ADU Builder in Great Falls, VA

Great Falls has the land that most of Northern Virginia doesn't. Two-acre and larger lots are common along Walker Road, Seneca Road and the side streets off Georgetown Pike, and that opens up an option most Fairfax County homeowners can't use: a detached second unit. It also brings problems most suburbs never deal with. Many homes run on a well and a septic field, and some back onto Difficult Run, the Potomac or one of their tributaries. DLA Design & Build plans accessory units around all of that, from a guest cottage across the lawn to a finished walkout lower level for a parent, and handles the county approvals.
Big lots, real constraints, and a builder who plans for both.

ADUs we build:

Find Out More

Tell Us About Your Project

Fill out the form below and we will call you back to talk about your project.

Why Great Falls Owners Build a Second Unit

Many Great Falls families bought decades ago and have no intention of leaving. The kids are grown, the grandchildren visit often, and an aging parent may need to be closer than a drive from Reston or across the river. On a lot this size, a separate cottage lets everyone keep their own front door. Others build for a live-in caregiver.
Resale is part of it too. A main house plus a separate place for family is a combination Great Falls buyers look for. A well-designed guest house that matches the main house can add real appeal, but only if it was permitted properly.
See Our Previous Work

ADU Types We Build in Great Falls, VA

Fairfax County splits these into two tracks: units inside the house, which can get an administrative permit, and detached units, which need a special permit. Here is where each type lands on a Great Falls property.

01Detached ADU

Detached ADUs in Great Falls

This is where Great Falls stands apart from most of Fairfax County. A detached accessory living unit is allowed only on lots of two acres or more, and only by special permit from the Board of Zoning Appeals after a public hearing. Plenty of Great Falls properties meet the acreage. The unit is capped at 1,200 square feet, two bedrooms and two occupants, and the owner has to live in either the cottage or the main house. On a well and septic lot, the Health Department signs off before the permit is approved. We site the cottage around the drainfield, the reserve area and any stream buffer first, then design the building.

02Attached ADU

Attached ADUs in Great Falls

Fairfax County's administrative path is built around units inside the house. To qualify, the unit must be wholly contained in the single-family home and connect to the main dwelling through finished, temperature-controlled, fully enclosed interior space. A unit that shares a wall but can only be reached by an outside door wouldn't meet that standard. Any new exterior entrance has to be on the side or rear. Size is limited to 800 square feet or 40 percent of the house, whichever is less. On a large Great Falls house, 40 percent is rarely the tighter number, so 800 square feet usually sets the size. A larger interior unit takes a special permit.

03Basement ADU

Basement ADUs in Great Falls

Fairfax gives basements a special allowance. An interior unit can use the entire existing basement or cellar, subject to a cutoff date in the ordinance that we confirm with the county, even when that's more than 800 square feet. Many Great Falls homes have big walkout lower levels facing the woods, and finishing one with a separate side or rear door is often the most direct path to a legal unit. The administrative permit still requires the interior connection to the main house, one extra off-street parking space, and Health Department approval on well or septic. For sizing, the county counts basement space with at least six feet six inches of structural headroom as floor area.

04Garage Conversion

Garage Conversion ADUs in Great Falls

Converting an attached garage can work as an interior unit, since it's already inside the house's structure. Two county rules shape it. The first is parking: the property needs the house's required spaces plus one, and a garage only counts if a car can actually park in it. Convert the garage, and those spaces have to exist on the driveway instead. The county also limits how much of a front yard can be paved in some residential districts, so widening the driveway isn't always the answer. The second is placement: any new garage or carport built with the unit must sit directly beside an existing garage or carport and use the same driveway and curb cut.

05In-Law Suite

In-Law Suites in Great Falls

Fairfax County notes that an accessory living unit is sometimes called an in-law suite, and that's the point to understand. The county describes an ALU as a living space with areas for eating, sleeping, living and sanitation. Once a suite has all of that, expect it to need the ALU permit, the recorded document and the renewals. A bedroom and bath that share the family kitchen generally stay part of the main house, and we confirm that with zoning staff. Since 2021 the unit no longer has to house someone 55 or older or with a disability, but many families still build it for a parent. We plan a step-free route from the car, a curbless shower and wide doors.

06Guest House

Guest Houses in Great Falls

Pool houses, studios and guest cottages show up on a lot of larger Great Falls properties. How the county treats one depends on what's inside and how it's used. A detached building with areas for eating, sleeping, living and sanitation, used as someone's home, is a detached ALU: two acres, a special permit and 1,200 square feet at most. A pool house with a changing room and half bath, or a painting studio with no kitchen, is a different kind of accessory structure. We confirm the classification with county zoning staff before drawing, because a kitchen added later can change the answer and the approval it needs.

07Above-Garage ADU

Above-Garage ADUs in Great Falls

A carriage house, meaning a detached garage with an apartment upstairs, suits the estate character of Great Falls. Because it stands apart from the house, the county treats the apartment as a detached ALU: a two-acre minimum, a special permit from the Board of Zoning Appeals and 1,200 square feet at most. Building over an existing attached garage is a different case. If the new space connects to the house through finished interior space, it can be an interior unit on the administrative track, within the 800 square foot limit, and the garage below doesn't count toward the unit's floor area. Either way, we check whether the garage walls and footings can carry a second floor before designing.

08ADU Addition

ADU Additions in Great Falls

The county's administrative permit covers units in an addition, so a new wing is often the most direct legal route to a comfortable unit on a lot under two acres. The addition has to connect through finished, conditioned interior space, meet the zoning district's setbacks, and keep the unit within 800 square feet or 40 percent of the house. On a Great Falls lot, the harder limits are usually physical: the septic field, the well and any Resource Protection Area near a stream. We map those, then design a side wing or rear addition that looks like it was built with the house. For added space that isn't a separate unit, see our Great Falls home additions page.

Talk to Us About Your Lot

Fairfax County Rules for Accessory Living Units in Great Falls

Great Falls is unincorporated Fairfax County, so the county Zoning Ordinance governs. The county's term is accessory living unit, or ALU. In Fairfax, ADU refers to affordable dwelling units. An ALU is allowed only with a single-family detached house, one per lot, with no more than two bedrooms and two occupants, and the owner must live in either the main house or the ALU. The 2021 zoning rewrite dropped the old requirement that one occupant be 55 or older or have a disability.
The permit path depends on where the unit sits. An interior unit, including one in a basement or in an addition, can be approved by administrative permit if it stays inside the house, connects to the main living space through finished, climate-controlled, fully enclosed interior space, and stays within the size limit: the lesser of 800 square feet or 40 percent of the house, or the whole basement as it existed on a cutoff date set in the ordinance. A detached unit needs a special permit from the Board of Zoning Appeals, which involves a public hearing, and is limited to lots of at least two acres and 1,200 square feet. We plan any detached ADU here around that hearing.

If the house is on a well or septic system, the Fairfax County Health Department must sign off before the county will approve the ALU permit, and adding bedrooms can mean the drainfield has to be evaluated or expanded. And land within 100 feet of a perennial stream is typically inside a Resource Protection Area, where most construction needs county approval and some needs a formal exception. Building permits come from Land Development Services.
Reach Out To Us

Frequently Asked Questions About ADUs in Great Falls, VA

The Health Department answers that one. Fairfax County requires Health Department approval before it will issue an administrative or special permit for an ALU on a property served by a well or septic system. A drainfield is sized for a certain number of bedrooms, so a unit that adds one or two may exceed its design. We start by finding the existing septic permit and locating the field and reserve area. Sometimes the answer shapes the whole project, including where a detached cottage can go.

Under the current Fairfax County rules, no, not as a detached accessory living unit. The two-acre minimum applies to detached units, and they also need a special permit from the Board of Zoning Appeals. The county has been studying changes to the ALU rules, including the detached lot size, but nothing had been adopted as of September 2026. On your lot, an interior unit is the realistic option now: a lower-level apartment or an attached wing connected through conditioned interior space.

Yes, through a special permit instead of the administrative one. According to the county's summary of the ALU rules, the Board of Zoning Appeals can approve a larger interior unit, changes to the entrance and street-access standards, and a unit where the required on-site parking isn't provided. That means a public hearing and a longer schedule, so it's worth deciding early whether the extra space is worth it. On many Great Falls houses the basement allowance solves the problem without a hearing, since an interior unit can use the whole existing basement, subject to the ordinance's cutoff date.

Yes. Fairfax County maps Resource Protection Areas along perennial streams, and the buffer includes land within 100 feet of the water plus major floodplain. Difficult Run and many of its tributaries run through Great Falls. Most construction, grading or impervious surface in an RPA needs county approval, and some work needs a formal exception with a public hearing. We check the county's RPA mapping against your plat early. Moving the cottage is usually easier than fighting for an exception.

The county requires the parking the main house already needs plus one more space. According to the county's summary of the rules, that works out to three off-street spaces on a public street and four on a private street. A garage can count if a car can actually park in it. A new outside entrance has to be on the side or rear of the house, and an interior unit has to share the main house's driveway and curb cut.

Yes. The owner records a copy of the permit in the county land records before anyone moves in. The initial approval runs for two years, and the Zoning Administrator can extend it for periods of up to five years based on the property's compliance record. The unit also needs a working fire extinguisher and interconnected smoke alarms, plus carbon monoxide detection where required. We build those in and hand you the paperwork.

GOOGLE REVIEWS

What Our Clients Say

Real feedback from homeowners across Northern Virginia. We're proud of the relationships we build, project by project.

5.0 Average Rating
★★★★★
45+ Five-Star Reviews
Read All Reviews on Google

How We Approach a Great Falls ADU

Site first, design second. On a Great Falls lot, the septic field and reserve area, the well, any RPA buffer and the slope decide where a second unit can go. We map those before sketching, then decide with you between an interior unit on the administrative track and a detached one that goes to the Board of Zoning Appeals.
Drawings then go to the Health Department, county zoning and Land Development Services, plus the special permit package if the unit is detached. Construction starts after approvals, with tree protection set before the first truck rolls up a long driveway.

What Great Falls Owners Get

A second home that suits the setting: a cottage at the edge of the woods, or a lower level that opens onto the back lawn. It comes with the county permit recorded and the septic questions answered, so the paperwork is ready when a buyer asks for it.
Contact Us Today

ADU Builder in Nearby Communities

We plan and build ADUs across Northern Virginia. See local details for nearby areas.

Plan an ADU on Your Great Falls Property

Call (703) 743-7777 or fill out the form. Your address and lot size are enough to start.
We'll look at the lot, the septic situation and any stream buffers, then tell you which path, administrative or special permit, fits what you want to build.

"*" indicates required fields

* indicates required fields