Estimating Guide
By Phazahn Odom | Founder of Phixmo | Former bookkeeper for residential GCs | Former NFL tight end, Pittsburgh Steelers
ADUs are one of the best opportunities in residential construction right now. Demand is high, margins are solid, and homeowners are motivated. But ADUs are also full of estimating traps: utility connections that cost more than expected, site work that varies wildly, and permit requirements that differ by jurisdiction.
After years of doing the books for residential GCs, here's how to estimate an ADU build that accounts for the surprises and protects your margin.
The estimate depends heavily on the ADU type. Three main categories.
A standalone unit built from the ground up. The most expensive type because it needs its own foundation, full framing, roofing, and utility connections. Also the cleanest to estimate because there's no existing structure to work around.
An ADU attached to the existing home. Shares a wall, possibly some utilities. Costs depend on how the connection to the existing structure works.
Converting existing space into an ADU. Often cheaper because the shell exists, but conversions hide surprises: inadequate foundations, code-compliance gaps, moisture issues, and utility upgrades the existing structure can't support.
Each type has a different cost profile. Know which one you're bidding before you start. For software built around the ADU workflow, see ADU contractor software.
A detached ADU is essentially a small house. The categories are comprehensive.
Excavation, grading, foundation. Varies enormously based on the lot: a flat, accessible lot is cheap, a sloped or tight-access lot is expensive. This is a major variable.
Walls, floor, roof structure. Relatively predictable per square foot.
Materials and labor for the roof system.
Siding, windows, exterior doors, weatherproofing.
Full rough-in and fixtures for kitchen and bathroom, plus connection to the main sewer or septic.
Full wiring, panel (often a new subpanel), fixtures, and connection to the main service. Many ADUs require a service upgrade on the main house.
Heating and cooling system, often a mini-split for ADUs.
Full thermal envelope and interior walls.
Flooring, cabinets, countertops, paint, interior doors, trim.
The surprise category. Water, sewer, gas, and electrical connections from the ADU to the mains can run $5,000 to $30,000+ depending on distance and whether the main service needs upgrading.
ADU permits, impact fees, and school fees vary dramatically by jurisdiction. Some areas have waived or reduced fees to encourage ADUs; others charge heavily.
GC markup (12 to 18%) and contingency (10% minimum given the variables).
These are the categories that turn a profitable ADU into a money loser if you miss them.
If the ADU is far from the main connections, trenching and running lines gets expensive. If the existing electrical service can't handle the additional load, you need a service upgrade, which can add $3,000 to $8,000. Always verify the existing service capacity before bidding.
A backyard ADU with no easy access for equipment and materials costs more in labor. If everything has to be carried through a narrow side yard, factor that in.
Sloped lots, poor soil, or high water tables drive foundation costs up. Get a sense of the soil and grade before committing to a foundation number.
Connecting to the main sewer or expanding a septic system can be a major cost. Some jurisdictions require sewer capacity studies.
Impact fees, school fees, and permit costs vary wildly. In some areas they're waived for ADUs; in others they add tens of thousands. Check before you bid.
Some jurisdictions require fire sprinklers, electrical upgrades, or other improvements to the main house when adding an ADU.
ADUs reward a site visit and punish a guess. Five habits keep the estimate honest.
ADUs are too variable to estimate from a description alone. You need to see the lot, the access, the existing utility connections, and the grade.
Check whether the existing electrical service, sewer, and water can handle the additional load. This is the most common source of cost surprises.
Detached ADUs commonly run $250 to $450+ per square foot depending on market and finishes. Use this to sanity-check your detailed estimate, not to replace it.
Once you've seen the site, estimate each category. The site-specific variables (foundation, utility connections, access) are where your detailed estimate beats a per-square-foot guess.
10% minimum. ADUs have more unknowns than a typical remodel.
AI handles the structure of the estimate so you can focus on the site-specific variables.
ADU plans are typically clean, single-story drawings, which work well for AI extraction. Upload the PDF, calibrate the scale in 2 clicks, and get dimensions, room sizes, window and door counts, and fixture locations, each badged with how it was derived. Generate a first-pass estimate from the measurements.
For early conversations before you have plans, describe the ADU: "600 square foot detached ADU, one bedroom one bath, full kitchen, mini-split HVAC, [city]." Get a categorized estimate to give the homeowner a ballpark while you arrange a site visit.
The AI handles the structure of the estimate. Your site visit and local knowledge handle the variables that make ADUs tricky: utility connections, site access, and foundation conditions. See how the AI takeoff works.
A sample 600 sq ft detached ADU in a mid-to-high cost market. The ranges are wide because ADU costs genuinely vary this much based on site, utilities, finishes, and jurisdiction.
Site work + foundation
Low
$15,000
Mid
$25,000
High
$45,000
Framing
Low
$18,000
Mid
$25,000
High
$35,000
Roofing
Low
$6,000
Mid
$9,000
High
$14,000
Exterior (siding, windows, doors)
Low
$12,000
Mid
$18,000
High
$28,000
Plumbing
Low
$10,000
Mid
$15,000
High
$22,000
Electrical (incl. possible service upgrade)
Low
$9,000
Mid
$15,000
High
$25,000
HVAC
Low
$5,000
Mid
$8,000
High
$12,000
Insulation + drywall
Low
$8,000
Mid
$12,000
High
$18,000
Interior finishes
Low
$20,000
Mid
$32,000
High
$50,000
Utility connections
Low
$5,000
Mid
$15,000
High
$30,000
Permits + fees
Low
$3,000
Mid
$10,000
High
$25,000
GC overhead + profit (15%)
Low
$17,000
Mid
$31,000
High
$50,000
Contingency (10%)
Low
$13,000
Mid
$24,000
High
$38,000
Total estimate
Low
$141,000
Mid
$239,000
High
$392,000
A flat lot with nearby utilities and modest finishes lands low. A sloped lot needing a service upgrade with high-end finishes lands high. Your numbers depend on your site and jurisdiction.
Phixmo turns a plan into an estimate in minutes, then runs the ADU from permits to final invoice with real-time job costing so the margin you bid is the margin you keep. Field workers are always free.
Detached ADUs commonly run $250 to $450+ per square foot depending on market, site conditions, and finishes. Conversions can be lower. Use per-square-foot as a sanity check, not a final estimate, because site-specific variables matter enormously.
Utility connections and electrical service upgrades. If the ADU is far from the mains or the existing service can't handle the load, costs climb fast. Always verify before bidding.
Both. The plans give you the structure and dimensions. The site visit reveals the variables that drive ADU costs: access, grade, soil, and utility connections. Don't bid an ADU without seeing the site.
10% minimum. ADUs have more unknowns than typical remodels, especially around foundations and utilities.
Research them before bidding. Impact fees, school fees, and permit costs vary dramatically. Some jurisdictions waive them for ADUs, others charge heavily. Include the actual fees for your jurisdiction in the estimate.