How to build a house on a steep slope

The biggest hurdles boil down to two main factors: the gradient of the slope and whether the site is upslope or downslope. 

You can get a rough idea of how challenging a particular site will be to build on by its gradient rating. Less than 10% incline is considered slight and is the easiest to build on, while 11-20% is considered moderate. Anything above 20% is deemed steep. Beyond about 15%, costs begin to increase significantly as the risks become greater and the work becomes more difficult.

An upslope plot (where what will be the front of the house rises up to the back) is also more challenging than a downslope plot. These types of plots usually require some amount of cutting or even blasting, plus transporting and disposing of rocks and soil.

Additional Foundation Work – Time & Cost

It’s not unheard of to have houses built on grades of as much as 50%; however, the cost of the complex foundation systems required for such houses is often more than the cost of an entire house on flat ground.

But even moderately sloped sites typically require more concrete, deeper excavation by specialized excavation equipment or blasting, retaining walls or terraces, and specialized solutions for drainage and septic systems. All that extra labour and materials needed to ensure the house is up to code and safe to inhabit mean higher costs and longer construction times. 

Water – Drainage & Sewage

If you’ve ever seen video clips of mudslides, you’ll understand why proper attention to drainage — for both surface and subsurface water — is absolutely necessary to building on a slope. 

Rainfall runoff has to be redirected away from your home’s foundation, without endangering any neighboring properties with water flows and without flooding public roads. Retaining walls have to be waterproofed and drained to prevent water from pooling there. Swales and culverts can usually be used to channel the runoff to stormwater drains or soakaways (holes dug in the ground and filled with rubble that allow surface water to percolate back into the ground).

Finally, sewage treatment can be tricky depending on where your home is relative to the sewage line. If the line is uphill, you may need to install a pump, and if it’s downhill, you may have to install tumble bays to slow the flow to a reasonable rate.

Storage of Fill Soil & Staging Areas

If your plot requires cutting, the resulting extra soil will either have to be transported out or stored somewhere nearby if it is to be used for filling. Even if you’re not cutting, you’ll need a staging area for vehicles, equipment, and deliveries. If you’re only filling, you’ll have to have soil trucked in to the site, a costly expense that varies widely with geography, so we highly recommended getting estimates.

Access to the House

Access to the property  for both you and your builders is an important factor to consider in your planning. Property that may seem cheap initially could run up high costs for additional grading if there’s no good way to access it. Switchback and curved driveways are helpful and attractive, but small properties that don’t have room for them may have no choice but to install a steep driveway that can be dangerous in bad weather.

Replenishing the Construction Site

Over the weeks or months it takes your home to be built, as trucks come and go, heavy equipment is parked, and construction materials are scattered about, any natural plants on your new home site are going to take a beating. Replanting with cultivated plants or waiting for the natural flora to grow back may require many months of waiting before the property gets the look you’d imagined when you first began the new home-building process.

Soil Type

The type of soil of a sloped property can be easily overlooked but could pose a problem for home builders. While granular soils (gravel, sand, or silt with little or no clay) drain well and can bear high loads, soils with high clay don’t do well at. Certain clays can damage foundations by swelling or expanding when wet and may require expensive engineering fixes or additional fill of granular soil.

Rock ledges near the surface can also present difficulties with drainage and septic systems. Often the most budget-friendly solution is to work around them, as blasting them out is costly and carries a liability risk to neighboring foundations.

Landscaping

Flat ground is of course easier to work with, but with the right planning, a sloped lot can be the canvas for landscaping that sets you apart from ordinary front yards. You’ll want to put down plants that can take root on an incline. Ground cover plants with deep roots can help reduce erosion and prevent weeds. For planting in retaining walls, use plants that grow well in crevices. Waterfalls, potted plants, and decorated switchbacks can greatly spruce up sloped yards. 

Source: Legal Eagle