Question 1

How much does it cost to build an ADU?

Accepted Answer

In 2024–2026, a typical US ADU costs $100,000–$300,000 all-in. Stick-built detached ADUs run about $330–$450 per square foot at mid-range finishes, so a 600 sq ft detached unit lands around $200,000–$270,000 before site work and permits. Garage conversions are far cheaper ($180–$250/sq ft), and JADUs carved from existing space are cheapest ($140–$200/sq ft). Your state, lot conditions, and finish level move these numbers a lot — the calculator above adjusts for your state.

Question 2

What is the cheapest type of ADU to build?

Accepted Answer

A JADU (junior ADU) inside your existing home is cheapest — roughly $90–$280 per square foot, often $40,000–$100,000 total — because the structure, roof, and utilities already exist. Next cheapest is a garage conversion ($120–$340/sq ft), then a basement conversion, then an attached ADU, and finally a detached ADU, which is the most expensive per square foot but gives the most design freedom and rental value.

Question 3

How much does it cost to convert a garage into an ADU?

Accepted Answer

A garage-to-ADU conversion typically costs $50,000–$150,000 for a 300–500 sq ft unit ($180–$250/sq ft at mid-range finishes). The existing slab, walls, and roof save a lot, but budget for insulation, a new electrical panel, plumbing for a kitchen and bath, egress windows, and bringing everything up to current code. Detached garages needing utility trenching cost more than attached ones.

Question 4

Is it cheaper to build a prefab ADU or stick-built?

Accepted Answer

Prefab usually wins on total cost and almost always on speed. A modular prefab runs about $140–$220 per square foot plus $20,000–$45,000 for delivery, crane, and setting — so a 600 sq ft unit totals roughly $104,000–$177,000 before site work, vs $198,000–$270,000 for an equivalent stick-built detached ADU. Prefab also cuts on-site build time to weeks instead of months. The trade-off: less design flexibility, crane access requirements, and factory lead times. Use the prefab comparator tab to compare side by side.

Question 5

How much are ADU permit fees?

Accepted Answer

ADU permit fees (plan check + building permit + typical impact fees) range from about $1,500 in low-cost states to $15,000 in California, with a national typical band of $3,000–$8,000. Several states, notably California under SB 13, cap impact fees on smaller ADUs. The fee table tab shows all 50 states plus DC — but always confirm with your city or county, because the city, not the state, sets the final fee schedule.

Question 6

How much rent can I charge for an ADU?

Accepted Answer

It depends entirely on your local market — check comparable small rentals (studios and 1-bedrooms) within a few miles of your home. As a rough national reference, ADUs commonly rent for $1,200–$2,800/month depending on size and metro. Enter your expected rent in the Rental ROI tab to see payback years, cap rate, and 10-year net operating income. Be conservative: use 5% vacancy and ~20% operating expenses (taxes, insurance, maintenance) in your first pass.

Question 7

How long until an ADU pays for itself?

Accepted Answer

Simple payback is total project cost divided by annual gross rent. A $180,000 ADU renting for $2,200/month pays back in about 6.8 years on paper ($180,000 ÷ $26,400). With 5% vacancy and 20% operating expenses, the realistic payback stretches to roughly 9 years. Financing changes the math — the ROI tab shows cash-on-cash return and levered payback for HELOC and construction-loan scenarios.

Question 8

Should I build an ADU or add square footage to my house?

Accepted Answer

An ADU usually wins on both cost and value. A home addition costs $300–$450/sq ft and historically recoups about 62% of its cost in appraised value, with no income. An ADU costs a similar amount per square foot but can generate rent — and appraisers credit rental income through a gross-rent multiplier, often valuing the ADU at 100–130% of its cost in strong rental markets. The ADU vs Addition tab compares both side by side with your numbers.

Question 9

Can I finance an ADU with a HELOC?

Accepted Answer

Yes — a HELOC is the most common ADU financing tool because you draw funds as construction progresses and pay interest only on what you've drawn (about $708/month per $100,000 at 8.5%). Alternatives: construction loans (higher rate, convert to a mortgage at completion), renovation loans, or cash-out refinance. The Financing tab compares monthly payments across these options. Check with your lender whether they count projected ADU rental income toward qualification.

Question 10

What is a JADU, and how is it different from an ADU?

Accepted Answer

A JADU (junior accessory dwelling unit) is a small unit — capped at 500 sq ft — created within the walls of an existing single-family home, with its own entrance and an efficiency kitchen. Unlike a full ADU, a JADU can share a bathroom with the main house and faces the simplest permitting. It's the cheapest path to a legal rental unit, but size and layout are constrained by your existing floor plan.

Question 11

How big can an ADU be?

Accepted Answer

Most states allow detached ADUs up to 800–1,200 sq ft, and many set a minimum allowance of at least 800 sq ft regardless of lot-size formulas. JADUs are capped at 500 sq ft. Your city may add height limits, setbacks, and lot-coverage rules on top of state law. The size presets in the Build Cost tab (250–1,200 sq ft) reflect the common legal range — confirm your local maximum before designing.

Question 12

Do ADUs increase property value?

Accepted Answer

Usually yes, especially where rents are high. Appraisers value income-producing ADUs using a gross-rent multiplier (roughly 8–12× annual rent), which in hot markets can exceed the construction cost. A non-rented ADU used as a home office or multigenerational housing still adds value, though appraisers credit it more conservatively. Detached ADUs with separate meters and addresses appraise best.

Question 13

What site-work costs should I budget for an ADU?

Accepted Answer

Site work is the #1 budget surprise. Common adders: sewer lateral $5,000–$15,000, water hookup $2,500–$9,000, electrical upgrade $2,500–$9,000, grading $3,000–$12,000, design/engineering $6,000–$18,000, survey and soils $1,500–$5,000, and fire sprinklers $2–$4/sq ft where required. Detached ADUs need nearly all of these; conversions need far fewer. The Site Work tab lets you tick the ones that apply and adds a contingency buffer.

Question 14

Can I build an ADU in my backyard?

Accepted Answer

Probably — most US states now have laws that override local bans on ADUs, but the details (setbacks, height, parking, owner-occupancy, short-term-rental rules) are set by your city or county. Start with your planning department's ADU handout or pre-approved plan program, which many California, Oregon, and Washington cities now offer. This tool estimates costs; it cannot confirm your lot's zoning — always verify locally before spending on design.

Question 15

Do I need fire sprinklers in my ADU?

Accepted Answer

It depends on your state and city. Some jurisdictions require sprinklers in all new detached ADUs (California does in many cases), while others exempt ADUs under a size threshold or when the main house lacks them. Budget $2–$4 per square foot if required — roughly $1,200–$4,800 for a typical unit. Your building department will confirm during plan check.

Question 16

Are these ADU cost estimates accurate?

Accepted Answer

They are planning estimates, not quotes. Every range on this page reflects 2024–2026 US data with an honest spread, adjusted by a regional labor multiplier — but your lot, soil, utility distances, and finish choices can move the final number 20–40% either way. Use this lab to set a realistic budget band and compare options, then get at least three local contractor bids. All calculations run 100% in your browser; no data ever leaves your device.

Factor	Prefab	Stick-built
Cost predictability	Fixed factory price; fewer change orders	Bid-based; change orders common
On-site build speed	Days to set modules once delivered	6–12 months of on-site construction
Design flexibility	Limited to factory catalog options	Fully custom floor plans
Site disruption	Weeks of heavy work, not months	Months of noise, trades & inspections
Crane & access needs	Requires crane access to the lot	No crane needed
Appraisal & resale	Appraises like site-built once complete	Longest track record with appraisers

ADU Cost Lab

ADU build-cost estimator

Prefab vs stick-built comparator

Prefab trade-offs at a glance

ADU permit fees by state

All 50 states + DC

Rental income ROI & payback

ADU vs home addition

ADU financing estimator

All three products, same amount

Site-work cost adders

ADU cost questions, answered

How this lab works

ADU costs across the US

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