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Adding an ADU onto your house
A middle path: cheaper per square foot than a separate building because it shares walls and services, but more freedom than working inside the footprint you already have. The constraint is that it has to work with the house you own — its structure, its roofline, and where it sits on the lot.
It follows the house’s rules, not an outbuilding’s
An addition is part of the principal dwelling, so it is generally held to the setbacks and height limits that apply to the house rather than the more permissive ones some jurisdictions grant a detached accessory building. That cuts both ways. Alexandria allows a detached building one foot from the lot line but an addition has to respect the zone’s ordinary setbacks; Fairfax applies the same yard requirements as the house.
Alexandria § 7-203(C); Fairfax § 4102.7.B · Read the sectionFairfax wants it genuinely connected
A Fairfax accessory living unit must be wholly contained within the structure of the house and have direct access to it through interior space that is finished, temperature controlled and fully enclosed. A breezeway does not satisfy that. Any new external entrance has to be on the side or the rear rather than the front elevation.
Fairfax § 4102.7.B(2), (3) · Read the sectionSizing is measured against the house
Most jurisdictions size an interior or attached unit as a proportion of the existing house, so a larger house buys a larger unit. Alexandria allows up to a third of the house’s gross floor area. Fairfax allows the lesser of 800 square feet or 40% of it. Loudoun allows up to 70% in its rural districts and 50% in the suburban ones. Work out your own number before deciding this route is too small.
Alexandria § 7-203(B)(3); Fairfax § 4102.7.B(4); Loudoun § 4.02.01.B · Read the sectionWhat it is like to live through
Worth planning for honestly. An addition means construction attached to the house you are living in, with the noise, dust and occasional loss of a wall that implies. A detached build is disruptive to the yard rather than the household. Neither is a reason to choose differently, but people are happier when they knew in advance.
Questions people ask
Does an addition have different setbacks from a backyard building?
Usually yes, and often stricter. An addition is part of the house and follows the house’s setbacks, while several jurisdictions give a detached accessory building more permissive ones — Alexandria allows one a foot from the lot line, which an addition could not use.
How big can an attached ADU be?
It is normally a proportion of your existing house. Alexandria allows up to a third of its gross floor area, Fairfax the lesser of 800 sq ft or 40%, and Loudoun up to 70% in rural districts or 50% in suburban ones.
Does it need to connect to the main house internally?
In Fairfax, yes. The unit must sit wholly within the structure and have direct access through interior space that is finished, temperature controlled and fully enclosed — a breezeway does not qualify. Any new external entrance must be on the side or rear.
Would an addition give you what you want?
It depends on how big your house is, where it sits on the lot, and whether the structure takes the change easily. Tell us about your property and we will tell you how large an addition your jurisdiction would allow, whether the house suits it, and what it would cost against the alternatives.
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.