ADU terms, in plain English

Accessory dwelling unit (ADU)

Also called: granny flat, in-law suite, backyard cottage, garage apartment, casita

A second, smaller home on the same lot as your house, with its own kitchen, bathroom and somewhere to sleep. It stays part of your property — you cannot sell it separately.

The three facilities matter legally, not just practically. Virginia law defines an ADU as a unit with complete independent living facilities: somewhere to live, sleep, eat, cook and wash. Miss one and it is not an ADU at all.

In Northern VirginiaCareful in Northern Virginia: "ADU" officially means Affordable Dwelling Unit in both Fairfax County and Loudoun County — an unrelated affordable-housing program. Fairfax calls the thing described here an accessory living unit; Arlington calls it an accessory dwelling.

Accessory structure

Also called: shed, outbuilding, detached studio, backyard studio

A building on your lot that nobody lives in — a shed, workshop, studio, home gym or detached office. It has no kitchen or no bathroom, which is what separates it from an accessory dwelling unit.

This distinction is usually good news. An accessory structure follows a simpler, cheaper permitting path, none of the ADU rules apply to it, and you do not have to wait for the 2027 state law to build one.

By right

Also called: permitted use, as of right

Something you are allowed to do without asking permission from a board — no hearing, no discretionary approval, no chance for a neighbour to object. You still need permits, but they must be issued if you meet the written rules.

This is the single biggest difference between an easy project and a hard one. A by-right project is a matter of paperwork and time. A discretionary approval can be refused.

Gross floor area

Also called: GFA, finished floor area

The total floor space of a building, measured to the outside of its walls. It is the number size limits are written against.

What counts varies by county, and the difference is rarely trivial. Fairfax includes basement or cellar space with at least six feet six inches of headroom, but excludes a garage.

Owner-occupancy requirement

A rule that you must live on the property yourself — in either the main house or the second unit — rather than renting out both.

It is the rule that most often stops an ADU being treated purely as an investment. Worth checking whether it applies only when you apply, or continuously afterwards.

In Northern VirginiaFairfax and Arlington both require you to occupy one of the two units. Loudoun’s accessory dwelling section does not impose it. From July 2027 the state permits localities to require it only at the point of application.

Setback

How far a building has to sit from the edge of your property. Measured separately from the front, sides and rear, and given in feet.

Setbacks are usually the rule that decides where a detached unit can physically go, more than the size limit does — a generous square-footage allowance is no use if there is nowhere on the lot it can legally sit.

In Northern VirginiaFrom 1 July 2027 the state caps rear and side setbacks for an ADU at whichever is smaller: the setback required for your house, or the one required for accessory structures on your lot.

Zoning district

Also called: zone, zoning designation

The category your lot sits in on the county zoning map, which determines what may be built there. Written as a code such as R-1 or PDC.

You will not usually know yours offhand. It is on the county GIS map, and it is on your property record.

In Northern VirginiaIn Loudoun County this decides your size limit directly: up to 2,500 sq ft in the rural and transition districts, but 1,200 sq ft in the urban and suburban ones.

Administrative permit

A permit a county officer issues by checking your application against the written rules. No hearing, no board vote — if you meet the standards, you get it.

Far quicker and more predictable than a special permit, and cheaper. Where an administrative route exists, it is almost always the one worth designing towards.

Parcel ID

Also called: PIN, tax map number, GPIN, parcel number

The county’s unique reference for your lot, used on tax records, permits and maps. Every locality formats it differently.

Useful shorthand when talking to a zoning office — it removes any doubt about which property you mean.

Plat

Also called: plat map, survey plat

The drawing of your lot showing its boundaries, dimensions and where buildings sit on it. You will usually have one in your closing paperwork.

It is the quickest way to answer questions about lot size and where a new building could go, and any designer will ask for it early.

Special permit

Also called: special exception, special use permit, SUP

Permission that a board grants case by case rather than something you are automatically entitled to. It involves an application, usually a public hearing, and it can be refused.

Expect months rather than weeks, and budget for the possibility of a no. It is not a formality.

In Northern VirginiaIn Fairfax County this is the only route to a freestanding accessory living unit, and only on a lot of at least two acres.

HOA and covenants

Also called: homeowners association, property owners association, POA, CC&Rs, deed restrictions

Private rules recorded against your property, enforced by a neighborhood association rather than by the county. They can forbid things the county would allow.

This is the most commonly missed obstacle, and the one no map or public record will answer for you. Check your deed and closing paperwork, or look the property up in the county land records — the declaration of covenants is normally recorded there and is public. Separately, Va. Code § 55.1-1815 gives members in good standing access to association books and records on written notice, for a stated purpose and at reasonable cost; that covers the association’s own records rather than the recorded declaration.

In Northern VirginiaVirginia’s 2027 ADU law expressly does not override recorded covenants or property owners’ association declarations. If your covenants prohibit a second dwelling, the state law does not change that.

SB 531

Also called: Virginia ADU law, § 15.2-2292.4

The 2026 Virginia law requiring localities to allow accessory dwelling units in single-family neighborhoods, capping the ADU permit fee at $500, and ending rules that the occupants be related. It takes effect on 1 July 2027.

It is narrower than most coverage suggests. Localities may still require parking, set size limits, and require a 30-day minimum lease. It also does not override an HOA.

In Northern VirginiaThe law does not apply to a locality that adopted an ADU ordinance before 1 January 2026, and several Northern Virginia jurisdictions had rules in place well before then. Whether that exempts them is an unsettled question of interpretation.

Zoning ordinance

The county’s own written law governing what can be built and where. It is public, numbered by section, and it is the thing that actually decides your answer.

Most online guidance summarises other summaries of it. Anything that matters is worth reading in the ordinance itself, which is why every rule on this site links to the section it came from.