Building into a gentle slope can enhance the efficiency of a design by providing living space inside the foundation and requiring less additional material and labor than the same amount of space above grade. And if properly designed, the in-ground living area can be bright and spacious.
Environmentally responsible site design begins with minimizing disturbance to any site, and working with the existing topography is an essential ingredient. A more measurable benefit of building into a slope is often reduced heating and cooling expenses -and added comfort- that result from the temperature of the soil five feet below grade (the “outside air temperature” of these spaces) is a constant 52 degrees year-round.
The ideal slope for in-ground building is a slope of 5 to 7 feet through the width of the house, or 15% to 20%. Building on steeper slopes often presents challenges, and this type of construction is carefully regulated by local jurisdictions that vary, as do local soil types.
Codes that restrict building exist to protect the environment and the people who share it. What are the concerns with building on a steep slope; how are they addressed; and what is the cost of addressing them?
This is a long post, so if you don't have the time to read this over a nice cup of tea, go directly to the last section, “Tips for building site hunters regarding steep slopes” … and please return later to read the entire piece!
What are the issues of building on steep slopes?
Soft soil not retained by ledge or deep-rooted trees on steep slopes is prone to dangerous landslides that can affect areas tens of times larger than a single home site and may impact hundreds of lives. Landslides occur when these soils become saturated and heavy, and gravity beckons. The steeper the slope, the greater the danger.
Any disturbance to the ground during construction- excavation and backfilling, especially during spring or any rainy period - disturbs the site’s static state and may pose a real and sudden danger to construction crews.
In these environments, special care must be taken to ensure that foundations are supported by more stable soil or bedrock and that the foundations are designed with adequate strength to resist the forces of shifting soil around them.
Since the footprint of every house (and its driveway and sidewalks) replace permeable (absorbent) ground surface with impermeable surface, water running off the impermeable surfaces can cause the soil to become over-saturated, weaken, and slide. Managing the runoff is a matter of great concern and may affect the safety of hundreds of people nearby.
How are these issues addressed
Let’s use a real example.
My clients purchased a site in a well-established neighborhood in a Seattle suburb. Neighboring houses built over the last fifteen years are typically over 3000 square feet with attached two and three car garages built on hilly terrain.
The site slopes down from the street, the portion closest to the street being the steepest. The ground falls 40 feet over the first 70 feet from the street, In the last several years as the pace of development in the area quickens, small landslides have occurred. The county now defines a steep slope as 30% or more and requires a special permit that imposes specific restrictions on developing steep sites.
The process, not yet complete, has gone smoothly:
Initial site visit
I study the regs and visit the site with the client. I advise:
Site the house about 50 feet from the street where the ground slopes only 10’ over a house depth of 27 feet.
Build a compact vertical house in which the floor levels match the contours and minimize disturbance to the site. Three full living levels, (the clients are young and energetic): enter the house on grade on the middle level, a 900 square foot great room with expansive views to the southwest through the treetops.
The upper level will be the primary bedrooms and a 400 square foot roof deck above the treetops, which I refer to as the” home’s, backyard,” since development of a ground-level yard for outdoor entertaining will likely be forbidden.
The lowest level will include a private guest suite, a large art studio, both with on- grade walk-outs , and mechanical and storage rooms .
Detach the garage so its elevation can be independent of the house.
We schedule an exploratory meeting with the City Engineer and senior members of the zoning and building staffs and the fire chief.
We preset our site plan concept, a sketched house plan and house plan, and a 3D massing model. They pronounce the concept sound with some reservations:
They warn us not to cross an on-site sewer easement with cars. It runs the full width of the site half way between the street and the proposed house location.
We’ll need to install a sprinkler system since the nearest fire hydrant is over 500 feet from the house location.
They express concern over where and how to channel rainfall from the impermeable house and garage roofs and driveway, potentially destabilizing the slope. And the nearest municipal storm sewer is over 200 feet away uphillfrom the site.
They remind us of the requirement to engage a geo-tech engineer who would take soil borings and advise us and the City on the design of a foundation that would bear on stable soil.
When approved, there will be a buffer of 15 feet around the structures and driveway that may be landscaped, but no alterations of any kind can be made to the remainder of the site.
My client had already engaged a geotechnical engineer. Soil borings are taken in several locations at and near our proposed house location. My client and I meet with the engineer to get the results and to strategize on the issues raised by the City. His report was mixed:
The soil is stable enough to support the foundation without many extra bells and whistles. The design of the house and the proposed siting fit each other nicely, with only a portion of one wall requiring a pour of over 8 feet. The geotech will work with Lindal’s structural engineer on a foundation design that will take the site conditions into account: a base for the house that is well supported (vertically) and also resistant to shifting soil from uphill (laterally).
The sewer line is near the surface. Making the necessary cuts for a driveway might in fact expose the line. The only way for cars to access the house will be to replace about 40 feet of the line with a deeper line at a huge cost borne by my clien
Rainwater from the impermeable roofs and driveway cannot be drained anywhere onto our site. The soil is not stable enough. And the nearest city storm sewer is over 200 feet away, uphill from the house (pumping required)!
A creative and realistic approach
This is a moment when some clients give up, and others angrily lash out and vow to fight the restrictions (they mean that) at any cost (they don’t mean that). Fortunately, this client wants to satisfy the City by addressing each issue proactively and creatively. Our case for the special permit includes:
We are designing a “vertical” house with a 900 square foot compact footprint that is roughly 40% of the size of the other homes in the subdivision. many of which are on sites now defined as steep slopes.
The foundation will be built to the spec advised by the geotech and approved by the City. No cutting corners here.
The idea of crossing the sewer easement Is nixed, and the client decides to create a parking area on the street side of the easement. This decision precludes a garage and a long driveway reducing the added impermeable surface on site by about 1200 square feet. Our total impermeable surface will about 25% of the neighborhood norm.
Rather than arguing for larger area around the house to develop, we’ve planned a 425 square foot "roof yard” that will enjoy full-day sunshine and be ideal for family activities and entertaining. The roof yard is also an appealing anecdote to parental concerns regarding the security of children playing alone in their yard.
The house will be built with a catchment system that will also purify the roof runoff so that it can be used for all domestic purposes. This will alleviate the concern of adding water to the site and the expense of under-street piping and pumping to a remote storm sewer. The system will cost $35,000-$40,000.
We have addressed the concerns of building on a steep slope, and the client will enjoy benefits from several aspects of the proposed solution.
What is the added cost of steep-slope construction?
By now you’ve figured that all the testing, proposal preparation, the precautionary measures to construct, and the inspection of this work of community-wide importance carries a substantial price tag. While many of the cost centers cannot be determined until the work is actually done, clients need to budget for these expenses. In this case, I’ve estimated:
|Geotech engineer, testing, special surveys etc.||$15,000|
|Additional foundation costs||$15,000|
|Special inspections performed by the City and billed to client||$10,000|
Often the added expense is more than the money you saved by purchasing a “site with issues” And the “expense” of added time to permit can also be considerable if you choose to “fight city hall”.
Ahhhh… but the result can be magical when a house and its site are in perfect harmony.
Warmmodern Living’s tips for building site hunters regarding steep slopes
The first seven tasks should be completed during the feasibility period of your site purchase... before closing on the site.
1. Ask the seller's agent if there are any known issues that will require a special permit (that covers more than steep slopes).
2. If your state requires seller disclosure forms, obtain one from the seller's agent.
3. Research the permitting jurisdiction’s website for their definition of a steep slope (e.g. any area that exceeds 30%) in the area you intend to place the house or any other area of the site that will be developed(driveway, landscaped yard, pool area etc.). Then check the site using a topographical study if one has been done or if there is no topo plan, hire a surveyor for the hour it will take to come to the site to measure the incline.
4. Meet with city or county zoning and building officers to gain insight into their concerns and suggested remedies
5. Hire a geotechnical engineer to test the soil and explain issues and possible remedies.
6. If the site is priced comparably to nearby sites with no issues, try to renegotiate the price. If there is a feasibility period, extend it to six months or a year.
7. Identify remedies, estimate their cost, and make sure you are still within your budget.
8. Design your home with an experienced professional who will guide you through this process and can guide you creatively.
9. Begin the special permitting process as early as possible. Agree upon deadlines with the professionals you rely on to contribute to an approval. Don’t hesitate to be aggressive with laggards.
An upcoming blog post will shed some light on catchment systems, why you may want to consider one, how they work, and when to begin planning one.