Building an accessory dwelling unit costs $150 to $300 per square foot on Angi’s latest figures, with a national average of $180,000 and most homeowners spending $60,000 to $285,000 (September 2026)2. HomeGuide’s ADU page, dated November 2023, publishes $100 to $300 per square foot and a $60,000 to $180,000 average for a 600-square-foot unit1 — the two pages carry different dates, and each is presented as published. Angi and HomeAdvisor are corporate siblings, and matching figures alone don’t establish independent corroboration; HomeAdvisor’s page here is labeled for guest houses, so its figures appear only in the clearly labeled guest-house section below3.
Cost by type
Angi’s totals by construction type, permits and fees excluded2:
| ADU type | Average cost range (Angi, 2026) |
|---|---|
| Basement conversion | $60,000–$150,000 |
| Garage conversion | $60,000–$150,000 |
| Attached new construction | $100,000–$216,000 |
| Above-garage construction | $128,000–$225,000 |
| Detached new construction | $110,000–$285,000 |
HomeGuide frames the same choice per square foot, with totals for a 600-square-foot unit: prefab modular $80 to $160 per square foot ($50,000 to $100,000), attached $125 to $225 ($75,000 to $135,000), detached $150 to $250 ($90,000 to $150,000, or $250,000+ with high-end finishes), above garage $200 to $500+ ($120,000 to $300,000), basement conversion $50 to $100+ ($30,000 to $60,000), garage conversion $50 to $200 ($20,000 to $140,000)1. Where the framings overlap they diverge — Angi’s detached range runs higher than HomeGuide’s 600-square-foot detached total, and its garage conversion starts well above HomeGuide’s $20,000 low — presented as published, with the pages carrying different dates12.
Cost by size
Both publishers tabulate cost by square footage, and the tables share every top end while Angi’s lows run higher — presented as published, with HomeGuide’s page dated 2023 against Angi’s 2026 update12:
| Size (sq ft) | HomeGuide (dated 2023) | Angi (2026) |
|---|---|---|
| 400 | $40,000–$120,000 | $60,000–$120,000 |
| 500 | $50,000–$150,000 | — |
| 600 | $60,000–$180,000 | $90,000–$180,000 |
| 750 | $75,000–$225,000 | $112,500–$225,000 |
| 1,000 | $100,000–$300,000 | $150,000–$300,000 |
| 1,200 | $120,000–$360,000 | $180,000–$360,000 |
Cost by location
Angi’s city-level figures2:
| Location | Average ADU cost (Angi, 2026) |
|---|---|
| Los Angeles | $150,000–$375,000 |
| New York | $135,000–$325,000 |
| Chicago | $150,000–$350,000 |
| Boston | $110,000–$285,000 |
| Atlanta | $110,000–$290,000 |
| Indianapolis | $115,000–$250,000 |
Where the money goes
Angi’s budget shares: materials at 45% to 50% of the total project, labor around 40%, and design fees 10% to 15% ($12,000 to $18,000 on a $120,000 project); the general contractor runs 10% to 20% of the total, quoted as $50 to $150 per hour or $300 to $500 per day2. HomeGuide puts labor alone at 40% to 60% of the project — a higher share than Angi’s 40%, presented as published12. Angi’s pro rates: architect $125 to $250 per hour, structural engineer $70 to $250, subcontractors $150 to $250 per day, electrician $50 to $100, plumber $45 to $2002.
Planning, site prep, and foundation
HomeGuide’s planning table1:
| Factor | Average cost (HomeGuide) |
|---|---|
| House plans | $1,200–$5,000 |
| Land survey | $400–$1,800 |
| Excavation | $1,500–$5,000 |
| Grading and leveling | $1,000–$5,000 |
| Foundation (concrete slab) | $2,400–$10,500 |
| Building permits | $500–$2,000 |
Angi prices site prep at $1,000 to $5,800 and a concrete foundation at $5 to $40 per square foot — $2,400 to $30,000 for a standard 600- to 1,200-square-foot ADU, sharing HomeGuide’s $2,400 low but running far higher at the top, presented as published12. On framing, Angi publishes a $25,000 average with lumber at $350 to $500 per 1,000 board feet and metal framing about 10% more2.
Systems, finishing, and cleanup
Angi puts electrical, plumbing, and HVAC together at $3,400 to $10,400 — electricity $1,000 to $3,800, a small furnace and AC at $6,000, plumbing at $900 — and finishing at $10 to $60 per square foot, with high-end materials pushing past $150 per square foot; finishing a basement conversion runs $3,500 to $34,5002. Post-construction cleanup costs $0.10 to $0.50 per square foot ($60 to $600 for a standard ADU), waste disposal $100 to $600, and a dumpster rental $300 to $800 per week, all on Angi2.
Permits
Permit requirements and fees vary by municipality, so confirm with your local building authority before budgeting — some areas restrict ADUs entirely. HomeGuide publishes $500 to $2,000, while Angi publishes a $1,350 national average and, for California specifically, $10 to $12 per square foot — $7,500 to $9,000 for a 750-square-foot unit on its figures. The ranges diverge widely by location, presented as published12.
Prefab vs. custom
HomeGuide prices a prefab modular ADU at $80 to $160 per square foot, or $50,000 to $100,000 for 600 square feet — the lowest per-square-foot band on its type table1. Angi claims prefab saves up to 20% on project cost — its own claim, attributed2.
Guest house vs. ADU
The definitional line, attributed to HomeAdvisor’s guest-house page: a guest house hosts visitors without the full amenities for permanent living, while an ADU includes a complete kitchen and bathroom for long-term residence3; HomeGuide draws the same distinction on its ADU page1. HomeAdvisor’s guest-house figures, on their own label: an $85,000 average on a $62,000 to $225,000 normal range, roughly $110 to $250 per square foot, custom builds at $100 to $500 per square foot, and garage or shed conversions at $110 to $120 per square foot ($15,000 to $30,000)3. HomeGuide’s own figures for adjacent alternatives, per square foot: modular guest house $80 to $160, modular home addition $90 to $210, tiny house $150 to $450, shipping container guest house $150 to $300, prefab log cabin $100 to $250, metal building conversion $25 to $80, pole barn conversion $20 to $60, yurt $25 to $1101.
Related cost guides
See home addition cost for attached builds, basement finishing cost and cost to build a garage for conversion starting points, and pole barn cost for the conversion alternative above.
The bottom line
Budget $150 to $300 per square foot on Angi’s 2026 figures, with its national average at $180,000; HomeGuide’s 2023-dated page publishes $100 to $300 per square foot and $60,000 to $180,000 for 600 square feet. Conversions sit at the low end on both pages and detached new construction at the top — $110,000 to $285,000 on Angi. Design, permits, systems, and finishing each carry their own line above, and the permit spread is the widest single variable between the two sources. Confirm zoning and permit rules with your local building authority before pricing the rest.
Sources (each fetched and read October 26, 2026; figures are each publisher’s latest published data):
1. HomeGuide, “How Much Does It Cost to Build an ADU?” (page dated November 13, 2023) — homeguide.com/costs/adu-cost
2. Angi, “How Much Does an Accessory Dwelling Unit (ADU) Cost to Build?” (updated September 29, 2026) — angi.com/articles/how-much-do-adu-costs.htm
3. HomeAdvisor, “How Much Does It Cost to Build a Guest House?” (guest-house page; updated June 17, 2026) — homeadvisor.com/cost/architects-and-engineers/build-a-guest-house
Related guides
Duplex Construction Cost
Cottage Cost
Pool House Cost
Mother-in-Law Suite Cost
Guest House Cost
Shipping Container Home Cost
Tiny House Cost (2026 Guide)