CostGuideHQ

ADU Cost: $100–$300 per Square Foot in 2026

Author: CostGuideHQ Editorial·✓ Fact-checked·Updated July 2026·5 min read·4 sources
Quick answerQuick answer: Building an ADU typically costs $100 to $300 per square foot, or roughly $62,000 to $225,000 for a full project, depending on size, construction type, and location. Detached, ground-up units cost the most per square foot; prefab and interior conversions cost the least.
  • ADUs typically cost $100 to $300 per square foot to build, with a reported national average project cost of $85,000 (HomeAdvisor).
  • Whole-project totals usually fall between $62,000 and $225,000, depending on size, construction type, and location (HomeAdvisor).
  • Prefab/modular ADUs are the cheapest option at $80 to $160 per square foot, while above-garage units can run $200 to $500 per square foot (HomeGuide).
  • Labor makes up 40% to 60% of total ADU project cost (HomeGuide).

How much does adu cost?

National average
$200
per square foot
Typical range
$100–$300
Most projects
Whole project
$62,000–$225,000
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Cost by material / type

TypeLowHigh
Prefab/modular ADU (per sq ft)$80$160
Interior conversion — garage, basement, or attic (per sq ft)$50$200
Attached ADU, stick-built (per sq ft)$125$225
Detached ADU, ground-up stick-built (per sq ft)$150$250
Above-garage ADU (per sq ft)$200$500
Estimate your cost
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Rough estimate only — get local quotes for exact pricing.

What affects the cost?

  • Construction type — ground-up detached builds run $150-$250/sq ft, attached builds $125-$225/sq ft, interior conversions $50-$200/sq ft, and prefab/modular $80-$160/sq ft (HomeGuide, Mesocore)
  • Labor, which typically makes up 40% to 60% of total project cost (HomeGuide)
  • Site preparation, including excavation ($1,500-$5,000) and land leveling ($1,000-$5,000) before the foundation goes in (Mesocore, HomeGuide)
  • Foundation work, which runs about $2,400-$10,500 depending on soil and unit size (HomeGuide)
  • Plumbing and electrical systems — plumbing alone can run $10,000 to $20,000+ and electrical work around $8,000 for a full build (HomeAdvisor)
  • Permits and fees, generally $500-$3,000 depending on jurisdiction and project scope (HomeGuide, HomeAdvisor)
  • Design and architectural plans, typically $1,200-$5,000 (HomeGuide, Mesocore)
  • Geographic location — per-square-foot pricing spans $100-$300 nationally, with higher-cost regions landing at the top of that range or above it (HomeGuide, HomeAdvisor, RenoFi)

Why does an ADU cost so much?

An accessory dwelling unit is essentially a small, fully-functioning house, so it carries most of the same cost drivers as building a home, just at a smaller scale. Sources put the per-square-foot range at $100 to $300 (HomeGuide, HomeAdvisor, RenoFi), and full-project totals commonly land between $62,000 and $225,000, per HomeAdvisor’s typical range for a guest house build (national average $85,000). The gap between the low and high end mostly comes down to construction type: a prefab or modular unit can come in at $80 to $160 per square foot, while a ground-up detached build with its own foundation, roof, and full utility hookups runs $150 to $250 or more (HomeGuide, Mesocore). Labor is the single biggest line item, typically eating up 40% to 60% of the total budget (HomeGuide), because a full kitchen, bathroom, and living space all need to be built and wired from scratch in a compressed footprint.

What affects the cost?

A handful of factors explain most of the spread between a $62,000 ADU and one that costs well over $225,000:

  • Construction type. Prefab/modular units run $80 to $160 per square foot, interior conversions (garage, basement, or attic) run $50 to $200, attached builds run $125 to $225, detached ground-up builds run $150 to $250, and above-garage units run $200 to $500 (HomeGuide, Mesocore).
  • Site preparation. Excavation typically runs $1,500 to $5,000 and land leveling $1,000 to $5,000 before foundation work can start (Mesocore, HomeGuide).
  • Foundation work. Depending on soil conditions and unit size, this runs about $2,400 to $10,500 (HomeGuide).
  • Plumbing and electrical. Plumbing alone can run $10,000 to $20,000 or more, and electrical work adds roughly $8,000, since a full ADU needs its own kitchen and bathroom systems (HomeAdvisor).
  • Permits and design. Architectural plans typically run $1,200 to $5,000 and permits $500 to $3,000, though this varies by jurisdiction (HomeGuide, HomeAdvisor).
  • Location. Per-square-foot pricing spans $100 to $300 nationally, and higher-cost markets tend to land at the top of that range or above it (HomeGuide, HomeAdvisor, RenoFi).

How to lower your ADU cost

  • Consider a prefab or modular unit. These run $80 to $160 per square foot versus $150 to $250-plus for a stick-built detached unit (HomeGuide, Mesocore).
  • Convert existing space instead of building new. Garage, basement, or attic conversions can cost as little as $50 to $100 per square foot for basement work, since you’re reusing an existing foundation, roof, and shell (HomeGuide).
  • Right-size the unit. Smaller ADUs cost less in absolute dollars even at the same per-square-foot rate — a 300 square foot unit can run $27,000 to $53,000 as a prefab build or $45,000 to $95,000 custom, while an 800 square foot unit can run $65,000 to $110,000 prefab or $115,000 to $210,000 custom (HomeAdvisor).
  • Get multiple bids on labor. Since labor makes up 40% to 60% of the total budget, comparing contractor quotes has an outsized impact on the final number (HomeGuide).
  • Handle site prep and permitting early. Locking in excavation, land survey, and permit costs upfront ($400 to $5,000 range for these individually) avoids surprise change orders once construction starts (HomeGuide, Mesocore).

Prices vary substantially by region, lot conditions, and local permitting requirements, so treat the figures above as a planning budget. Get at least two or three local quotes, and confirm whether design, permits, and utility hookups are included before comparing bids.

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Frequently asked questions

How much does it cost to build an ADU?
Most ADUs cost $100 to $300 per square foot to build (HomeGuide), or about $62,000 to $225,000 for the full project depending on size and type (HomeAdvisor).
What's the cheapest type of ADU to build?
Prefab/modular units and interior conversions (garage, basement, or attic) are the least expensive, running $50 to $200 per square foot versus $150 to $250 or more for a ground-up detached build (HomeGuide, Mesocore).
How much does a garage conversion ADU cost?
Estimates vary by source: about $15,000 to $30,000 per HomeAdvisor or $20,000 to $140,000 per HomeGuide, depending on how much interior finish and system work the conversion needs.
Is it cheaper to build a detached or attached ADU?
Attached ADUs are generally less expensive, running $125 to $225 per square foot versus $150 to $250 per square foot for a detached, ground-up build (HomeGuide).
How much of an ADU budget goes to labor?
Labor typically accounts for 40% to 60% of the total project cost (HomeGuide).
Does location significantly affect ADU cost?
Yes. Per-square-foot pricing spans $100 to $300 nationally, and higher-cost regions tend to land at the top of that range or above it (HomeGuide, HomeAdvisor, RenoFi).

Sources

Figures synthesized and cross-checked across these sources (retrieved 2026-07-24):

  • HomeGuide — Overall average $100 to $300 per square foot. By type (per sq ft / for a 600 sq ft unit): prefab modular $80-$160/sf ($50,000-$100,000); attached $125-$225/sf ($75,000-$135,000); detached $150-$250/sf ($90,000-$150,000, up to $250,000 with high-end finishes); above garage $200-$500/sf ($120,000-$300,000); basement conversion $50-$100/sf (avg $30,000-$60,000); garage conversion $50-$200/sf ($20,000-$140,000 total). Planning costs: house plans $1,200-$5,000; land survey $400-$1,800; excavation $1,500-$5,000; foundation $2,400-$10,500; building permits $500-$2,000. Labor makes up 40% to 60% of total project cost.
  • HomeAdvisor — Average cost $85,000; typical range $62,000-$225,000 (low end $29,000, high end $320,000). Per square foot: $110 to $250 depending on build type. Custom build: $100-$500/sq ft (600 sq ft example $60,000-$300,000). Garage/shed conversion: $110-$120/sq ft, most conversions $15,000-$30,000. Prefabricated: $5,000-$100,000 for a ready-made kit plus up to $10,000+ for install. By size: 300 sq ft prefab $27,000-$53,000 / custom $45,000-$95,000; 800 sq ft prefab $65,000-$110,000 / custom $115,000-$210,000. Components: plumbing at least $10,000 (larger layouts $20,000+); electrical roughly $8,000; roofing about $8,000; general contractor fee 15%-25% of total; permits $600-$3,000; site prep/demolition $2,000-$9,000.
  • RenoFi — Per square foot $150-$300. Labor is about 40% of total. Detached ADU cost breakdown: foundation/site work $15,000-$40,000; structural systems $80,000-$150,000; MEP systems $30,000-$60,000; interior finishes $40,000-$80,000; permits/fees $5,000-$15,000. Note: figures previously recorded here for a national average, a $60,000-$285,000 project range, detached/attached project averages, size-based breakdowns, California/regional breakdowns, a prefab savings percentage, and a garage-conversion figure could not be confirmed on the live source page as of 2026-07-24 and have been removed pending re-verification.
  • Mesocore — Building permits $500-$2,000. House plans $1,200-$5,000. Site preparation: excavation $1,500-$5,000, leveling $1,000-$5,000. Land survey $400-$1,800. Stick-built detached ADU $150-$250 per square foot. Prefab/modular ADU $80-$160 per square foot. Interior conversions (garage, basement, or attic) around $50-$200 per square foot. Planning and permitting phases alone can reach $5,000 or more.

Sources report per-square-foot pricing fairly consistently ($100-$300/sq ft on average) but whole-project totals vary more because 'ADU' and 'guest house' data sets skew toward different unit sizes and regions. The national_average_usd figure ($200/sq ft) is the synthesized midpoint of the $100-$300/sq ft span reported across HomeGuide, HomeAdvisor, and RenoFi. The project_low/project_high figures use HomeAdvisor's stated 'typical range' ($62,000-$225,000; average $85,000, extremes $29,000-$320,000) as the anchor. RenoFi's ADU page states a per-square-foot range of $150-$300 and that labor runs about 40% of total cost, both of which line up with the other sources, but a live re-check of the RenoFi source page on 2026-07-24 could not verify several other figures previously attributed to it (a $180,000 national average and $60,000-$285,000 project range, detached/attached project averages, size-based breakdowns, California/regional breakdowns, a 10%-25% prefab savings figure, and a $60,000-$120,000 garage-conversion figure) — those claims have been removed pending re-verification rather than left uncited. The labor figure is cited to HomeGuide only: HomeGuide states labor is 40% to 60% of total cost, while RenoFi's page supports only the ~40% lower end, not 60%. The $500-$3,000 permits range blends HomeGuide's stated $500-$2,000 with HomeAdvisor's stated $600-$3,000 (low end from HomeGuide, high end from HomeAdvisor). Figures generally cover design, permits, site work, and construction of the unit itself; they exclude land acquisition and can exclude utility hookup/upgrade costs unless a source states otherwise. Garage-conversion pricing showed real source disagreement even after removing the unverified RenoFi figure ($15,000-$30,000 per HomeAdvisor vs. $20,000-$140,000 per HomeGuide) — treat garage-conversion numbers as a wide planning range rather than a tight estimate.