Inside town limits, Vienna allows only attached units, so several of these types work differently here than anywhere else in Fairfax County. If your Vienna address is outside the town line, the county rules apply instead.
01Detached ADU
Detached ADUs and Vienna's Town Limits
Inside the Town of Vienna, a detached backyard cottage is not allowed as a living unit. The town's ALU standard requires the unit to sit within the house or share at least one wall with it, and the zoning code says no accessory structure may be used for dwelling purposes. That rules out a freestanding cottage on an in-town lot of any size.
The realistic alternatives are an attached wing, a lower-level apartment or a second story over an attached garage, all covered below. If your Vienna mailing address is outside town limits, you are under Fairfax County rules, where a detached unit is possible on two or more acres with a special permit. Our McLean ADU page explains those county rules.
02Attached ADU
Attached ADUs in Vienna
This is the type Vienna's code was written for. The town allows an attached ALU in the RS-16, RS-12.5 and RS-10 zones, up to 800 square feet or 40 percent of the house, whichever is less. The Board of Zoning Appeals can approve a larger unit as a conditional use, but never more than 1,000 square feet or 40 percent of the house, whichever is less. Any new outside door goes on the side or rear.
The unit needs its own areas for living, sleeping, eating, cooking and a bathroom, and the owner has to live in one of the two homes. We build the shared wall with fire and sound separation and give the unit its own heating and cooling, so both households can close their doors and forget the other is there.
03Basement ADU
Lower-Level Units in Vienna Split Levels
Vienna's split levels and raised ranches often have a lower level that already opens to the backyard, which makes them strong candidates. Because the space is inside the house, it adds nothing to lot coverage. The town's size cap still applies, and its standard does not include the county's allowance for using an entire existing basement. A large lower level may have to be divided, with part going to the ALU and part staying with the main house.
We add egress windows sized for bedrooms, deal with moisture at the foundation walls and create a side or rear entrance. It is the same basement remodeling work we do across Northern Virginia, plus the systems a second household needs.
04Garage Conversion
Garage Conversions in Vienna
Converting an attached garage keeps the unit inside the house, which the town allows. Parking is where these projects succeed or fail. The main house needs two off-street spaces for up to three bedrooms and three for four, and the ALU adds one more space per bedroom. Take away the garage and all of that has to fit on the driveway, which must be asphalt, concrete or grid pavers and at least 12 feet wide. New paving also eats into the 25 percent lot coverage cap.
On an RS-10 lot, a one-bedroom conversion is often the most practical version. Inside, we build an insulated floor over the slab and replace the door with a wall and windows that match the house.
05In-Law Suite
In-Law Suites in Vienna
In Vienna, the line between an in-law suite and an ALU is the kitchen. The town defines an ALU as a unit with areas for living, sleeping, eating, preparing food and a bathroom. A bedroom suite with a sitting room and bath that shares the family kitchen does not meet that definition, so it is simply part of the house. Add a kitchen and it becomes an ALU, with the certificate of occupancy, the owner-occupancy affidavit, the compliance form every two years and possible town inspections.
For parents who want to stay near Church Street and the Town Green, we plan step-free entries, wide hallways and a curbless shower either way.
06Guest House
Guest Houses and Studios in Vienna
A backyard guest house where visitors sleep is not an option inside the town, since Vienna's code does not let any accessory structure be used for dwelling purposes. What you can build is a detached garage, shed or similar non-residential structure, subject to the town's accessory structure rules and, for less common uses, a Zoning Administrator determination. Those structures have to sit behind the rear wall of the house, cannot cover more than 30 percent of it, and cannot be taller than the main house.
For overnight guests, an attached suite or a lower-level guest room does the job, and it can become a permitted ALU later if you add a kitchen and apply. Outside town limits, Fairfax County rules govern guest structures instead.
07Above-Garage ADU
Suites Over an Attached Garage in Vienna
Vienna's ALU standard names a second story above an attached garage as an allowed location, which makes this one of the cleaner ways to add a unit on a small in-town lot. It adds living space without adding lot coverage. The garage has to be attached. An apartment over a detached garage is out, because detached structures cannot be used as dwellings in town.
The limits are the town's 35-foot height cap and two-and-a-half-story maximum, plus the structure underneath, since a garage built for one story may need new footings. We check the foundation and framing first, then plan the stair, the fire-rated ceiling over the cars and a side or rear entrance. If the garage itself needs rebuilding, see our garage additions work.
08ADU Addition
ADU Additions in Vienna
Adding new footprint for the unit runs straight into Vienna's setback and coverage math. All three single-family zones require a 35-foot rear yard, side yards are 15 feet in RS-16 and RS-12.5 and 12 feet in RS-10, and lot coverage tops out at 25 percent. A rear wing that looks easy on paper can run out of room once the existing house, driveway and patio are counted.
A second-story addition sidesteps coverage entirely, and a unit on the new floor still counts as attached. We plot the existing footprint first and design the unit to the space that is actually left.
