{
  "slug": "400-sq-ft-adu-calculator",
  "title": "400 Sq Ft ADU Calculator",
  "heading": "400 Sq Ft ADU Calculator",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/400-sq-ft-adu-calculator",
  "summary": "Rough construction cost estimate for a 400 sq ft accessory dwelling unit.",
  "description": "A 400 sq ft ADU (accessory dwelling unit) is a common size for a backyard studio or in-law suite. Construction costs vary widely by region and finish level, so this calculator lets you set your own price per square foot and returns a total estimated build cost along with a breakdown between shell and finish work using an editable split.",
  "formula": "Total Cost = Area × Price per Sq Ft. Shell Cost = Total × Shell%. Finish Cost = Total × (1 - Shell%).",
  "dateModified": "2026-09-06",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=400-sq-ft-adu-calculator",
  "inputs": [
    {
      "id": "area",
      "label": "ADU area",
      "kind": "number",
      "hint": null,
      "default": 400,
      "unit": "sq ft",
      "min": 100,
      "max": null
    },
    {
      "id": "pricePerSqFt",
      "label": "Price per sq ft",
      "kind": "number",
      "hint": null,
      "default": 300,
      "unit": "$",
      "min": 50,
      "max": null
    },
    {
      "id": "shellPercent",
      "label": "Shell (structure) share of cost",
      "kind": "number",
      "hint": null,
      "default": 55,
      "unit": "%",
      "min": 20,
      "max": 80
    }
  ],
  "outputs": [
    {
      "id": "totalCost",
      "label": "Estimated total build cost",
      "format": "currency",
      "hint": null,
      "primary": true
    },
    {
      "id": "shellCost",
      "label": "Shell/structure cost",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "finishCost",
      "label": "Interior finish cost",
      "format": "currency",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "ADU area: 400 sq ft",
      "Price per sq ft: 300 $",
      "Shell (structure) share of cost: 55 %"
    ],
    "outputs": [
      "Estimated total build cost: $120,000",
      "Shell/structure cost: $66,000",
      "Interior finish cost: $54,000"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter adu area (sq ft).",
      "Enter price per sq ft ($).",
      "Enter shell (structure) share of cost (%).",
      "Read your estimated total build cost on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "area": 240,
        "pricePerSqFt": 180,
        "shellPercent": 35
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "area": 400,
        "pricePerSqFt": 300,
        "shellPercent": 55
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "area": 640,
        "pricePerSqFt": 480,
        "shellPercent": 80
      }
    }
  ],
  "limitations": [
    "Estimates assume standard, average conditions — local rules, pricing, and materials vary.",
    "Results are rounded for readability; add a buffer before ordering, booking, or committing.",
    "Double-check anything with a real cost attached against a local quote."
  ],
  "faq": [
    {
      "q": "How much does a 400 sq ft ADU cost to build?",
      "a": "Costs vary heavily by region and finish quality, commonly ranging from $150 to $500+ per square foot — adjust the price field to a local contractor quote for accuracy."
    },
    {
      "q": "What's included in 'shell' cost?",
      "a": "Shell typically covers foundation, framing, roofing, exterior siding, windows, and rough plumbing/electrical, while finish covers drywall, flooring, cabinets, fixtures, and paint."
    },
    {
      "q": "Does this include permits and site work?",
      "a": "No, permits, utility hookups, and site prep can add significant cost on top of this estimate and vary greatly by jurisdiction."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/500-sq-ft-basement-finish-calculator",
    "https://www.revenuelab.fyi/toolbox/24x24-garage-calculator"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — 400 Sq Ft ADU Calculator (https://www.revenuelab.fyi/toolbox/400-sq-ft-adu-calculator)"
}