4 minute read
Turning a basement into an ADU
Usually the cheapest and quickest route, and in Fairfax it is effectively the only one available on a normal lot. The space and the services are already there. What you are buying is a compliant exit, enough ceiling height, and keeping the water out.
In Fairfax this is the route
Because a Fairfax accessory living unit has to sit wholly within the house, a basement conversion is the practical option for anyone without two acres. It is also treated generously: as an alternative to the usual size cap you may use the entire basement or cellar, up to its size as of 10 May 2023. That is a much larger allowance than the 800 square feet the standard rule permits.
Fairfax § 4102.7.B(4) · Read the sectionArlington puts no size limit on it at all
Arlington caps an accessory dwelling at 750 square feet, or 35% of the two units’ combined floor area, whichever is smaller — except in a basement, where no specific size limit applies. If you have a large basement in Arlington, this is comfortably the most generous route available to you.
Arlington ACZO § 12.9.2 · Read the sectionEgress is the requirement that decides feasibility
A room used for sleeping needs a compliant way out, which in a basement usually means an egress window and a window well, and sometimes cutting into the foundation to make one. It is not optional and it is not cosmetic. Get this assessed early, because it is the single item most likely to change whether the project makes sense.
Height, damp, and how it will feel
Check the ceiling height before anything else — once you add a floor build-up, insulation and any ducting, a basement that felt fine can end up under the minimum. Deal with water properly rather than cosmetically; a finished basement that damps out is a much worse outcome than an unfinished one. And think honestly about light. The units people are happy to live in have real windows and a proper entrance, not a hatch off the utility room.
Questions people ask
Is a basement ADU allowed in Fairfax County?
Yes, and it is the main route available. Fairfax requires the unit to sit wholly within the house, and it allows the entire basement or cellar to be used, up to its size as of 10 May 2023 — larger than the standard 800 sq ft cap.
How big can a basement ADU be in Arlington?
There is no specific size limit for one in a basement. Arlington’s usual cap — 750 sq ft or 35% of the two units combined — does not apply to basement conversions, making it the most generous route there.
What usually stops a basement conversion?
Egress and ceiling height, in that order. A sleeping room needs a compliant exit, which normally means an egress window and window well, sometimes cut into the foundation. And once flooring, insulation and ducting go in, a basement that seemed tall enough often is not.
Would your basement actually work?
It comes down to egress, ceiling height and water, and all three are assessable quickly by someone who has done it before. Tell us about your house and we will tell you whether it is viable, what your jurisdiction allows, and what it would take.
See what you can buildAnything here that quotes the law links to the section it came from, so you can read the original. Rules differ by jurisdiction — check what applies where you are. This is general information, not a determination about your property.