Cost to Build an ADU (Accessory Dwelling Unit)

What Drives the Cost to Build an ADU?

The cost to build an ADU typically runs $150–$400 per square foot for the structure itself, with total project cost commonly landing between $100,000 and $400,000 depending on type, size, site conditions, and region. That range is wide because an accessory dwelling unit is not a single product. It is a small house or a converted space, and the scope behind it changes the number more than any finish selection does.

ADU pricing is not one number. Hard costs cover site work, foundation, framing, roofing, mechanical systems, electrical systems, plumbing systems, and interior finishes. Soft costs stack on top: design, structural engineering, energy calculations, permits and fees, impact fees, and utility connections. Then you add contingency for the unknowns a small project always seems to find.

An accessory dwelling unit is a second, smaller residence on a lot with its own kitchen, bath, and sleeping area. A junior ADU (JADU) is different: it is a converted space within the primary home, usually under 500 sq ft, and it may share a bathroom with the main house. The adu floor plan drives cost as much as square footage does, because a compact plan with stacked plumbing and a simple roof saves real money over a sprawling layout with multiple wet walls.

The biggest single variable is whether you build new or convert existing space. A detached ADU or attached ADU starts from raw ground and needs its own foundation, framing, roofing, and MEP systems. A garage conversion or basement conversion reuses the slab, walls, and roof, which removes the most expensive structural scope from the estimate. If you are pricing a cost to build a garage apartment, the existing shell usually means you skip excavation, foundation, and part of the framing, but you still need to add a full kitchen, bath, and separate utility metering.

Utility connections, site work, and permit fees are the line items most often underestimated by first-time ADU builders. A detached unit may need a new sewer lateral, water service, gas line, and electrical feed trenched from the main house or the street. Those trades are invisible in a per-square-foot figure but very real on the invoice, and the adu site work cost alone can swing by tens of thousands of dollars depending on trench length, soil, and whether the main panel has capacity. If you want the structural side priced line by line, ADU construction cost estimating breaks it into measurable quantities rather than a lump sum, and the same discipline applies across construction cost estimating for any small residential project. Local code also matters: under the IBC, egress, fire separation, and accessibility rules for a detached dwelling differ from those for a converted space, and those requirements translate directly into scope and cost.

ADU Cost by Type: Detached, Attached, Garage, Basement, JADU

The table below compares the five main ADU types. Ranges are typical U.S. figures for the structure and basic finishes; they vary by region, scope, and date, and they exclude land, major site work, and utility trenching unless noted.

Detached ADU cost is the highest per square foot because the unit stands alone. It needs its own foundation, framing, roofing, and full mechanical, electrical, and plumbing systems, plus utility trenching from the main service. Nothing is shared except the lot.

Attached ADU cost sits lower because one wall and sometimes the utility runs are already there. Garage conversion and basement conversion are cheaper again per square foot, but the savings come with conditions: a garage slab may not be rated for living loads, and a basement needs egress and moisture control before finishes go in. A junior ADU is the smallest and cheapest option, but the 500 sq ft cap and shared-bathroom rule limit how you can use it. For conversion scopes, remodeling estimating services price the existing conditions work that new construction never carries, and residential estimating services cover the new-build side.

ADU Cost Breakdown: Where the Money Actually Goes

A useful ADU cost breakdown follows CSI MasterFormat divisions so every line has a home. For a detached unit, the major divisions are 02 Existing Conditions (demolition and clearing), 03 Concrete (foundation and slab), 06 Wood, Plastics, and Composites (framing), 07 Thermal and Moisture Protection (roofing, insulation, waterproofing), 22 Plumbing, 23 HVAC, 26 Electrical, and 09 Finishes. That structure is what lets a contractor compare bids apples to apples instead of guessing at a lump sum.

As a share of hard cost for a detached ADU, typical splits run: site work and foundation 15–25%, framing and roofing 20–30%, MEP rough-in and trim 20–30%, interior finishes 15–25%, and exterior finishes 5–10%. Those percentages shift with soil conditions, roof geometry, and how much of the MEP work is new versus extended from the main house.

Soft costs commonly add 10–25% on top of hard costs. That bucket includes design, structural engineering, energy calcs, permit fees, impact fees, and utility connection fees. On a small project these are proportionally larger than on a full house, which is why a 600 sq ft ADU rarely hits the same per-square-foot number as a 2,500 sq ft home.

For early conceptual budgets, UniFormat is often the better lens: A Substructure, B Shell, C Interiors, D Services. It groups cost by element rather than trade, which helps at the feasibility stage. Once you move to a bid-level estimate, CSI MasterFormat governs because subcontractors price by division. A proper construction estimate separates labor, material, equipment, and subcontractor costs, not just a lump-sum per square foot. If you need the quantities behind those divisions, construction takeoff services measure them from the drawings, and MEP estimating services price the mechanical, electrical, and plumbing scope that drives so much of an ADU budget.

ADU Site Work and Utility Connections Cost

Site work for a detached ADU covers everything before the foundation formwork goes up: clearing and grubbing, excavation for footings and utilities, grading to subgrade, backfill, and compaction in lifts. On a flat, accessible lot with sandy soil, this scope can run $8,000–$15,000. Add a slope, clay or expansive soils, a narrow side-yard access, or a need for a retaining wall, and the same scope can reach $35,000 or more. Because site work is priced from measured quantities rather than a per-square-foot rate, the more useful comparison between bidders is their cut, fill, and trench takeoff, not their bottom-line allowance. An excavation and earthwork estimating takeoff tied to the grading plan gives you that comparison.

Utility connections are where the ADU budget most often breaks. Trenching from the main house or the street to the ADU for water, sewer, gas, and electrical commonly lands at $10,000–$40,000 combined, and the upper end arrives quickly if you need a new sewer tap at the main, a longer run under a driveway, or a utility transformer upgrade. Electrical service may require a new meter, a subpanel, or a full service upgrade; gas may require a new line and meter; sewer may require a new lateral, cleanouts, and a jurisdiction inspection before backfill. Treat each as a separate line in the adu construction estimate rather than one utility allowance.

Stormwater management is the third piece. Impervious-surface limits, lot coverage caps, and drainage rules can trigger a drywell, infiltration trench, or on-site detention, which is civil scope with its own design and inspection fees. If your lot is close to the coverage limit, confirm the ADU footprint against the local ordinance before you price concrete. A sitework estimating review of the civil sheets will surface those items early, when they are still cheap to change.

ADU Permit Fees and Impact Fees

Permit fees for an ADU are set by the local building department and are usually calculated from project valuation, square footage, or a flat schedule. Across U.S. jurisdictions, building permit fees commonly fall in the $2,000–$15,000 range for a small residential structure, but the number is entirely local. Plan check, fire review, energy compliance documentation, and separate trade permits are often billed on top of the base building permit, so read the fee schedule line by line before you set your soft cost budget.

Impact fees are the larger variable. School, park, transportation, water, and sewer impact fees can add $0 to $30,000 or more, and many states have reduced or waived them for ADUs below a certain size or when the unit is within an existing envelope. Those rules change, sometimes annually, so verify the current ordinance rather than relying on a number from an older project. Water and sewer connection fees are separate from impact fees and are usually paid to the utility, not the building department.

Timelines carry cost too. A three- to nine-month approval window is common, and every month of delay adds interest, insurance, and site security to a construction loan. Because permit and impact fees are jurisdiction-specific, list them as their own line in the adu construction estimate instead of spreading them through soft costs. A budget estimating structure that separates hard costs, soft costs, and fees makes it obvious when a jurisdiction is the reason a project does not pencil. If you are testing a site before purchase, a feasibility study can put a defensible range on those fees before you commit.

How to Use an ADU Cost Calculator and Per-Square-Foot Numbers

1. Collect the inputs first. An adu cost calculator is only as good as what you feed it: adu square footage, ADU type (detached, attached, garage conversion, basement, or JADU), finish level, site conditions, and region. Change any one of those and the output moves. If the calculator does not ask for site conditions or utility distance, it is not pricing your project.

2. Apply the formula. Total cost = (hard cost per square foot × ADU square footage) + soft costs + site work + utility connections + permits and fees + contingency. The adu construction cost per square foot covers only the first term. Everything after the plus sign is where ADU budgets actually fail.

3. Classify the estimate before you trust it. A per-square-foot output from a calculator is a conceptual estimate, roughly AACE Class 5 or 4. It is useful for feasibility and for comparing sites. It is not a bid, and no responsible contractor will hold a price built from it. Move to a Class 3 or better estimate once you have drawings and a scope narrative.

4. Adjust for finish level. Builder-grade, mid-range, and high-end finishes can swing the interior finishes line by 50% or more. Cabinets, countertops, plumbing fixtures, lighting, and flooring drive most of that spread. A calculator that uses one finish level for every project will be wrong for most of them.

5. Add contingency. Carry 10–20% for unknown conditions. Conversions are the highest-risk ADU type: hidden water damage, asbestos in older flooring or duct wrap, and knob-and-tube wiring are common once walls open. A preliminary estimating pass with a stated contingency is more honest than a single hard number.

6. Know when to stop using the calculator. Once you have a plan set, switch to a quantity takeoff. Line-item quantities priced in estimating software give you a bid-ready number and a basis you can defend when a subcontractor's quote comes in high.

Worked Example: 600 Sq Ft Detached ADU Estimate

Example only. The numbers below are illustrative, not a quote, and they will move with your region, soil, and finish level.

Assume a 600 sq ft detached ADU, mid-range finishes, a flat lot, and utilities stubbed 40 ft from the main house. A full adu construction estimate starts with hard costs at $300 per sq ft:

• Hard cost: $300 × 600 = $180,000
• Site work: $18,000
• Utility connections: $22,000
• Permits and fees: $8,000
• Soft costs (design, engineering, energy calcs): $15,000
• Subtotal: $180,000 + $18,000 + $22,000 + $8,000 + $15,000 = $243,000
• Contingency at 15%: $243,000 × 0.15 = $36,450
• Total: $243,000 + $36,450 = $279,450

The $180,000 hard cost splits roughly like this:

Divide the total by the floor area to get the all-in adu construction cost per square foot: $279,450 ÷ 600 = $465.75 per sq ft. That figure is higher than the $300 hard cost rate because site work, utility connections, permits, soft costs, and contingency all ride on top of the building itself. A quantity takeoff and a material takeoff of the actual drawings will replace these allowances with measured quantities.

For comparison, a garage conversion of the same 600 sq ft often lands around $150,000–$190,000 all-in, because the foundation, framing, and roofing are reused. The detached adu cost is driven by building those three scopes from scratch.

How to Prepare an ADU Construction Estimate

1. Assemble the full drawing set. You need architectural, structural, and MEP sheets plus a site plan showing utility routes, setbacks, and easements. Missing sheets turn into allowances, and allowances turn into change orders.

2. Do the quantity takeoff by CSI MasterFormat division. Count and measure concrete, framing lumber, sheathing, roofing squares, drywall sheets, fixtures, devices, and linear feet of pipe and wire. A blueprint takeoff that follows the division structure keeps the estimate auditable line by line.

3. Apply waste factors to every measured quantity. Use 5–10% for lumber and sheathing, 10% for tile and flooring, 5% for drywall, 2–5% for rebar, and 10% for roofing shingles. Waste is a real cost, not a rounding error.

4. Price labor separately from material. Use local wage rates and crew productivity for self-performed work. For subcontracted scopes, request at least three bids per trade and compare them scope-to-scope, not just by bottom line.

5. Build the estimate with one row per line item. Columns for quantity, unit, material unit cost, labor unit cost, and extended total. This is the same structure used in a construction estimate prepared by a professional estimator, and it makes updates fast when a dimension changes.

6. Add general conditions, overhead and profit, and contingency as separate lines. The owner should see what is markup and what is construction cost. Burying these inside unit rates hides risk from both sides.

ADU Cost for Contractors: Bidding and Risk

The adu cost for contractors is not the same number the owner sees in a magazine. Your bid has to cover direct cost, indirect cost, overhead, profit, and risk. A 10–20% overhead and profit is typical for residential work, but it varies by market and contract type, so set your rate from your own financials rather than a rule of thumb.

Most ADU bid risk sits in five places: unknown soil at the foundation, utility trench conflicts with existing lines, surprises inside an existing structure during a conversion, permit conditions that add scope after award, and owner-directed changes. Each of these shows up as a hard cost, not a soft one, so price them or exclude them in writing.

Every bid should carry a scope narrative and an exclusions list. State what is included, such as trenching to 40 ft, and what is not, such as rock excavation or a new transformer. That single page prevents most disputes about utility connections and site work.

Change orders on ADUs cluster around utility upgrades, structural repairs found during conversion, and finish upgrades. Price them with the same unit rates as the base bid so the owner can see the change is scope, not markup. A general contractor estimating approach that keeps unit rates consistent makes change order estimating fast and defensible.

Finally, tie a schedule of values to the estimate so progress payments match installed work. If the estimate is built line by line, the schedule of values writes itself.

ADU Cost by Region and Market Conditions

The adu construction cost per square foot you carry in your budget is a local number, not a national one. Labor rates, material availability, permit fees, and local code requirements all push the same 600 sq ft detached unit to very different totals depending on where it is built. High-cost coastal metros typically sit at the top of published ranges, while lower-cost inland and rural markets sit at the bottom.

Labor is usually the largest regional variable. In tight markets where framers, electricians, and plumbers are booked out, subcontractor pricing can run 20–40% above the national average for the same scope. That gap shows up fastest in finish trades and MEP rough-in, where crew availability sets the price more than material does. If you are pricing in a market you do not know well, review our construction estimating locations coverage before you assume a rate.

Material costs move with lumber, steel, and copper markets. A framing package priced in a soft lumber quarter can be 15% cheaper than the same package six months later, and copper wire and pipe pricing swings independently. Always date your estimate and note the pricing basis — vendor quote date, index month, or bid date — so anyone reading the adu cost breakdown later knows what market it reflects.

Do not apply a national average to a local project. Get local subcontractor bids for the major trades, or use a regional cost database such as RSMeans cost data adjusted to your city. The cost per square foot that matters is the one your local subs will actually sign. If you are still testing feasibility, compare the regional range against your pro forma before you commit to a design.

Common ADU Cost Mistakes to Avoid

• Underestimating site work and utility connections. On detached ADUs these are the most common overruns. Trenching, a new sewer lateral, a panel upgrade, and a water service run can add five figures before a single stud is set. Price them from a real site work scope, not a line-item allowance.
• Leaving impact fees out of the comparison. adu permit fees vary wildly by jurisdiction, and school fees and utility connection fees often sit on top. Two cities with identical construction costs can differ by tens of thousands in fees alone.
• Treating a per-square-foot calculator as a bid. A calculator gives you a conceptual budget. It does not know your soil, your panel capacity, or your finish level. Use it for go/no-go screening, then build a real adu cost breakdown from quantities.
• Ignoring existing conditions in conversions. In a garage conversion, check slab thickness and elevation, ceiling height, moisture and vapor control, asbestos in old drywall or flooring, and whether the existing wiring can carry the new load. In a basement conversion, add dewatering, egress window cutting, and ceiling height for ducts. Each one can add cost after the contract is signed.
• Skipping contingency. 10–20% is a standard range for ADUs because soil, utilities, and existing conditions are rarely fully known at bid time. A zero-contingency budget is a change-order budget.
• Not checking local ADU rules first. Size limits, height, setbacks, parking requirements, and owner-occupancy rules can force a redesign. A design change after permit submittal is the most expensive kind. Run a value engineering review before you lock the plans, and an estimate review before you lock the price.

ADU Cost Checklist Before You Sign a Contract

1. Confirm type, size, and finish level in writing. Detached, attached, garage conversion, basement, or JADU — plus square footage and finish tier. Every line in the adu construction estimate should trace back to that scope statement.
2. Verify what the scope includes. Site work, utility connections, permits and fees, and impact fees should be either in the contract or explicitly listed as owner costs. Ambiguity here is where disputes start.
3. Check the cost structure. A clean estimate separates hard costs, soft costs, and contingency. If they are blended into one number, you cannot audit it or compare bids.
4. Ask for quantity takeoff backup. For concrete, framing, roofing, drywall, and MEP, you should see quantities, units, and unit costs. A quantity takeoff is the difference between a bid and a guess.
5. Confirm waste factors and unit costs are stated. Framing waste is commonly 5–10%, drywall 10–15%, and tile 10–15% depending on layout. If waste is buried in the unit price, you cannot check it.
6. Review the schedule of values and payment terms. Payments should track completed work, not a front-loaded deposit. Our schedule of values preparation guide explains how to structure it.
7. Confirm who pays each fee. Permit fees, impact fees, and utility connection fees should each have a named payer. Utility connection fees in particular are often assumed by the owner and missed by the contractor.
8. Check the change order process and unit rates. Ask for unit rates for common changes — framing per linear foot, drywall per square foot, concrete per cubic yard — before you sign. That is what change order estimating discipline looks like up front. If you want a second set of eyes on the whole package, bid estimating services can review it before award.

When to Get a Professional ADU Estimate or Takeoff

A professional ADU construction estimate is worth the fee at three specific points: when you are applying for a construction loan or refinance, when you are comparing two or more contractor bids, and when you are still deciding between a detached unit, an attached addition, a garage conversion or a JADU. At each of those points the number you are working with has to survive scrutiny from a lender, an appraiser or a competing bidder, and a rough per-square-foot figure will not do that.

A third-party quantity takeoff and estimate gives you a line-item baseline. Instead of comparing one contractor's lump sum against another's, you compare both against the same measured quantities: cubic yards of concrete, linear feet of framing, square feet of drywall, fixture counts, and hours by trade. That baseline lets you check bids, spot missing scope, and negotiate from facts rather than from the lowest number on the page.

A professional adu building cost estimator works from the drawings and specifications, measures quantities, applies correct waste factors, and prices labor and material with current local rates. Deliverables are organized under CSI MasterFormat divisions for trade-level detail and can be rolled up into UniFormat elements for a systems-level view. That structure is what makes the estimate usable for both bidding and cost control later, and it aligns with the AACE estimate classification system so you know whether you are looking at a Class 4 screening number or a Class 2 bid-ready estimate.

Conversions carry the most hidden scope. A garage or basement conversion can hide slab reinforcement, underslab plumbing, egress window cutting, shear wall upgrades, and panel or service changes. A professional estimate surfaces that scope before you sign, not after demolition. For new detached construction, the same discipline applies to site work, utility trenching, and foundation details that owners routinely underestimate. If you are wondering how much does an adu cost, the answer depends on which of these scopes your project actually triggers, and a line-item takeoff is the only way to see that clearly.

Scope Precision Estimate provides same-day quotes, bid-ready estimates in 48 hours, and 20% off for new clients. Most projects turn around in 24–48 hours, with rush available. If you need a second opinion on a contractor's number, an estimate review works alongside a full ADU construction cost estimating package. For measured quantities only, a quantity takeoff gives you the material and labor counts without pricing, and a residential estimating scope covers the full house or ADU build.