Steep slopes and problem soils across the DC area
Do steep slopes or soils like marine clay change what I can build? The rules differ at every jurisdiction line. Here is the short answer for each.
Disclaimer: This is general information, not legal or professional advice for any one property. Codes change. SetbackIQ is not a zoning determination, a permit, or a survey, and it does not replace an architect, engineer, or lawyer. Confirm with your building and zoning offices before you build, buy, or sign a contract.
| Where | Short answer |
|---|---|
| Washington, DC | DC has no steep-slope zoning for houses. SetbackIQ flags lots the DC soil survey puts in the 15 to 40 percent slope class. On those lots, expect DOEE erosion controls (any work moving more than 50 sq ft of soil) and engineered walls: retaining walls over 4 ft need a DC-licensed design professional, and DOB may ask for a soil report. |
| Montgomery County, MD | Montgomery has no steep-slope permit for an ordinary house lot. Slopes of 25% or more (15% in Ten Mile Creek and in Special Protection Areas next to wetlands) count as steep in Planning's Environmental Guidelines, which apply when a development plan is reviewed: stream buffers grow to take in the whole slope, and septic fields are banned on slopes over 25%. For a building permit, steep ground mainly shows up through the sediment control permit and engineered foundation and wall designs. |
| Prince George's County, MD | DPIE can require a sealed soils report based on slopes and soil type (§ 32-131), and no cut or fill may be steeper than 2:1 without a soil engineer's report (§ 32-160). Retaining walls on steep ground are capped at 6 ft with terraces. |
| Alexandria, VA | New houses always need a sealed soil report, which must flag expansive marine clay. Additions and in-ground pools need a soil report or a 1,500 psf design. Foundation repairs need a soil report. |
| Arlington County, VA | Steep slopes of 25% or more next to stream buffers are part of the RPA, so work there needs county review. Disturbing 2,500 sq ft or more anywhere needs a Land Disturbing Activity permit. |
| Fairfax County, VA | Fairfax maps marine clay (Marumsco soils) and other shrink-swell soils. On those soils the Problem Soils Ordinance requires a geotechnical report for new construction and grading, with footings at least 4 ft deep. |
Towns and cities inside these counties (for example, Rockville, Gaithersburg, Bowie, Laurel, Herndon, Vienna, and Falls Church) have their own rules.