How to Compare ADU Builder Bids in Fairfax County (2026)

The short version

A low ADU bid is usually an incomplete one. The gap between two quotes is almost always scope, design, engineering, permits, utilities, finish level, not the builder being cheaper.
Compare inclusions line by line, not bottom-line totals. Put every bid on the same checklist below.
In Fairfax County, confirm what your lot allows first: a detached ALU needs a 2+ acre lot and special permit; most lots take the interior or attached route.
Well or septic changes everything, the Health Department must approve before a permit, and capacity can decide feasibility.

Why ADU bids are so hard to compare

When homeowners send us a competing quote, the question is almost always “why is this one cheaper?” The honest answer is usually that it is not cheaper, it is smaller. One bid includes structural engineering and permits; another leaves them for you to sort out. One assumes mid-range finishes; another quietly assumes builder-grade. Until the two bids cover the same scope, the totals are not comparable, and the “savings” on the low one tends to reappear as change orders once the walls are open.

Line these up apples-to-apples

What to compare	Why it matters
Design & engineering	Included, or a separate cost you will owe later?
Permits (who files)	A turnkey builder files and manages the Fairfax PLUS submission for you.
Site work & utilities	Foundation, trenching, and water/sewer/electrical connections add up fast.
Finish level & allowances	Two bids can “include finishes” at wildly different quality.
Change-order policy	How are surprises priced, and what triggers a change order?
Timeline & warranty	A realistic schedule and a written warranty signal a serious builder.
License, insurance, references	Virginia license, insurance, and completed ALU projects nearby.

Red flags in a low bid

A lump-sum price with no itemized scope.
No mention of who pulls the permits.
Allowances that are obviously too low to hit the finishes you discussed.
A timeline that ignores permit review entirely.
No written change-order policy.

The Fairfax-specific questions to ask first

Before you even compare numbers, make sure each builder has confirmed what your lot can support. In Fairfax County, a detached Accessory Living Unit is only allowed on lots of at least 2 acres and needs a special permit; on smaller lots, the interior or attached route (up to 800 square feet or 40 percent of the home) is the option. If you are on well or septic, the Health Department must sign off before a permit is issued. A builder who hand-waves these questions has not actually scoped your project. Our Fairfax County ADU zoning guide covers the rules in full.

Frequently asked questions

Why are two ADU bids for the same project so different?

Almost always scope, not price. One bid may include design, engineering, permits, and utility connections while another quietly leaves them out, or assumes cheaper finishes. A lower number is usually a less complete number, which is why comparing inclusions line by line matters more than comparing totals.

What should a complete Fairfax County ADU bid include?

Design and structural engineering, Fairfax County permits filed through the PLUS portal, site work and utility connections, the full build and a defined finish level, and, if the property is on well or septic, any Health Department approval and septic work. Anything missing tends to return as a change order.

How do I check a Fairfax ADU builder's credentials?

Confirm a valid Virginia contractor license and insurance, ask for real local references and completed Accessory Living Unit permits, and require a written contract with a clear change-order policy and warranty. A builder who has run the PLUS portal before will not be learning on your project.

What Fairfax-specific questions should I ask?

Whether your lot supports a detached ALU (only allowed on lots of at least 2 acres, with a special permit) or an interior/attached one (up to 800 square feet or 40 percent of the home), and whether you are on well or septic, which requires Health Department approval before a permit is issued.

Is the lowest ADU bid the best deal?

Rarely. If a bid is well below the others, the gap usually reappears later as change orders for work that should have been in the original scope. Compare what each bid actually includes, and treat a suspiciously low number as a warning, not a win.

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