Building a house on sloping land

If you have been looking at plots and already have some price references, you will have noticed that sloping plots are always cheaper than flat ones. This is because building a house on sloping land is somewhat more expensive—let’s see why.

While a flat plot is more convenient, nowadays we have sufficient technical resources to level land by carrying out earthworks and retaining walls. So why is a sloping plot more expensive?

Precisely for this reason: building a house on sloping land increases the cost of earthworks and the corresponding retaining walls, and in certain cases it can even increase the cost of the foundations.

In fact, for this reason, we always recommend contacting an architect before buying the plot. A professional can help you assess the additional cost of earthworks and retaining structures and check whether the foundations could also increase the budget, based on geologist test pits.

At our studio, we always support the client before they purchase the plot to assess the feasibility of the build, especially when it involves building a house on sloping land.

So, in today’s post, we analyse what it means to build a house on sloping land and what additional costs you should take into account, so you can assess whether the savings on the plot price are worth it. This will depend on each case, but I can share some key recommendations.

We will look at a set of recommendations for building a house on sloping land and making the most of it, since the slope can shape the project, but it can also work in your favour—by taking advantage of the sun or the ground’s thermal mass.

1 Assess the plot’s orientation

The slope’s orientation will be essential when building a house on sloping land.

South-facing orientation

This is the best-case scenario. A south-facing plot allows us to place a large façade on this side and open up large windows to capture sunlight.

In addition, if the house is adapted to the slope and built on different levels, we must bear in mind that this is an advantage, as we will have several south-facing façades.

Other orientations

Of course, we would not rule out a plot outright because it is not south-facing. There are also options for building a house on sloping land facing east, west, or north.

In these cases, the presence of courtyards or other strategies that allow us to capture southern light will be very important, especially in winter.

Recently, we designed a house in Cervelló where the plot sloped to the north, so the sun reaches it from the upper part of the site.

In this case, to achieve direct sunlight, we opened a courtyard in the centre of the house, creating a second façade that captures the sun’s light and heat.

In addition, the upper part of the house is designed with a large skylight also oriented to the south, again with the aim of increasing the capture of solar light and heat.

Thermal Mass

When building a house on sloping land, we should also anticipate that part of it is likely to be partially buried. What may seem like a drawback can be an advantage in terms of energy efficiency.

One of the characteristics of the ground is its high thermal mass. This means the earth stores heat and, in fact, by burying the home by just a couple of metres, we can achieve a stable temperature of around 20°C all year round.

Ideally, the north façade—the coldest one—should be buried to keep it at a stable temperature. There is no need to fear potential damp, as today this can be resolved constructively with cavities that allow ventilation and water drainage into the ground without the risk of moisture.

On the other hand, the excavation carried out on the slope can also be used to bury a Canadian well. Let us remember that Spanish regulations require us to ventilate homes directly. Burying a Canadian well is a very cost-effective strategy that allows us to take advantage of the ground’s thermal mass and preheat the air we introduce for ventilation through a pipe embedded in the earth.

In summer, this 20°C air will be sufficient to ventilate the house at a comfortable temperature, while in winter it can be warmed with a small heat input up to 22°C.

2 Check what type of ground makes up the subsoil

The type of ground will determine our strategy for siting the home and designing its foundations.

Foundations on sloping land can follow two strategies. On the one hand, and generally in the case of steep slopes, we can choose not to modify the land’s natural slope and simply anchor the house with deep piles into the ground.

This is a common strategy when building a house on sloping land, but one we should be cautious about, as a house of this type loses any relationship with the natural ground and turns into a flat-like house suspended from the terrain and connected to the outdoor space by stairs.

In fact, this typology would only be advisable in the case of very steep slopes or poorly cohesive ground. At the studio, we recently encountered the case of a house on an extremely sloping plot with very low-bearing soil; in this case, we opted for the strategy of raising the house on deep micropiles.

As a second strategy for siting a home on sloping land, there is the possibility of carrying out earthworks and retaining structures.

This strategy allows better integration of the home into the site, as well as taking advantage of the ground’s thermal mass.

In this case, once the earthworks have been carried out, the foundations will be similar to those on flat land and, as always, will depend on the type of subsoil.

The reason a house on sloping land is usually somewhat more expensive than the same house on flat land is precisely this prior earthworks and retaining work, which should not concern us if the ground is sufficiently firm. For this reason, it is always advisable to commission a geotechnical study, or at least a few test pits, to check the ground’s firmness before buying.

Possibility of carrying out a geotechnical study

If we have not yet bought the plot, it is normal to want to check whether it is suitable without having to spend a large amount of money (a full geotechnical study costs around €1,000). For this reason, it is possible to commission a geologist to carry out a couple of test pits in the area where the house is expected to be located (here again, it is advisable to consult an architect to review the regulations).

Test pits provide information about the soil’s strength and type and allow us to get an idea of whether a plot is suitable for building or could cause problems.

If, after the test pits, we are still not sure, then there is no alternative but to commission a geotechnical study. In some cases, it may be possible to agree with the owner on who will bear the costs, but it is important to know that the owner is not obliged to provide it for the sale.

The geotechnical study is, in a way, the continuation of the previous test pit. It is information obtained through a series of controlled boreholes for site investigation in relation to construction. In other words, we will obtain all the data needed to design the foundations of a house or building.

A full geotechnical study gives us confidence about the ground’s composition and its capacity to bear the loads of the construction.

In fact, carrying out a geotechnical study is mandatory before starting foundations on any construction.

Earthworks and retaining structures

As mentioned in previous sections, when building a house on sloping land, one of the most common and recommended strategies is earthworks and retaining structures.

Although it depends on many factors, one of the best solutions is to adapt to the slope of the land, seeking, as far as possible, the most stable and flattest area to locate the main spaces of the home. In this way, we would avoid excessive retaining walls or earthworks.

The less earth we move and the fewer retaining walls we build, the lower the costs. We will also avoid major earthworks that, beyond causing a significant visual impact, are not consistent with the site’s morphology. If we have bought a sloping plot, we must adapt to it, respecting its water runoff and integrating our home into the surroundings.

3 Plan the possible siting and layout of the house

Finally, we reach the point we have all been waiting for: How do you build a house on sloping land?

Let us look at the factors to consider when planning to build a house on sloping land in relation to its layout.

The first thing we need to check is at what level the access to the house is located.

Sometimes we can access the plot both from above and from below, but in general the access from the street will be either at the top or the bottom of the plot.

Secondly, we should check which direction the slope faces.

Based on these two pieces of information, we will outline the first strategies for siting the house, depending on privacy in relation to the street as well as solar gain. The best-case scenario, of course, is a south-facing slope with the street at the top, so that if we open the house to the south, we will not have views from the street.

The so-called daytime spaces—such as the living room, lounge, dining room and even the kitchen—are best located at the lowest part of the slope. If the house is built over two storeys, these would correspond to the ground floor, so they can connect directly to the outdoors, linked to the garden and allowing, in summer, their area to be doubled with porches, pergolas or terraces.

Another important issue to consider when building a house on sloping land is the presence of courtyards. As we have already mentioned, especially on slopes that are not south-facing, it will be very important to achieve light and solar radiation.

In addition, in the Mediterranean climate, courtyards tend to work very well, as we do not have very extreme temperatures. For this reason, ventilating from an outdoor space with a slightly more controlled temperature is often beneficial for conditioning the house.

Finally, we should consider the placement of stairs. Building a house on sloping land involves creating different levels for access, living spaces and the garden. It is important—and mandatory—to ensure accessibility to the essential spaces of a kitchen, a bedroom and at least one bathroom for a person with reduced mobility. While we often think of a wheelchair, it may also be an older person or someone who is temporarily in a cast.

It is also important to think about routes with stairs that we will have to take while carrying shopping bags or pushing a child’s stroller, for example. It is important that arriving home is comfortable; perhaps arriving by car or on foot can be directly connected to a storage room or pantry that allows you to unload weight and later organise the shopping from there to the kitchen.

The same applies when we arrive with travel suitcases or a baby stroller. If it is not easy to access the house, it is important to have a covered space where you can unload belongings without getting wet.