Local Permitting Guide

Planning a Home Addition or ADU in Fairfax County

Fairfax County has some of the more detailed permitting requirements in Northern Virginia, particularly when it comes to environmental protections near streams and waterways. Before you commit to a design, it helps to understand a few of the county-specific rules that shape what's actually buildable on your lot. This page summarizes the basics — for anything project-specific, always confirm directly with Fairfax County's Land Development Services.

What Is a Resource Protection Area (RPA)?

Resource Protection Areas are environmentally sensitive corridors of land along streams, rivers, and other waterways that eventually drain into the Chesapeake Bay. Fairfax County regulates development within RPAs under the Chesapeake Bay Preservation Ordinance (Chapter 118 of the County Code). In their natural state, RPAs filter pollutants from stormwater, reduce runoff volume, and prevent erosion — which is why the county restricts land disturbance and vegetation removal within them.

Generally, no development, land disturbance, or vegetation removal is allowed within an RPA boundary without prior approval from the county's Department of Land Development Services. Non-compliance is a violation of the ordinance and can carry civil penalties of up to $5,000 per day, in addition to requiring a restoration plan.

Can You Still Build Near an RPA?

Not automatically — but there are real exceptions for homeowners, particularly for smaller additions to existing homes. Fairfax County allows a streamlined "minor addition" waiver within an RPA if several conditions are met:

• The primary home was built before the RPA was established on that property (generally before July 1, 1993, or November 18, 2003, depending on the RPA's mapping designation)
• The addition is attached to the existing home — detached structures like garages, gazebos, or pools don't qualify
• The addition's footprint is no larger than 1,000 sq ft, or 2% of the total lot area up to a maximum of 2,500 sq ft, whichever is greater
• Total land disturbance for the project stays under 2,500 sq ft

If your project is within 15 feet of a floodplain boundary, a separate Floodplain Use Determination is also required before the RPA waiver can be approved. Projects that exceed the 2,500 sq ft land-disturbance threshold require a full grading plan instead of the streamlined waiver process.

Setbacks for Accessory Dwelling Units (ADUs)

Fairfax County refers to ADUs as Accessory Living Units (ALUs) in its zoning ordinance. General guidelines for most residential zones include:

• Size: generally capped at 800 sq ft or 40% of the primary home's finished floor area, whichever is smaller
• Front yard: an ADU typically cannot extend forward of the rear wall of the primary dwelling
• Side yard: setback minimums vary by zoning district — commonly in the range of 8 to 12 feet from the side property line
• Lot coverage: total impervious surface (house, driveway, ADU, patios) is generally limited to 25–30% of the lot
• Parking: one additional off-street parking space is typically required, which can be a driveway spot rather than a garage

These figures vary by zoning district and specific property, so they should be treated as a starting point for the conversation, not a guarantee for your lot.

The Permitting Path

Most addition and ADU projects in Fairfax County move through a few common stages: a zoning determination confirming your property is eligible, submission of full construction documents (architectural, structural, mechanical, and energy compliance), and — if your property is on well or septic — a Health Department review to confirm the system can support the addition. Everything is submitted through the county's online PLUS permitting portal.