ADU Cost Guide: What $300,000 Actually Buys You in 2026
Most ADU cost articles give you a range. “Detached ADUs run $200,000 to $500,000 in 2026.” That’s true and it’s useless. A $200k build and a $500k build are different products in different markets, and “your project might be either” doesn’t help you decide if you can afford one.
So instead of a range, here’s a single dollar figure broken into the line items that actually show up on the bid. A 600-square-foot detached ADU, turnkey, permitted, built in 2026 in a typical mid-cost US metro: roughly $300,000. Then we’ll walk through what makes a real project cheaper or more expensive than that.
If your number is $250k or $400k, that’s fine. Read this anyway, because the line items don’t change much; the unit costs in each line do.
Why $300k is the right anchor
A few reasons.
First, 600 square feet is the size most homeowners actually end up at after one round of “what do we really need.” It’s enough for a real bedroom, a small but functional kitchen, a full bathroom, and a living/dining area that doesn’t feel like a hotel room. People who start at 400 sq ft tend to push up to 600. People who start at 800 sq ft tend to come back down for cost reasons.
Second, “mid-cost US metro” (places like Phoenix, Denver, Raleigh, Nashville, Salt Lake City, Minneapolis) is where most of the country actually lives. Coastal California adds 30-50% to every line item. Rural Mississippi shaves 20% off most of them. The middle of the country is the middle of the cost curve, and that’s where the $300k anchor lives.
Third, $300k is roughly what HELOC math and cash-out refis can produce for a typical homeowner with 50%+ equity. It’s the budget people actually have, not the budget they wish they had. This is also why so many ADU projects stall at this number. Homeowners run the math, hit $300k, and have to decide whether to push higher or scale down. Knowing where the dollars go inside that $300k is what tells you which lever to pull.
(Cost figures triangulated from contractor cost-guide pages including aduwizard.com, angi.com, and maisonremodeling.com as of May 2026, and from RSMeans 2026 residential construction unit costs. Confirm with two or three local builders before you budget.)
The $300,000, line by line
Here’s where the money goes on a turnkey 600-sq-ft detached ADU at the $300k anchor. All figures are installed cost: materials plus labor plus the contractor’s overhead and margin on that line.
| Phase | Line item | Cost |
|---|---|---|
| Pre-construction | Architectural design + engineering | $14,000 |
| Permit fees (city, plan check, school district, utility) | $11,000 | |
| Surveying + soils report | $3,500 | |
| Site work | Demolition + clearing | $4,000 |
| Excavation + grading | $5,500 | |
| Utility trenching (water, sewer, electrical, gas) | $9,000 | |
| Foundation | Concrete slab + footings | $14,000 |
| Shell | Framing (lumber + labor) | $32,000 |
| Roofing (asphalt shingle) | $8,500 | |
| Exterior siding (fiber cement) | $11,000 | |
| Windows + exterior doors | $9,500 | |
| Mechanical/Electrical/Plumbing | Plumbing rough-in + fixtures | $14,000 |
| Electrical rough-in + panel + fixtures | $13,000 | |
| HVAC (mini-split, single zone) | $7,500 | |
| Insulation + drywall | Insulation (walls + ceiling + slab) | $6,000 |
| Drywall + tape + texture + paint (interior) | $13,500 | |
| Finishes | Flooring (LVP throughout) | $7,500 |
| Kitchen cabinets + counters + appliances | $18,000 | |
| Bathroom (tile, vanity, fixtures) | $9,000 | |
| Interior doors + trim + hardware | $5,500 | |
| Site finish | Landscaping + walkway + utility connections finish | $7,500 |
| Soft costs | General contractor overhead + margin (~20% of build) | $50,000 |
| Construction loan interest + insurance | $5,500 | |
| Contingency (8% of hard costs) | $20,000 | |
| Total | ~$298,500 |
Round to $300k. A few things worth noticing.
The contractor’s overhead and margin is the single largest line item. A 20% margin on a quarter-million-dollar hard-cost build is $50k. That’s not a markup the GC is hiding. That’s the going rate, and most reputable GCs will show it as a separate line. If a bid doesn’t break it out, ask. If a bid is below 15%, the contractor is either underpricing themselves (you’ll see them squeeze you on changes later) or carrying it inside the line items where you can’t see it.
Permit fees are eleven thousand dollars. That number surprises people. It includes the building permit itself, but also plan check fees, school-district impact fees, water and sewer connection fees (sometimes called “capacity charges”), and electrical-meter fees. In some California cities, ADU-specific impact-fee waivers from state law will knock several thousand off this. In some Northeast cities with old water systems, sewer connection alone can be $8,000-$15,000.
The kitchen and bathroom together are $27,000. That’s mid-range: IKEA cabinets, butcher block or low-end stone counters, basic appliances, ceramic tile. The same two rooms can hit $50k on a build with shaker custom cabinets, quartz, and a tiled walk-in shower. It’s a place to spend or save up to about $20k without changing the structure.
Contingency is $20,000. This is not optional. The number of builds that come in under-budget without dipping into contingency is small. Plan to use it. If you don’t, you spent it on landscaping you weren’t going to do otherwise, and that’s a fine outcome.
What makes it cheaper
You can build a 600-sq-ft detached ADU for $220-260k if some combination of these is true:
- You’re in a low-cost metro (Memphis, San Antonio, Oklahoma City, much of the rural Midwest). Subtract roughly 15-20% across labor lines.
- Your lot is easy to build on: flat, accessible, existing utilities close by, no demolition needed. Saves $5-10k vs. the anchor.
- You go with prefab or panelized construction. A factory-built 600 sq ft ADU lands at $180-230k turnkey in many markets, but the trade-off is design constraint and the foundation/utilities work still being site-built. Lead times are shorter, surprises are fewer.
- You act as the GC yourself. This saves the $50k contractor margin but you take on the risk, the scheduling, the inspections, and the dealing-with-subs that the GC was earning that money to do. If you’ve done this before, fine. If you haven’t, it’s a hard project to learn on.
- You scale down. A 400 sq ft studio (no separate bedroom, smaller kitchen) lands at $200-240k. The fixed costs (permits, foundation labor mobilization, utilities) are similar; the variable costs (framing, finishes, fixtures) scale roughly with square footage.
What makes it more expensive
You’re looking at $400-500k+ if:
- You’re in coastal California, Seattle, NYC metro, or Boston. Add 30-50% to labor lines and another $5-15k to permits.
- Your lot is hard. Sloped, hard-to-access, no utilities nearby (a 200-foot trench for sewer is a real cost), big trees that need removal or root-protection, or a soil report that comes back requiring deeper footings or a stem-wall foundation.
- You upsize. An 800-1000 sq ft ADU with a second bedroom, separate office, or a real laundry room runs $380-500k.
- You want the finishes. Custom cabinets, quartz, tiled wet rooms, hardwood floors, vaulted ceilings, premium windows. Easy $30-60k of upgrades on a 600 sq ft footprint.
- You’re attaching to the main house. Counterintuitively, attached ADUs can be more expensive than detached ones because of fire-separation requirements, structural tie-ins, and utility re-routing inside existing walls. Don’t assume “attached = cheaper.”
- You hit a permitting snag. Variances, design review boards, historical districts, HOA approvals, neighbor objections. Any of these can delay you 3-6 months and add interim financing costs.
The hidden costs people miss
Even at the $300k anchor, here’s what tends to surprise homeowners:
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Property tax reassessment. A permitted ADU adds appraised value to your property and most counties will reassess. Plan for $500-2,500/year in additional property tax depending on your jurisdiction’s millage rate. Some states (California’s Prop 13 environment, for instance) limit reassessment to the new portion only. Check your state.
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Insurance cost increase. A second dwelling on your lot is a coverage change. Expect $300-800/year in additional premium, and tell your insurer before construction starts. Claims during a build that the insurer didn’t know about are the thing that kills coverage.
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Utility rate changes. Many cities charge a separate water/sewer base rate per dwelling unit, even if the ADU is on the main meter. That’s $20-60/month of new fixed cost.
-
Landscaping you didn’t budget for. The construction crew will tear up part of the yard. The grading and walkway line items in the table cover the bare minimum. The “make the yard look nice again” budget is usually another $5-15k that lives outside the construction contract.
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Furnishing the unit. Especially if you’re renting it out. Appliances are in the contract, but beds, sofa, table, kitchenware are not. $5-10k for a basic, livable furnish.
The $300k anchor doesn’t include any of these. Figure another $10-20k of first-year additional cost beyond the construction contract.
Financing math, briefly
A few common ways homeowners pay for an ADU at this price point:
- Cash-out refinance. Refinance your primary mortgage at a higher principal, take the difference. Works if mortgage rates are below your current rate or you have substantial equity. Builds the cost into a 30-year amortization at mortgage rates.
- HELOC (home equity line of credit). Borrow against equity, draw as needed during construction, pay interest-only during the draw period. Variable rate. Good for staged builds. As of mid-2026, HELOC rates are running 7-9% APR for borrowers with strong credit.
- Construction loan. Purpose-built for builds, converts to a mortgage at completion. Higher interest during construction (8-10%), lower paperwork burden than refinancing, but stricter draw schedules and inspection-driven disbursement.
- Cash + small HELOC. Common for homeowners with $150-200k in savings. Pay most in cash, cover the contingency and last-mile finishes with a HELOC. Lowest total interest cost.
This article doesn’t recommend a financing path. Talk to a mortgage broker who has done ADU-specific financing before. Many haven’t, and the wrong product can cost you tens of thousands over the life of the loan.
The bottom line
$300k buys you a 600-sq-ft detached ADU in a typical US metro in 2026, finished, permitted, ready to live in. About a sixth of that goes to permits, design, and soft costs before construction even starts. About a sixth goes to the contractor’s overhead and margin. Roughly $200k is the actual hard cost of materials and labor in the building.
If your budget is under $260k, the conversion or prefab paths are likely the better fit. If your budget is over $400k, you can have a real upgrade in size, finishes, or both. The $300k middle is where most people land, and most of them are happy with the result, provided they didn’t expect $200k of unit for $200k of budget.
Where to go next
- ADU vs. garage conversion: which is right for you: if you’re not yet sure detached new-build is the right path.
- ADU permit timeline: how the calendar actually plays out from “we want to do this” to “we move in.”
- Layouts for 600 sq ft: what fits, what doesn’t, and the floor plans that don’t waste space at this size.
This article gets updated when the underlying numbers change. RSMeans publishes new unit costs every quarter, ADU-specific permit-fee schedules tend to update at the city budget cycle each summer, and we’ll re-anchor the table when we see meaningful drift. If you spot a number that looks off, the contact link is on every page; tell us and we’ll dig in.