Virginia’s New ADU Law: What Loudoun County Homeowners Should Know

Updated August 13, 2026. Virginia has enacted a new statewide accessory-dwelling-unit law, but its delayed effective date matters. Senate Bill 531 was approved as Chapter 895 and becomes effective July 1, 2027. Until then, current local rules continue to shape what can be approved.

For Loudoun County homeowners, that means planning against today’s zoning ordinance while watching how the county implements the state framework. Building code, wastewater, access, stormwater and site constraints remain important under either set of rules.

First: “ADU” can mean two different things in Loudoun

In housing programs, ADU often means Affordable Dwelling Unit. This article is about an accessory dwelling: a secondary independent dwelling on the same residential lot as a principal home.

Use the full phrase when searching county pages or speaking with staff so you reach the correct zoning and permit information.

What Loudoun County’s current ordinance says

Loudoun’s current Zoning Ordinance Section 4.02.01 contains use-specific standards for accessory dwellings. The permitted maximum size varies by zoning-district family.

In Transition, Rural and Joint Land Management Area districts, the maximum is the lesser of 70% of the principal structure’s gross floor area—with a footprint no more than 70% of the principal structure’s footprint—or 2,500 square feet.

In Urban districts, the maximum is the lesser of 70% of the principal structure’s gross floor area—with a footprint no more than 70% of the principal structure’s footprint—or 1,200 square feet.

In Suburban districts, the accessory dwelling’s footprint cannot exceed 50% of the principal structure’s footprint, and its gross floor area is limited to the lesser of 50% of the principal structure’s gross floor area or 1,200 square feet.

Wastewater approval is a threshold issue

For lots served by individual sewage disposal systems, Loudoun’s ordinance requires Health Department approval. For lots served by a community wastewater system, it requires approval from Loudoun Water.

An existing septic system may not support another dwelling or additional bedrooms without evaluation or modification. Resolve wastewater capacity before investing heavily in a detached-unit design or modular-home deposit.

How many accessory dwellings are currently allowed

The current ordinance permits one accessory dwelling on a lot smaller than 20 acres. One additional accessory dwelling is permitted on a parcel of 20 acres or more. In AR-1 and AR-2, an additional unit is permitted for each 25 acres beyond 20 acres; additional units may also be considered through Special Exception.

Placement and yard requirements also apply. An accessory dwelling may be inside the principal structure or in an accessory building, with standards tied to the zoning district and building configuration.

Properties inside incorporated towns are different

Loudoun County’s zoning ordinance does not govern properties inside its incorporated towns; each town has its own zoning ordinance. Recorded covenants or HOA rules may also impose private restrictions even when zoning allows a use.

What Virginia SB 531 changes in 2027

The enacted law creates a statewide ADU permit framework and requires a locality to issue the ADU permit when the proposal meets the statute and applicable local codes and ordinances. The law caps the locality’s ADU permit fee at $500, but expressly allows separate fees such as building, inspection or site-related fees.

The law allows localities to retain objective requirements in areas such as parking, size, building code, water, sewer, septic, emergency access, flood zones, stormwater, historic districts and the relationship between the accessory and primary dwelling.

The effective date is July 1, 2027—not the 2026 enactment date. County ordinance amendments, procedures and application materials may evolve before then.

What the new law does not make automatic

A state ADU right does not prove that a particular corner of a lot is buildable. A project can still be constrained by setbacks, easements, topography, floodplain, septic reserve area, utilities, entrance access, drainage, fire access and the building code.

Using a prefab or modular unit also does not bypass the site approvals. The unit’s construction classification, foundation, local installation, utility connections and certificate of occupancy still need a coordinated permit path.

What Loudoun homeowners can do now

Confirm the property’s jurisdiction and zoning district. Obtain the current plat or survey. Identify setbacks, easements, wastewater service, drainfield and reserve areas, well location, utilities, access and plausible building area.

Decide whether the ADU would be an interior conversion, addition, garage apartment or detached unit. Compare that concept against current Loudoun standards and the site’s physical constraints.

If timing may extend into 2027, avoid designing to an assumed future rule. Track the county’s implementation work and verify the requirements in effect when the application is filed.

Official sources

See the official Virginia Legislative Information System bill page for SB 531 for status, chapter number and effective date.

Review Loudoun County’s current Accessory Dwelling standards in Zoning Ordinance Chapter 4.

Use the county’s Zoning Ordinance page for the current ordinance, jurisdiction caveats and future amendment updates.

This article summarizes current public information and is not legal, zoning, engineering, wastewater or permit advice. Confirm current requirements with the responsible locality and licensed professionals before acting.

Considering a prefab or modular ADU?

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