seem like fun at first, it is going to cost you more and take longer than a simple flat section. You will need to pay specific attention to the types of plants you choose and will likely have to build up areas or flatten off areas to form suitable useable outdoor spaces. It is particularly important to consider the design of the house in relation to the landscaping – on an exposed site the house can be positioned to create sheltered outdoor living areas where you can escape the wind.

How Much Extra Does It Cost To Build On A Slope?

Yes, it is more expensive to build on a slope. But how much extra will it cost? When it comes to building on a hill, figuring out the ‘how’ is the most important part of pricing the build. Once you know how you are going to build it – then you can more accurately estimate the cost of the build. Quantity surveyors are experienced in pricing all types of house construction, and will be able to make allowances for factors such as site access, steepness of slope, foundation type etc. when preparing a cost estimate at an early stage of the project.

7 Recommendations For Building On A Hill

If it costs more and takes longer, you might be wondering why bother to build on a slope? The views from the hillside suburbs of Christchurch such as Cashmere, Clifton, Huntsbury, Mount Pleasant, Redcliffs and Sumner speak for themselves! No matter where you live, building on a hill almost always means the guarantee of spectacular views throughout the home. Here are some tips on pulling it all together and proceeding with a confident and successful hillside build.

  1. Hire A Quantity Surveyor. In general, hill builds are lengthy and more expensive. Getting a quantity surveyor on board early on will make sure everyone is on the same page with cost estimates.
  2. Ensure Everyone Is Working Together. If the client, architect, engineer and builder are working together as a team throughout the project then you gain the benefit of everyone’s knowledge when it comes to designing your home and planning how it can be most efficiently built.
  3. Have A Contingency Fund. Setting up an agreed-upon contingency plan in the contract will give both the homeowner and builder a good buffer to work with when the unexpected occurs.
  4. Allow Extra Time. Hill builds take time, most around 12-15 months just for the build, while a standard house on the flat could realistically be completed within 6-9 months. Ideally the work should be planned to avoid excavation work during the wetter winter months.
  5. Get A Geotechnical Report. Understanding the ground in which you wish to build is even more important when building on a slope – you need to know what you are dealing with underneath the surface. Your structural engineer can usually help coordinate this report if you don’t know where to begin.
  6. Understand Local Council Regulations. Most houses on the hill will require resource consent – even if you manage to keep below height limits and recession planes you will usually require resource consent for the excavation work anyway. Understanding how the District Plan works will help you to maximise the potential of your new home.
  7. Choose Your Team Carefully. As we have discussed above, sloped sections do involve some extra challenges. It makes sense to choose an engineer, builder and architect, with considerable experience in designing and building on a slope for best results.

Building on a hill means innovative design and a home that is unique. Working with the slope can turn it into a distinct advantage for both everyday living and resale value. Thinking of building on a sloped section and need some advice from an experienced architect? Call Chaplin Crooks Architects today for a free consultation to discuss your project.