{
  "slug": "cost-to-build-a-home-addition-calculator",
  "title": "Cost to Build a Home Addition Calculator",
  "heading": "Cost to Build a Home Addition Calculator",
  "category": "financial",
  "url": "https://www.revenuelab.fyi/toolbox/cost-to-build-a-home-addition-calculator",
  "summary": "Foundation, tie-in, and finish costs for a ground-floor room addition.",
  "description": "An addition costs more per square foot than new construction, and the reason is structural rather than cosmetic: you pay for a new foundation and roof on a small footprint, you pay a crew to open and re-tie the existing wall, and you pay for a week of protecting a house someone is living in. Kitchens and bathrooms inside an addition carry their own premium because of the plumbing and cabinetry, which is why one number per square foot never fits every addition. This calculator prices the shell per square foot with a regional index, then adds foundation type, the wall tie-in, HVAC extension, and an optional wet-room premium. Worked assumption for the defaults: a 400 sq ft family-room addition at $260/sq ft on a crawl space, $9,000 tie-in, $6,500 HVAC extension, no plumbing, 15% contingency — about $155,000 in a national-average market and roughly $205,000 on the West Coast.",
  "formula": "Total = (area × shell rate × regional index + foundation + tie-in + HVAC + wet-room premium) × (1 + contingency).",
  "dateModified": "2026-09-06",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=cost-to-build-a-home-addition-calculator",
  "inputs": [
    {
      "id": "area",
      "label": "Addition footprint",
      "kind": "number",
      "hint": null,
      "default": 400,
      "unit": "sq ft",
      "min": 40,
      "max": 3000
    },
    {
      "id": "shellRate",
      "label": "Shell and finish rate",
      "kind": "number",
      "hint": null,
      "default": 260,
      "unit": "$/sq ft",
      "min": 120,
      "max": 800
    },
    {
      "id": "region",
      "label": "Region",
      "kind": "select",
      "hint": "Applies a regional labor-and-bid multiplier to the whole job.",
      "default": "1",
      "options": [
        {
          "value": "1",
          "label": "US national average — 1.00×"
        },
        {
          "value": "0.86",
          "label": "Rural / small town — 0.86×"
        },
        {
          "value": "0.94",
          "label": "South metro (Atlanta, Dallas, Tampa) — 0.94×"
        },
        {
          "value": "1.02",
          "label": "Midwest metro (Columbus, KC, Minneapolis) — 1.02×"
        },
        {
          "value": "1.14",
          "label": "Mountain West (Denver, Phoenix, Salt Lake) — 1.14×"
        },
        {
          "value": "1.28",
          "label": "Northeast metro (Boston, Philly, DC) — 1.28×"
        },
        {
          "value": "1.38",
          "label": "West Coast metro (Seattle, LA, Portland) — 1.38×"
        },
        {
          "value": "1.55",
          "label": "Highest-cost metro (SF Bay, Manhattan) — 1.55×"
        }
      ]
    },
    {
      "id": "foundation",
      "label": "Foundation type",
      "kind": "select",
      "hint": null,
      "default": "38",
      "options": [
        {
          "value": "22",
          "label": "Slab on grade — $22/sq ft"
        },
        {
          "value": "38",
          "label": "Crawl space — $38/sq ft"
        },
        {
          "value": "62",
          "label": "Full basement — $62/sq ft"
        }
      ]
    },
    {
      "id": "tieIn",
      "label": "Structural tie-in to existing wall/roof",
      "kind": "number",
      "hint": null,
      "default": 9000,
      "unit": "$",
      "min": 0,
      "max": 80000
    },
    {
      "id": "hvac",
      "label": "HVAC extension or mini-split",
      "kind": "number",
      "hint": null,
      "default": 6500,
      "unit": "$",
      "min": 0,
      "max": 40000
    },
    {
      "id": "wetRooms",
      "label": "Bathrooms or kitchens inside the addition",
      "kind": "number",
      "hint": null,
      "default": 0,
      "unit": "rooms",
      "min": 0,
      "max": 4
    },
    {
      "id": "wetPremium",
      "label": "Cost per wet room",
      "kind": "number",
      "hint": null,
      "default": 24000,
      "unit": "$",
      "min": 0,
      "max": 120000
    },
    {
      "id": "contingency",
      "label": "Contingency",
      "kind": "number",
      "hint": null,
      "default": 15,
      "unit": "%",
      "min": 0,
      "max": 35
    }
  ],
  "outputs": [
    {
      "id": "total",
      "label": "Estimated addition cost",
      "format": "currency",
      "hint": null,
      "primary": true
    },
    {
      "id": "perSqFt",
      "label": "Cost per square foot",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "shell",
      "label": "Shell and finishes",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "foundation",
      "label": "Foundation",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "wet",
      "label": "Bathroom / kitchen premium",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "reserve",
      "label": "Contingency reserve",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "low",
      "label": "Low end of band",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "high",
      "label": "High end of band",
      "format": "currency",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Addition footprint: 400 sq ft",
      "Shell and finish rate: 260 $/sq ft",
      "Region: US national average — 1.00×",
      "Foundation type: Crawl space — $38/sq ft",
      "Structural tie-in to existing wall/roof: 9000 $",
      "HVAC extension or mini-split: 6500 $",
      "Bathrooms or kitchens inside the addition: 0 rooms",
      "Cost per wet room: 24000 $",
      "Contingency: 15 %"
    ],
    "outputs": [
      "Estimated addition cost: $154,905",
      "Cost per square foot: $387",
      "Shell and finishes: $104,000",
      "Foundation: $15,200",
      "Bathroom / kitchen premium: $0",
      "Contingency reserve: $20,205",
      "Low end of band: $136,316",
      "High end of band: $185,886"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter addition footprint (sq ft).",
      "Enter shell and finish rate ($/sq ft).",
      "Enter region — Applies a regional labor-and-bid multiplier to the whole job..",
      "Enter foundation type.",
      "Enter structural tie-in to existing wall/roof ($).",
      "Enter hvac extension or mini-split ($).",
      "Enter bathrooms or kitchens inside the addition (rooms).",
      "Enter cost per wet room ($).",
      "Enter contingency (%).",
      "Read your estimated addition 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,
        "shellRate": 160,
        "region": "1",
        "foundation": "38",
        "tieIn": 5500,
        "hvac": 4000,
        "wetRooms": 0,
        "wetPremium": 14000,
        "contingency": 9
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "area": 400,
        "shellRate": 260,
        "region": "1",
        "foundation": "38",
        "tieIn": 9000,
        "hvac": 6500,
        "wetRooms": 0,
        "wetPremium": 24000,
        "contingency": 15
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "area": 640,
        "shellRate": 420,
        "region": "1",
        "foundation": "38",
        "tieIn": 14500,
        "hvac": 10500,
        "wetRooms": 0,
        "wetPremium": 38000,
        "contingency": 24
      }
    }
  ],
  "limitations": [
    "Results are estimates before tax, fees, and inflation unless an input explicitly covers them.",
    "Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.",
    "This is educational maths, not financial advice. Check anything contractual with the lender or your accountant."
  ],
  "faq": [
    {
      "q": "Where does the regional multiplier come from?",
      "a": "It is a job-level index against a US national average of 1.00, built from the spread contractors quote between low-cost rural markets and the highest-cost coastal metros. Materials vary less than labor, but bids move together, so one index tracks real quotes better than splitting them. Swap in your own multiplier by picking the tier closest to two bids you already have."
    },
    {
      "q": "Why is an addition pricier per square foot than a new house?",
      "a": "Fixed costs land on a tiny footprint. A 400 sq ft addition needs its own foundation, roof, and crew mobilisation, and the crew works around a family living in the house. Doubling the size rarely doubles the price."
    },
    {
      "q": "Is building up cheaper than building out?",
      "a": "Sometimes — you skip the foundation but pay for structural reinforcement, stairs, and often a full re-roof. Run the second-story tool and compare the two totals for your own house."
    },
    {
      "q": "Why does the tool add a contingency?",
      "a": "Because renovation scopes change once walls are open. On finished-space work 10–15% is normal; on anything touching foundations, sewer lines, or 1970s-and-older wiring, 20% is the honest number. If your contingency is never spent, you finished under budget — that is the point."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/cost-to-add-a-second-story-calculator",
    "https://www.revenuelab.fyi/toolbox/cost-to-build-a-house-calculator"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Cost to Build a Home Addition Calculator (https://www.revenuelab.fyi/toolbox/cost-to-build-a-home-addition-calculator)"
}