Cost: published figures on both sides
Convert: Foster Remodeling publishes $50,000–$200,000 for conversion-type ADU projects in Northern Virginia. The structure exists — you pay for transformation: insulation, plumbing, electrical, windows, finishes.
Build new: Makara ADU publishes $150,000–$350,000 for a detached ADU (quoted in The Banner); Excell Homes publishes $150,000–$200,000 for a turnkey 600-square-foot detached unit. Foundation to roof, all new.
The crossover: when the existing structure needs a new foundation, major reframing, a panel upgrade, and long plumbing runs, conversion pricing climbs into new-construction territory — for a compromised result. That is the number to watch.
A note on prices: material costs move up and down, and no two lots need the same work. Treat any numbers on this page as starting points, not quotes. Your written estimate after our free lot walk is the real number for your project.
Sources: Foster Remodeling (fosterremodeling.com, published figures); Makara ADU (quoted in The Banner); Excell Homes (excellhomes.us/adu, published figures).
What converts well (and what does not)
Garages convert best with solid foundations, adequate ceiling height, plumbing within reach, and electrical panel capacity — see garage conversion cost.
Basements need ceiling height, egress windows for bedrooms (code requirement), and moisture control — see basement ADUs.
Attics rarely pencil out: stairs, dormers, structural reinforcement, and HVAC for a space the house never meant to be lived in.
The rule: the better the bones, the better the conversion. Cosmetic transformation is cheap; structural rescue is not.
Timeline and disruption
Conversions skip foundation and framing — the longest phases — so construction runs shorter and the disruption footprint is smaller (no excavation, no crane, less noise). But the permit path is identical: health-department sign-off where it applies, ADU permit, building permit. And living through a conversion means construction inside your home's orbit; a detached build keeps the mess in the backyard.
Design freedom
New construction wins outright: any size within the caps, any layout, any style, placed for sun and privacy. Conversions inherit the box — its dimensions, its ceiling, its window placements, its relationship to the house. If the mission needs a specific design (accessible living for a parent, a rental that photographs well), weigh what the box gives up.
The decision
Convert when the structure has good bones and the budget leads.
Build new when the structure fights you, the design matters, or the lot has room to spare.
Price both always — the crossover number is specific to your building, and guessing it is the most expensive mistake in this decision. We will price both paths free on the walk-through.