What is the cheapest type of ADU to build in Virginia?

Converting existing space, such as a garage or basement, is usually the least expensive ADU because the structure already exists. New detached backyard homes cost more because they add foundation, framing, and utilities.

2026 price guide

How much does an ADU cost in Virginia?

Q: How much does an ADU cost in Virginia?

A garage or interior conversion typically runs $60,000 to $150,000, and a new detached ADU runs about $180,000 to $400,000 or more, roughly $250 to $400 per square foot, depending on size, site conditions, and finishes.

A garage or interior conversion typically runs $60,000–$150,000, and a new detached backyard home runs about $180,000–$400,000+ — roughly $250–$400 per square foot. Your final number depends on size, site conditions (grading, utilities, septic), and finish level. The single biggest swing is whether you are converting existing space or building new and detached.

Cost by ADU type

$60k–$150k

Garage / interior conversion

Reuses an existing structure — the value path.

$180k–$400k+

Detached backyard home

New standalone build, ~$250–$400 / sq ft.

$250–$400

Per square foot (new)

Finishes and site work move this most.

The costs most builders don't mention

The per-square-foot number is only part of the story. Budget for site and utility work (water, sewer or septic tie-ins, grading, and drainfield work can add roughly $10,000–$25,000), professional and design fees (often $8,000–$15,000), and finish upgrades. One piece of good news specific to Virginia: under SB 531, the local ADU permit fee is capped at $500 starting July 1, 2027 — a real saving versus discretionary review fees.

What drives your number up or down

Size and layout, whether the lot is on public sewer or septic, how far utilities have to run, site grading and access, and finish level are the main levers. A flat lot on public sewer with a simple rectangular plan sits at the low end; a sloped, septic lot with high-end finishes sits at the top. This is exactly what a site-specific Readiness Assessment prices — a real budget instead of a guess.

Rule of thumb: if you can convert existing square footage, start there. If you want a private, rentable, standalone unit, plan for the detached range — and remember it can return $1,500–$2,500+/month in many Virginia markets. See how to finance it.