ADU cost comparisons you can inspect.

How do the city presets compare?

Each row below uses an 800 sq ft detached ADU, one bathroom, standard finishes, one story, typical sitework, moderate utilities and 12% contingency. There are no extras. These are reproducible model scenarios, not a city cost survey.

How is the budget calculated?

1. Construction: (fixed allowance + area × base rate) × market factor × finish factor × story factor. Add $12,000 × market factor for each bathroom beyond the first; a half bath adds half that amount.
2. Finishes and stories: budget ×0.90, standard ×1.00, premium ×1.18 or luxury ×1.35. One story ×1.00; two stories ×1.08. Extra-bath allowances are added after these multipliers.
3. Design and engineering: effective construction cost (including any construction override) × the design percentage in the table, with a minimum of $7,000 × market factor and a maximum of $38,000 × market factor. A design override replaces this calculation.
4. Sitework: $12,000 for easy, $28,000 for typical or $55,000 for difficult conditions × the type’s site factor × (0.86 + 0.14 × market factor).
5. Utilities: $8,000 for light, $18,000 for moderate or $35,000 for major work × the type’s utility factor × (0.86 + 0.14 × market factor).
6. Permit allowance: area × the cost preset’s fee rate, with a $3,000 floor. When the selected state is California, scenarios of 750 sq ft or less use a 0.58 multiplier before the floor. That multiplier is an editorial assumption for remaining fees, not a statutory discount or a jurisdiction’s fee schedule. An entered fee amount replaces this entire calculation, including the floor.
7. Extras: solar $12,000, accessibility $10,000 and appliances $9,000, each × market factor; landscape $8,000 × (0.86 + 0.14 × market factor). Add any custom extra cost once.
8. Contingency and total: add the selected 5–20% reserve to the sum of all categories above. The default is 12%.
9. Low–high range: regional presets use −7.8% / +12% of the total, or −11.7% / +18% for difficult sites or major utilities. The U.S. baseline uses a wider −16.25% / +25%, or −22.75% / +35% for those difficult conditions. These editorial sensitivity bands include contingency and are not measured error margins or statistical confidence intervals. Quote overrides do not automatically narrow them.

The range rounds to the nearest $1,000, while still enclosing the unrounded total for unusually small entered budgets. Budget lines and the planning total round to dollars; rounding differences are balanced so items add up without negative allowances. The planning total is not the arithmetic midpoint of the asymmetric range.

How do quote overrides improve a budget?

A nonblank amount replaces its whole category; 0 is a valid waiver or work included elsewhere, and blank restores the model. Six amounts cover construction, design, permits, sitework, utilities and extras. Construction overrides affect the default design calculation. Extras override replaces all extra checkboxes and custom extras. Contingency is added after replacements. Entering amounts does not verify their scope or turn the estimate into a guaranteed price.

For a bundled quote, enter the complete amount once and zero the included categories. Check exclusions before doing so. Quote amounts persist when you change size or location until you clear them. Invalid shared amounts (negative, nonnumeric, infinite or above $10 million per category) fall back to the model. Shared links store the inputs, not a locked historical price; later model updates can change a restored result. Print the budget to retain today’s figures.

How are rental income and financing calculated?

Gross annual rent = monthly rent × 12. Net operating income = gross rent × (1 − your operating allowance). Net operating yield = net operating income ÷ project total. Simple payback = project total ÷ net operating income; it is unavailable if net operating income or project cost is zero. Yield is unavailable at zero project cost. Include expected expenses in your allowance; income taxes, appreciation and future changes are not modeled.

Loan principal = project total × financed percentage. For monthly interest rate r and number of monthly payments n, monthly principal and interest = loan principal × r ÷ (1 − (1 + r)⁻ⁿ). At 0% interest, payment = loan principal ÷ n. Cash portion = project total − loan principal. Monthly cash flow = net operating income ÷ 12 − monthly loan payment. Loan fees, construction-period interest, income taxes, major replacements and reserves are excluded.

Which official rules affect the assumptions?

California’s Government Code §66311.5 exempts ADUs of 750 sq ft of interior livable space or less from impact fees, with proportional fees above that threshold. Other charges can remain. Under §66313, a JADU is limited to 500 sq ft of interior livable space. The calculator flags larger California JADU scenarios.

Review the California HCD ADU Handbook and ask the local authority for applicable rules and fees. The calculator’s 200–1,800 sq ft input range does not establish that any size or configuration is permitted.

Which locations and activity records are included?

The 2025 U.S. Census Gazetteer supplies 32,058 places and 16,193 additional functioning county subdivisions: 48,251 location records across 50 states and Washington, D.C. These include cities, towns, townships and census-designated places (CDPs). The directory is not a list of issued ADU permits or an exhaustive address database; it excludes U.S. territories. New or renamed places after the dataset vintage may be missing.

We preserve Census names and identifiers, include functioning county subdivisions, and omit subdivisions with the same official name as a place in that state. County labels distinguish similarly named townships. CDPs are statistical areas and can be governed by a county. Select the state without a city for an unlisted or unincorporated property. Download provenance and source checksums.

Fifteen named cities map to the existing Los Angeles / Orange County, San Diego, Bay Area, Sacramento, Seattle, Portland, Denver or Austin presets. Other California locations use the California preset; remaining locations use the U.S. baseline. Preset choice is editable and separate from the selected jurisdiction. Geography supplies no cost multiplier. These mappings and the base rates are editorial judgments, not measured local prices.

The Massachusetts HLC tracker provides 351 municipality records covering 2025 and January–June 2026. The downloaded survey reports 2,084 approvals and 549 occupancy certificates. We show its application, approval and occupancy-certificate fields separately and retain missing values and each year’s reporting status. Some 2025 responses cover only midyear; the dataset is not a complete census of construction. A municipality’s records are not assigned to a similarly named CDP. Boston has separate zoning authority. Download the official survey snapshot (CSV) · Provenance.

Other locations currently show policy or program guidance where researched, or “not yet verified.” Missing activity data does not mean no ADUs are being built. State statutes and program pages establish neither actual local construction counts nor parcel approval.

• California HCD ADU Handbook ↗
• HCD annual housing progress dashboard ↗
• Massachusetts ADU guidance ↗
• Massachusetts ADU activity tracker ↗
• Washington ADU framework · RCW 36.70A.681 ↗
• Arizona ADU statute · A.R.S. 9-461.18 ↗
• Seattle ADU permit guide ↗
• Austin HOME amendments ↗
• NYC Plus One ADU program ↗

How does the model compare with a published builder example?

SnapADU’s cost page, updated March 2026 and checked September 8, 2026, gives the detached-ADU examples below. This is one San Diego builder’s published pricing, not a representative survey or a quote for your property. Its all-in scope includes design, permits, sitework and utilities; contingency treatment may differ from our model.

The model column uses standard finishes, one story, typical sitework, moderate utilities and 12% contingency. This comparison exposes differences; we have not calibrated every preset to this builder or validated the model against a representative market dataset. Obtain current property-specific quotes before a budget commitment. Open the current calculator →