ADU Construction in Lorton, Springfield & South Fairfax County (2026)
Regulations last verified: July 2026
The short version
- Lorton, Springfield, and South Fairfax are all in Fairfax County,so the county’s Accessory Living Unit (ALU) rules apply.
- Interior/attached ALU: up to 800 sq ft or 40% of the home, by administrative permit. Detached ALU: up to 1,200 sq ft, but only on lots of 2+ acres, with a special permit.
- This submarket’s larger lots mean the 2-acre detached threshold is actually reachable in parts of Lorton and Fairfax Station, unlike closer-in NoVA.
- Septic is common here, so Health Department approval, and septic capacity, often drive feasibility.
Can you build an ADU in Lorton or Springfield?
Yes. Lorton, Springfield, Burke, and Fairfax Station all sit within Fairfax County, so the county’s Accessory Living Unit rules, its term for an ADU, are what govern. You can build an interior or attached ALU, or, on a large enough lot, a detached one. What makes South Fairfax worth its own guide is not different rules but a different kind of lot, and that changes which option is on the table.
What’s different about South Fairfax
Two things set this submarket apart from Arlington, Alexandria, or the inner Fairfax suburbs. First, lot size. Fairfax County only permits a detached ALU on a lot of at least 2 acres (with a special permit), a bar that is nearly impossible to clear on a typical inner-NoVA lot. In parts of Lorton and Fairfax Station, larger properties actually reach it, which puts a detached backyard unit genuinely in play. Second, septic. A lot of South Fairfax is on well and septic rather than public water and sewer, and that means the Health Department has to approve your project before an ALU permit is issued. Septic capacity can be the deciding factor in whether an ADU is feasible, which is why we check it early.
Interior vs detached in South Fairfax
| Interior / attached ALU | Detached ALU | |
|---|---|---|
| Max size | 800 sq ft or 40% of the home, whichever is less | Up to 1,200 sq ft |
| Lot requirement | Any qualifying single-family lot | At least 2 acres |
| Approval | Administrative permit ($285, ~30 days) | Special permit (Board of Zoning Appeals) |
| If on septic | Health Department approval required first | Health Department approval required first |
For the full rulebook, see our Fairfax County ADU zoning guide and the permit process guide. For what these units cost, our Northern Virginia ADU cost guide breaks it down by type.
Building an ADU in South Fairfax with Excell
We design, permit, and build ADUs across Fairfax County, including Lorton, Springfield, Burke, and Fairfax Station, under one contract. That means we handle the zoning read on your lot, the septic and Health Department piece if it applies, the PLUS permitting, and the full build, so you get one team and one price from first drawing to final inspection.
Frequently asked questions
Can you build an ADU in Lorton, VA?
Yes. Lorton is in Fairfax County, so the county's Accessory Living Unit (ALU) rules apply: an interior or attached unit up to 800 square feet or 40 percent of the home (whichever is less) by administrative permit, or a detached unit up to 1,200 square feet on a lot of at least 2 acres with a special permit.
Are detached ADUs more feasible in South Fairfax?
Sometimes, yes. Parts of Lorton and Fairfax Station have larger lots that can meet the 2-acre minimum Fairfax County requires for a detached ALU, which is rare closer in. On a qualifying lot, a detached unit becomes an option, subject to a special permit before the Board of Zoning Appeals.
What about septic in Lorton and Springfield?
Many South Fairfax properties are on well or septic. In that case the Health Department must approve the project before an ALU permit is issued, and septic capacity can determine whether an addition or ADU is feasible at all. We evaluate this early so it is never a mid-project surprise.
How much does a Fairfax County ALU permit cost?
The administrative permit fee is $285, with a $75 renewal, and the county estimates roughly 30 days to process. A detached unit needs a special permit through the Board of Zoning Appeals, which is a separate, longer approval. Permits are filed through the county's PLUS portal.
Do you build ADUs in Springfield and Burke too?
Yes. We build across South Fairfax County, including Springfield, Burke, Lorton, and Fairfax Station, along with the wider Northern Virginia market. The Fairfax County ALU rules are the same across these areas; what changes is your specific lot, its size, and whether it is on public water and sewer or septic.
Related guides
- Fairfax County ADU zoning guide
- Fairfax County ADU permit process
- How much does an ADU cost in Northern Virginia?
- Our turnkey ADU service: design, permits & build
Where we build
Excell Homes designs, permits, and builds ADUs across the DC metro. Explore our work by area:
This guide is for general informational purposes only and does not constitute legal, zoning, or professional advice. Accessory dwelling unit rules vary by property, zone, and lot size, and they are actively changing. Several jurisdictions in the DC metro are currently reviewing or revising their ADU regulations, including Fairfax County. The information here reflects our understanding at the time of writing and may become outdated. Nothing here is a guarantee that a given project is permittable. Always confirm current requirements for your specific property directly with the relevant authority (the DC Office of Zoning and Department of Buildings, the Montgomery County Department of Housing and Community Affairs and Department of Permitting Services, or Fairfax County Planning and Development) before making decisions or beginning a project. Excell Homes is not responsible for actions taken based on this information.