Preface
A steep slope in regulatory parlance is defined as one with slope of 15 degrees. To build on a steep slope usually requires a zoning variance. A slope of 15 degrees requires some testing to ensure that the ground is stable and will support a foundation. It most often is, but knowing is security.
This degree of slope is actually ideal for a hillside home, a very efficient way to build. The warmmodern living website features several homes we’ve designed for 15 degree slopes.
A slope of 30 degrees is more likely to require some form of remediation to stabilize the soil. Deeper foundations, retaining walls and subsurface drainage development can translate into fairly substantial added cost as required by the variance.
A slope of 40 degrees is likely to have significant remediation in the form of retaining walls, deep foundations, restricted use of the site and ground water diversion, and navigating the site with equipment and material, all of which add costs, sometimes well into six figures. Unless your budget is very flexible (as in bottomless), keep looking.
The analysis of a steep slope site should begin in earnest with a geotechnical report before you purchase the site.
The initial Geotech report will indicate:
That bearing soil (soil stable enough to carry the weight of a house without shifting) is within 8-10 feet.
That bearing soil is well below the lowest level of your intended house. This typically means that the foundation needs to be deeper in order to rest on bearing soil.
If the soil around the house site is stable enough not too shift and slide when the soil is saturated with rain or groundwater
Steep slope sites sometimes require remediation that can mushroom into higher costs of many sorts,.
These unanticipated costs can turn a dream home project into a veritable nightmare. Can you tolerate that risk?
Let me share with you two steep slope experiences I have had in the last thirty days.
A site that shakes every thirty minutes on schedule
A prospective client is searching for a site with a water view in a suburban community. The site being considered is classified as a steep slope, and the rear lot line is a sheer cliff with a commuter rail at the base of that cliff.
A stunning house was designed for a previous buyer, who abandoned the project and the site.The selling agent, when asked, cites illness as the reason.
A geotechnical engineer was engaged by the previous buyer to analyze the site and describes the stability of the soil as “fair” and prescribes the use of a geo-grip beneath the house to add stability. We contacted the geotechnical engineer and asked if he had an estimate of cost, and he did not.
A call to the architect of the previously proposed residence revealed that the geogrid had an estimated cost of roughly $300,000. The architect did not, however, mention the retaining wall that was required at the edge of the cliff to project anyone on the site from falling 60 feet. I estimate the cost of a 6’ deep in-grade poured concrete wall with a stainless steel rod rail along the rear of the property to be $75,000 to $100,000.
Some quick research into geogrids gives them an estimated lifespan of 60 to 100 years. Any Lindal I design should serve generations of owners for more than 60 to 100 years.
Stand near a railway as a train goes by at full speed. The ground shakes! I worry that the train will roar by this site 10,750 times each year. The Geotech report does not quantify the effect of this vibration of the steep slope.
My recommendation to my client is to find another site.
The Best Laid Plans…
a Case Study
The site is very steep. Over 40% ! Nice neighborhood: bargain priced site (always the sign of an issue). Client closes without consulting any professionals. I visit the site after the closing and refuse to step on the slope. Client is in denial.
We schedule the required pre-construction meeting early in the process. The strategy: unearth the issues as seen by those who regulate permits prior to construction. We leave the Town Hall, client silent, His only words: “The broker told me that the $300,000 I‘d save on the land could be invested in the house instead.” A truer lie was never spoken.
The client engages a geotechnical engineer and law firm of considerable experience with the town officials. The process is to gain a variance from the zoning bylaws with specific site-based requirements to ensure that the foundation will sit on bearing soil and will withstand the historic seismic activity. It takes over eighteen months for approval and the issuance of a building permit.
Construction is not uneventful. The site is so steep (the yellow line) that excavation equipment cannot navigate the site when the soil is wet. The job is shut down for the five winter months. The foundation is very complex. The geotech reports that bearing soil is a full story below the footings at the lowest level of the foundation. The footing is installed 8 feet lower into the slope, and above that, 8 feet of foundation wall is added; the void is filled with clean fill, and the slab completed (at the level of the red line.
The broker’s words still resonate: The $300,000 that the client saved on the land was invested in the house… before the start of framing.
Too Steep for a four-wheel drive vehicle.
In another location, a client who engaged me to analyze perspective sites is about to make an offer on a site that looks steeper than steep from the topographic survey. Virtually the entire two-acre site is a slope of over 30 degrees.
The site is on a pond, and a buffer of non-disturbance from the water encroaches onto the site, restricting what look like the only buildable area.
A house that previously occupied the site had fallen into disrepair and was demolished.
This site, too, was the intended location of a custom 4000 square foot brick manse, that was never built, as the buyers backed out.
The site is totally wooded with mature trees and looks gorgeous.
While driving out to the site, the client mentions that the driveway for the previous house, the only access into the site was too steep for the four-wheel drive Ultimate Driving Machine after a recent minor snowfall.
We met on-site with the civil engineer who had worked with the previous intended buyer whose architect designed the brick house. Standing at the house site we all agree that the ground beneath our feet looks like demolition debris.
The broker is not a source of useful information, so we decide to meet with the head of the local building department to see what he knows about the site. He assures us that the village is not going to impose undue restrictions on the project. That is cold comfort because if the soil is not stable, it will need to be dealt with.
Last year I completed a project within a few miles of this site. I called the builder of that project to see if he was aware of this site. His response was that he had “turned down a number of jobs in that area due to poor site access for both equipment, material, and more importantly, utilities.
My client was prepared to hire a geotechnical engineer to visit the site, do borings, and analyze the soil. But he became aware of another site in the area that is free of major question marks about zoning variances, wetlands, steep slopes and the time and expense to satisfy the requisites of the variance.
Proceed aggressively with caution.
So what do I advise anyone who is considering the purchase of a site with steep slopes?
We invite prospective clients to contact us when they find a site to their liking, before they purchase that site. We refer to the local zoning regulations, study the topography to help them understand what kind of configuration works well with the existing grades. We look for wetlands and steep slopes where the information is available. We help clients understand what type of house belongs on the site and warn them of any concerns we have. This is one of many benefits of working with a local Lindal representatives like Warmmodern Living.
What should you do if you are seriously interested in a site that contains steep slopes?
Proceed with caution!
Unless you are crazy about the site, keep looking.
On a survey of the site, draw the zoning setbacks, any wetlands plus a 50 or100 foot buffer, and locate the steep slopes.
Asking for a zoning variance is an expensive time- consuming task. If there is anywhere to build on the site that will avoid a variance, strongly consider that. As a point of fact, if there is such a lop place, you may be denied the variance because you have an alternative on the same site.
Engage a geotechnical engineer to do the preliminary study to determine the stability of the soil at the intended house location.
If a geotech study has already been prepared , interview the geotechnical engineer who authored the probe and probe for the cost of the recommended remediation.
Steep slopes generally require hillside houses with the lower levels built into the slope. Does that work for you?
Do you have plans to build a pool, an accessory dwelling? Zoning variances typically prohibit any further development of the site
If the site requires a septic system will that be on a steep slope?
Make an appointment to visit the local building dept (zoning) and inquire about any history on this site; the typical approach taken by the Zoning Board of Appeals.
If a previous buyer backed out, try to reach out to the buyer or their architect and find out why the deal didn’t go through. Probe about the steep slope
If you do proceed and are successful at obtaining a variance, set aside a contingency of 20% of your total budget for remediation.
It can be designed so artfully and executed so skillfully that the house looks like it grew out of the slope!