Land records · Parcel maps · October 7, 2026
Steep Slope Ordinance: Building on Hillsides
What a steep slope ordinance does: limits grading, clearing, and building on hillsides above set slope percentages to reduce erosion and landslide risk.
Steep slope ordinance is a local regulation that limits or sets special standards for development on hillsides above certain slope percentages. Building on steep land can cause erosion, landslides, runoff problems, and visual scarring, so many communities restrict how much land can be disturbed, require engineering studies, or prohibit building on the steepest areas. Slopes are usually measured as percent grade, which is the vertical rise divided by horizontal distance, multiplied by 100. Ordinances may be part of zoning, subdivision rules, or a separate hillside overlay district. This page is general land-records background, not legal advice.
Typical slope categories
| Slope | Common treatment |
|---|---|
| Under 15 percent | Usually normal development standards |
| 15 to 25 percent | Limited disturbance, extra review |
| Over 25 percent | Often prohibited or heavily restricted |
Thresholds vary; some codes start restrictions at 10 or 20 percent.
Common requirements
- Maximum area of disturbance on steep portions.
- Geotechnical reports on soil stability.
- Engineered grading plans and retaining walls.
- Erosion and sediment control.
- Limits on tree clearing.
- Reduced density, requiring larger lots.
- Building height measured from natural grade.
Measuring slope
Surveyors calculate slopes from topographic surveys or LiDAR-based contours. A slope analysis map shades areas by slope category. Planners use these maps to determine buildable area.
Effect on buildable area
Some codes exclude steep slopes from the land counted toward density, so a 10-acre parcel with 4 acres over 25 percent may be treated as having only 6 developable acres.
Roads and driveways
Ordinances often limit driveway grades, such as no steeper than 15 percent, for emergency access. Roads may need slope easements for cuts and fills.
Finding out if rules apply
Check the zoning code and GIS for hillside or steep slope overlays. See how to find zoning of a property. Topographic layers can help estimate slopes.
Variances and exceptions
Owners of lots that are mostly steep may seek variances to build a reasonable home. Engineering studies help show safety.
Example
A buyer looks at a wooded hillside lot. The town’s ordinance prohibits disturbance on slopes over 25 percent and limits disturbance to 20 percent of areas between 15 and 25 percent. A slope analysis shows a small flat area near the road where a house can fit, with a driveway under 15 percent grade.
Why these rules exist
Hillside failures can damage homes below, block roads, and send sediment into streams. Clearing vegetation on steep land increases runoff and erosion risk. Ordinances aim to keep development where soils and slopes can support it.
Costs to expect
Building on slopes often involves geotechnical engineering, retaining walls, special foundations, and longer driveways, all of which raise costs. Buyers should get estimates before purchasing hillside land.
Before buying hillside land
Order a topographic survey or slope analysis, talk to planning staff about the ordinance, and confirm that a practical building site and driveway route exist within the rules.
Bottom line
A steep slope ordinance restricts grading and building on hillsides above set percentages, often requiring engineering studies and limiting disturbance. It can reduce buildable area and raise costs. Check slope analyses and overlays before buying hillside land. Explore topography and parcels via the Platbookmapper map.
Steep slope ordinance FAQ
What is considered a steep slope?
Often over 15 to 25 percent, depending on local rules.
Can I build on a steep slope?
Possibly, with limits and engineering review.
How is slope measured?
As rise over run, expressed as a percent.
What is a slope analysis?
A map showing areas by slope category.
Do steep slopes reduce allowed density?
In some codes, yes.