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Steep slopes and problem soils in 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.

A person last checked these rules on October 20, 2026. This is guidance only, so confirm with Fairfax County before you build.

The details

  • SetbackIQ checks the county's Marumsco and previously mapped marine clay layers. The maps are approximate, and only test borings confirm the soil.High confidence
  • A soil report with a slope stability analysis is required for new construction and grading in marine clay areas. Foundation repairs need a permit and engineer inspection.High confidence
  • Steep slopes and marine clay weigh against special permits for setback reductions and extra rear-yard coverage.High confidence

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SetbackIQ reads the zoning, flood zone, historic district, and environmental maps for your lot. Then it applies these rules.

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Common questions

Do steep slopes or soils like marine clay change what I can build in Fairfax County?
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.

Sources

  1. Fairfax County LDS, Homeowner's Guide to Marine Clay. Checked 2026-10-21
    Under the Problem Soils Ordinance (Chapter 107), a soil report prepared by a qualified geotechnical engineer is required for new construction and grading work in marine clay areas... Footings in marine clay must be at least 4 feet deep... A building permit …
  2. Fairfax County Zoning Ordinance § 5100.2.D, setback regulations (corner lots, sight distance, permitted extensions). Checked 2026-10-21
    [Sight distance] Points must be 30 feet from the property lines extended... Sight distance must be maintained between two horizontal planes, one of which is three and one-half feet, and the other ten feet above the established grade... [Extensions] heating,…
  3. Fairfax County Zoning Ordinance § 4102.7.A(5), (6), (17), rear setback coverage, freestanding accessory structures, swimming pools. Checked 2026-10-21
    All accessory structures and uses may cumulatively cover no more than 30 percent of the minimum rear setback on any lot located in a conventional residential district... [Coverage includes] children's play equipment, sports court, pool and associated deckin…

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.