Local Permitting Guide
Building an Accessory Dwelling Unit in the Town of Herndon
⚠ Important Notice
This page summarizes publicly available government information for general educational purposes only. HomeSprout LLC did not create this information, does not guarantee its accuracy, completeness, or currency, and is not responsible for decisions made based on it. Rules change and vary by property — always verify current requirements directly with the official source below before proceeding with a project.
Official source: Town of Herndon — Residential Zoning Information (see also Reston Now coverage of the November 2021 Town Council ordinance update)
Does This Guide Apply to You?
This guide applies specifically to the Town of Herndon — a separate, incorporated town with its own zoning authority, distinct from surrounding unincorporated Fairfax County. If your address is near Herndon but you're actually in unincorporated Fairfax County (including nearby Reston), confirm your zoning jurisdiction first — see our Fairfax County guide if that's you.
Herndon Town Council eased its ADU rules in November 2021, moving from a special-exception process to by-right approval for single-family units. Here's a plain-language summary — for project-specific eligibility, always confirm directly with Herndon's Department of Community Development.
Size Limits
- Attached or internal ADUs: up to 40% of the main dwelling unit's floor area, or 1,200 sq ft, whichever is less
- Detached ADUs: capped at 900 sq ft
Occupancy & Parking
- Up to 3 people may live in an ADU, in addition to one family in the main unit
- The age/disability occupancy restriction that used to apply has been removed
- One additional off-street parking space is required for attached/internal ADUs; two spaces for detached units
Setbacks
Detached ADUs require a 10 ft setback from the rear and side property lines (expanded from a prior 2 ft standard as part of the 2021 update).
Permitting
ADUs in eligible single-family zones are now permitted by right rather than requiring a special exception — homeowners still need to go through the town's standard zoning and permitting process, including code-compliant construction and a pre-occupancy property inspection.
Not sure what applies to your property?
Zoning district and lot specifics affect what's actually buildable. We handle the zoning determination and permitting process as part of our design-build service.
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