Independent homeowner planning guide
How Much Does an ADU Cost?
Build a complete first-pass budget for an accessory dwelling unit—construction, design, permits, site work, utilities, financing, contingency, and the costs that broad price-per-square-foot estimates often leave out.
On this page
The short answer
Start with the project scope, then build the budget.
First, an ADU budget depends on the project type, local labor and material prices, access, slope, utilities, code requirements, permit fees, finish choices, financing, and what already exists on the site. Therefore, two projects with the same floor area can have very different totals.
Building the ADU
Labor, materials, foundation, structure, roofing, windows, mechanical systems, interiors, fixtures and contractor overhead.
Getting it designed and approved
Survey, architecture, engineering, energy work, permit documents, reviews, inspections and applicable agency fees.
Making the site work
Demolition, excavation, access, drainage, utility trenches, service upgrades, retaining work, landscaping restoration and temporary facilities.
Start here if you do not know the numbers
Find evidence before choosing calculator inputs.
You do not need a finished design to begin. However, you do need a few local facts. Gather them in this order, then replace the calculator placeholders one by one.
| Budget input | Who to ask | What to request |
|---|---|---|
| Construction | Local contractors familiar with your proposed ADU type | An allowance based on the same size, finish level and site facts. Ask which work is included in the quoted rate. |
| Design and engineering | Your prospective designer and relevant specialists | A scope covering drawings, required studies, revisions and any services charged separately. |
| Permits and agency fees | Planning/building department and other reviewing agencies | Current written fee information for your proposal. Ask which fees are estimates and which depend on final plans. |
| Site work and utilities | Contractor, utility providers and relevant site specialists | What must be investigated, the allowance for known work, and exclusions for unknown conditions. |
| Financing and carrying costs | Prospective lender; your own project records | Loan fees, interest assumptions, draw charges and the time covered. Separately record temporary housing or other carrying expenses. |
| Contingency | Your project team | A reserve tied to unresolved conditions and scope. The sample percentage below is not a recommendation for your project. |
Keep a simple record: amount or range · what it includes · source and date · unknown / allowance / written estimate. An unknown cost is not zero.
Transparent planning tool
Build your first-pass ADU budget
Enter assumptions you can explain and replace them as local bids arrive. The calculator does not choose a hidden regional multiplier or claim to know your jurisdiction.
Choose your starting point
New to ADU budgeting? Start with the project, not the spreadsheet.
The guided path collects a few project facts and any cost information you already know. Unknown dollar inputs remain visible sample assumptions.
1 · Describe the project
Choose the option closest to what you are considering. These are representative illustrations, not project plans.
2 · Add known property conditions
Choose what you know today. These answers flag budget risks and questions to verify; they do not automatically add hidden costs.
3 · Add numbers you already know
Enter only figures you actually have. Leave a field blank to keep the calculator’s visible sample assumption; we’ll mark that item as unresolved so you know what still needs verification.
Your assumptions
Planning result
Construction $225,000 · Contingency $35,760 · Utilities $25,000
No cost figures have been replaced yet. All seven tracked budget assumptions are still sample values.
- Replace the sample construction allowance with a local builder allowance.
- Verify current permit and agency fees for the actual jurisdiction.
- Get a utility routing and capacity allowance.
- What foundation, access and utility-routing work is included in the construction allowance?
- Which permit, school, utility or other agency charges apply to this exact jurisdiction and project size?
Sample inputs, not a local quote. Until you replace the prefilled values, the total is an illustration. The plus/minus range is a sensitivity calculation, not a statistical confidence interval or a prediction of the final cost.
Formula: (600 × $375) + $18,000 + $10,000 + $20,000 + $25,000 + 12% contingency + $10,000.
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Keep your planning file
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How the sample total is calculated
This fictional 600-square-foot example uses $375 per square foot for construction ($225,000), then adds $18,000 design, $10,000 fees, $20,000 site work and $25,000 utilities. That subtotal is $298,000.
A sample 12% contingency adds $35,760. Adding $10,000 financing/carrying costs produces $343,760. These sample inputs explain the calculation; they are planning placeholders, not a contractor quote or recommended allowances.
A planning total also does not establish affordability. Compare the complete scope, borrowing terms, cash needed during construction and your household commitments before proceeding.
Choose assumptions carefully
What changes ADU costs most?
| Decision or condition | Why it matters | What to verify |
|---|---|---|
| Detached new construction | Requires a complete structure, foundation, exterior envelope and independent systems. | Verify access, foundation, fire separation, trenching distance and the electrical route. Under California’s Energy Code, a newly constructed detached ADU is treated as a new building and generally must meet solar PV requirements, subject to applicable exceptions. Get a project-specific Title 24 analysis rather than assuming a fixed solar cost. |
| Attached addition | Can require structural modification and careful integration with the primary home. | Existing plans, load paths, roof connection, fire separation and occupied-home disruption. |
| Garage conversion | The shell exists, but it may not meet residential structural, moisture, insulation or life-safety standards. | Evaluate slab moisture protection, ceiling height, structural framing, drainage, utilities and unpermitted alterations. California ADU law also limits when an agency may require replacement off-street parking after a garage, carport or covered parking structure is demolished or converted for an ADU; verify the current statutory and local pathway for the property. |
| Interior conversion | May reduce exterior construction but can trigger egress, sound, fire and utility work. | Verify legal ceiling height, required emergency escape/openings where applicable, fire and sound separation, ventilation, space-conditioning and independent living facilities. For a JADU, confirm the separate JADU requirements rather than applying detached-ADU assumptions. |
| Difficult site | Slope, limited equipment access, trees, drainage and long utility runs can outweigh savings from a smaller unit. | Survey, geotechnical needs, arborist constraints, trench routes and staging area. |
Get more useful answers
A message you can adapt for local professionals
Send the same project brief to each professional so the responses can be compared. Ask whether early estimating or a site visit carries a fee before booking it.
Subject: Early ADU budget discussion — [city/county], [ADU type]
Hello, I am exploring a [detached / attached / conversion] ADU in [city/county], approximately [size] square feet, for [intended use]. I am currently at [planning stage].
Known property conditions: [access, existing structure, slope and utility information]. Unconfirmed items: [list what still needs investigation]. Desired finish level: [brief description].
Could you explain what information you need to provide a preliminary allowance, and whether that service or a site visit has a fee?
Please distinguish construction from design, permits, site work, utility connections and owner-supplied items. For each amount, please state the scope, assumptions, exclusions, date and how long it remains useful. Please flag any costs you cannot estimate without further investigation.
I am gathering early planning information rather than requesting a fixed-price commitment before the scope is defined. What should I confirm before our next conversation?
The FTC recommends obtaining multiple written estimates and examining differences rather than automatically choosing the lowest price. Read the FTC’s contractor guidance.
Before treating an estimate as your working budget
- Can you explain what each amount includes and excludes?
- Have unknown items stayed visible rather than becoming zero?
- Have you checked for overlap between the contractor’s scope and your separate budget lines?
- Do you know what site investigation, design work or agency response could change the total?
Questions to resolve before comparing prices
ADU costs that broad averages do not answer
Use these questions to uncover exclusions, clarify allowances and understand why two prices may describe different projects.
What does the advertised price exclude?
Confirm whether the quote includes design adaptation, foundation, delivery, crane work, assembly, permits, utility connections, site work, taxes and finish items. A factory price is not automatically an installed project price.
Is the existing shell actually usable?
Budget for investigation of the slab, drainage, moisture, ceiling height, structure, fire separation, insulation, utilities and unpermitted work. Therefore, conversion can save money, but the garage is not a free residential shell.
Which fees apply to this address?
Request an itemized schedule covering plan review, building permits, impact charges, utilities, schools and other agencies. Do not apply one city’s fee exemption or estimate to another jurisdiction.
What does the capital cost?
Add origination and closing fees, appraisal, inspections, interest during design and construction, draw charges, rate changes and carrying time. Compare total borrowing cost—not only the monthly payment.
Why can a smaller ADU cost more per foot?
Design, permits, kitchens, bathrooms, utility work and mobilization contain fixed costs. Reducing area can lower the total while raising the cost per finished square foot.
What changed after the first estimate?
Track scope changes, concealed conditions, allowances, agency requirements, schedule effects and owner upgrades separately. This produces evidence you can use on the next decision.
Jurisdiction matters
Fees and requirements must be checked locally.
California Gov. Code § 66311.5 generally bars an impact fee on an ADU 750 square feet of interior livable space or less and applies proportionality rules to impact fees for larger ADUs. That rule does not mean every permit, school, utility or construction-related charge is waived. Verify the current itemized charges with the city or county, serving utilities and other applicable agencies.
Therefore, before fixing a budget, request the current written fee schedule and an itemized estimate from the authority having jurisdiction. Record the source, date and person or department contacted.
Continue planning
Connect the budget to the property and rules.
PROJECT-SPECIFIC EVIDENCE
Build an evidence file for your own budget.
This calculator is not based on a verified dataset of completed local ADU projects. Its sample amounts demonstrate arithmetic only.
For each real estimate, record jurisdiction, quote date, project type and size, included work, exclusions and how long the price is valid. Keep unknown costs separate from confirmed amounts.
Methodology and sources
What this calculator does—and does not do
In other words, the calculator performs arithmetic on assumptions entered by the user. The prefilled values are placeholders that demonstrate the framework; they are not a national average, market forecast, bid or representation of likely cost for a particular property. No hidden regional or project-type multiplier is applied.
- California Department of Housing and Community Development — ADU Handbook
- HUD USER — Accessory Dwelling Units case study
- Fannie Mae Selling Guide — Improvements and Accessory Dwelling Units
Important: This page provides independent educational planning information. Costs, laws, fees, lending requirements and construction conditions vary by property, jurisdiction, date and provider. Verify figures with the relevant local agencies and qualified professionals before making commitments.