The Cottage, The Code, And The Two Acres
You’ve got a parent who shouldn’t be living alone anymore, or a kid moving back, and a backyard that looks like it has room for a small second house.
Can you build one here?
In this post, you’ll learn what Fairfax County calls these units, the size and occupancy limits, the parking and recording rules nobody warns you about, and what we at WISA Solutions can build for you today.
What Are The Rules For Building An ADU In Fairfax County?
Fairfax County allows a second living unit on a single-family lot, but the rules are tighter than expected.
One unit per lot, alongside a single-family detached house.
Interior units cap at 800 square feet or 40% of the main house, whichever is smaller.
Two bedrooms, two occupants, and either the unit or the main house has to be owner-occupied.
A detached unit needs a special permit and a lot of at least two acres.
Most plans end there.
Interior conversions and attached additions clear the bar; a freestanding cottage in the backyard almost never does, and the reason is a single line in the ordinance.
Why Fairfax County Doesn’t Call It An ADU
Search “ADU,” and you’ll find plenty about Fairfax County.
Search the county’s zoning ordinance for ADU, and you’ll find something else entirely.
In Fairfax County, ADU means Affordable Dwelling Unit, a separate program under a separate ordinance.
What you’re picturing is called an Accessory Living Unit, an ALU, and the county’s own guidance says the ADU abbreviation shouldn’t be used for it.
Two different things, one acronym.
This matters the day you file, because you’ll be looking for the Accessory Living Unit permit in the county’s PLUS portal, and a plat labeled for the wrong use is a plat that comes back.
We use those two terms differently in-house, and we’ve been wrong to.
We’ve described an ALU as an attached unit and an ADU as a standalone one, which is how a lot of builders talk.
The county doesn’t.
An ALU covers the interior conversion, the attached addition, and the freestanding cottage; the only differences among them are the permits required.
Size, Bedrooms, And Who Can Live There
Two permit paths exist.
Which one you land on decides almost everything about the project, including whether your lot can legally support it at all.
Interior Units Under An Administrative Permit
An accessory living unit inside the main house can go through an administrative permit, which is the faster path.
The unit has to be wholly contained within the house and connect to the main dwelling through an interior space that’s finished, temperature-controlled, and fully enclosed.
Any new exterior entrance goes on the side or the rear, never the front.
Size is 800 square feet or 40% of the main house’s gross floor area, whichever is less, though a basement conversion can use the whole basement as it existed on July 1, 2021.
Gross floor area includes basement space with at least 6 feet 6 inches of headroom and excludes the garage.
Two bedrooms maximum.
Two occupants maximum.
Either the unit or the main house has to be owner-occupied, so you can’t rent out both.
The county quotes roughly 30 calendar days to process, with a $285 application fee. If construction requires a building permit, the zoning approval waits until that permit passes final inspection.
You file through the county’s PLUS portal with a plat, a floor plan labeling every room by use and square footage, and photos of the interior and every exterior side of the house.
The floor plan does not have to be drawn by a licensed professional, which surprises people.
The unit also needs a working multi-purpose fire extinguisher plus smoke and carbon monoxide detectors interconnected with the main house.
Detached Units And The Two Acre Problem
Building a freestanding unit in the backyard requires a special permit from the Board of Zoning Appeals, not an administrative one.
The Board can waive the wholly-contained rule, but only on a lot of at least two acres.
Two acres.
That single line disqualifies the large majority of residential lots across Fairfax County, Vienna, and Falls Church, including, and it’s the reason so many backyard cottage plans die at the research stage.
If your lot clears two acres, a detached unit caps at 1,200 square feet and has to stay clearly subordinate to the main house.
The Board can also modify entrance placement, allow an interior unit larger than the standard cap, or waive the parking requirement, and it can add parking requirements instead.
One more rule catches garage conversions: a new garage or carport has to sit directly adjacent to the existing one and share the same driveway and curb cut as the main house.
Parking, Recording, And Renewal
Approval isn’t a one-time event, and this is the part that surprises people most.
Parking comes first: you provide the spaces required for the main house plus one more, and your plat has to show three total spaces if you’re on a public street or four on a private one, with dimensions labeled.
Those spaces have to stay available for parking, so if you identify the garage on your plat, you can’t fill it with storage.
Then there’s recording.
Before anyone moves in, you record a copy of the permit in Fairfax County land records, indexed under your name in the Grantor Index.
The permit initially runs for two years, and the Zoning Administrator can extend it for up to five years based on your compliance record.
Renewals cost $75.
If you fall out of compliance, the unit can’t be occupied as a dwelling, though nobody makes you rip out the kitchen.
On a well or septic, the Health Department has to sign off before zoning approval, and expect requests for a stove permit, an egress point for a bedroom, or an electrical permit covering work somebody did without one years ago.
What We Can And Can’t Build For You Right Now
We’ve built accessory living units attached to the main house, and we haven’t yet built a freestanding one.
Partly the two-acre rule.
Mostly, it’s what our clients ask for, which is usually a main floor suite for a parent rather than a cottage out back.
The build itself is the same as any addition: a bedroom, bathroom, kitchen, laundry, and living space, so somebody can live there without needing anything from the main house.
We hold a Class A license in Virginia, along with residential and commercial builder’s licenses, permitting goes into your scope of work, and we run the Fairfax County pre-construction meeting with the inspector ourselves.
Our additions run north of $200,000 and take 4 to 6 months, with a basic addition starting at $400 per square foot for the shell.
What that returns is a separate question, covered in home addition ROI.
What we’d rather do first is review your lot size and zoning district during a discovery call.
The county can also inspect the property during reasonable hours with notice, so this is a permit you keep, not one you get.
Five minutes on the phone beats five weeks of design on a project your lot can’t support.
Ready To Check Your Lot Against The Code? Reach Out To WISA Solutions
The short version is that the interior path is realistic for most Fairfax County homeowners, but the backyard cottage usually isn’t; the deciding factor is your lot size, not your design.
Knowing which side of that line you’re on saves months. Fill out our contact form or give us a call, and we’ll review your lot and zoning district with you.
