Acreage opens up types that are rare closer in, but every one of them runs through the same checks here: the septic field, the well and the county's two-acre line.
01Detached ADU
Detached Cottages in Fairfax Station
This is where Fairfax Station stands apart from most of the county. Many properties here clear the two-acre minimum Fairfax County sets for a detached accessory living unit, so a separate cottage is a real option. It still takes a special permit from the Board of Zoning Appeals and a public hearing, and the unit is capped at 1,200 square feet.
What is underground usually decides the site. The cottage has to stay clear of the drainfield and its reserve area and keep its distance from the well, and every foot away from the house adds trenching for power, water and sewer. We map all of it on the plat before drawing the building. Owners weighing similar acreage elsewhere can compare our Great Falls ADU page.
02Attached ADU
Attached ADUs in Fairfax Station
On lots under two acres, including many in Crosspointe and South Run, an attached unit is the way to add a full second home under today's county rules. It shares a wall with the house, has its own kitchen, bath and entrance, and connects to the main house through finished, conditioned interior space, so it qualifies for the administrative permit instead of a hearing. The new exterior door has to face the side or rear.
Where there is an HOA, as in Crosspointe, architectural review usually applies on top of the county permit. Inside, we separate the two households with fire-rated walls and floors, sound insulation and separate heating and cooling.
03Basement ADU
Walkout Basement Units in Fairfax Station
Sloped, wooded lots here often give houses a full daylight lower level, and that space can become a unit through the county's administrative permit. An existing basement can be used in its entirety, subject to the ordinance's date cutoff, which frees a big lower level from the usual 800-square-foot cap.
On a septic system, the bedroom count matters more than the square footage. The Health Department reviews any change that adds bedrooms to see whether the system needs more capacity, and it has to sign off before the county approves the ALU. We pull the existing septic permit first, then decide how many bedrooms the unit can realistically have.
04Garage Conversion
Garage Conversions in Fairfax Station
Some larger houses here have a third garage bay that rarely sees a car, and converting part of an attached garage keeps the unit inside the house. Parking still has to work. The county requires the spaces the house needs plus one more for the ALU, which its summary puts at three off-street spaces on a public street and four on a private street, and a garage bay only counts if a car can actually park in it.
Long driveways usually have room for extra parking, but new paving adds land disturbance and impervious surface. We insulate and raise the floor, close the door opening with walls and windows that match the house, and check the septic impact of any new bedroom.
05In-Law Suite
In-Law Suites in Fairfax Station
Plenty of families here want a parent close by without creating a separate rental unit. An in-law suite with a bedroom, bath and sitting room that shares the family kitchen stays part of the house, so it skips the ALU permit, the owner-occupancy rule and the renewals. Add a full kitchen and it becomes an accessory living unit.
One rule applies either way on a septic property: the county says any change that increases the bedroom count gets Health Department review, ALU or not. We put these suites on the main level or a walkout, with a step-free path from the car, wide doors and a curbless shower.
06Guest House
Guest Houses and Pool Houses in Fairfax Station
A guest cottage, pool house or studio out by the tree line suits these properties. Without a kitchen, it does not meet Fairfax County's definition of an accessory living unit, so it is built as an accessory structure within the setbacks for your zoning district. Put a kitchen in and it becomes a detached ALU, with the two-acre minimum, the special permit and the 1,200-square-foot cap.
Plumbing decides a lot here. A bathroom in a guest building has to drain somewhere, and on a septic property that means talking to the Health Department early. We settle the classification with you before designing the plumbing, because changing it later means reopening walls and possibly permits.
07Above-Garage ADU
Barn and Garage Apartments in Fairfax Station
An apartment above a detached garage or a horse barn is a natural fit for an equestrian property, housing a barn manager, caregiver or relative close to the animals. Fairfax County treats a unit in a freestanding building as a detached ALU, so the lot needs two acres and a special permit, and the unit is limited to 1,200 square feet. Above an attached garage, the unit can be interior instead, and garage area does not count toward its size.
Over a barn, expect fire-rated separation from the stalls and storage below, insulation, proper egress and framing engineered for residential loads. For a new building, our garage additions team designs it to carry the floor above from day one.
08ADU Addition
ADU Additions in Fairfax Station
On a big lot, setbacks rarely stop an addition. The septic field, the well and the trees usually do. A new wing or rear addition can hold an interior ALU if it stays connected to the house through finished, conditioned space and within the county's size cap. Before placing it, we locate the drainfield and reserve area, the well and any Resource Protection Area along the nearest stream.
Disturbing more than 2,500 square feet of land triggers a grading plan, and tree protection has to be installed at the limits of clearing before work starts. Our Fairfax Station home additions page covers matching the main house and working through HOA review.
