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Calculation report
A focused summary of the selected scope, active cost lines and estimate. Changes are saved automatically in this browser.
Project and quote notes are included in shared links. Leave out private client details. Long projects may exceed the share-link limit.
Where the budget goes
| Budget component | Low | Typical | High |
|---|
Itemized cost ranges
| Included scope | Quantity | Combined factor | Low | Typical | High | Price basis |
|---|
All rates and factors are visible under Advanced. Category totals use unrounded arithmetic; individually displayed currency amounts can differ from their rounded total by a small amount.
Compare up to three addition budgets
Keep the current scenario, change the size or scope, and compare its range, cost per area and typical trade breakdown. Scenarios remain in this tab only; export projects for long-term storage.
Worked example: a 400 ft² bedroom addition
This is the calculator’s illustrative US-dollar scenario, not a published contractor quote: one story, standard finishes, 8 ft ceiling, 1× local factor, four windows and one exterior door. It includes a slab, framing, standard services, design/engineering and permit allowances. It does not add a bathroom.
The engine gives direct construction of $88,800 in the typical scenario. Add 15% contractor markup ($13,320) and $9,300 soft costs. With no additional tax and 15% contingency ($16,713), the typical planning total is $128,133, or $320/ft². The selected low/high rates produce $76,915–$206,710.
For the slab line alone: 400 ft² × $30/ft² × 1 local factor = $12,000 typical. For roofing: 400 × 1.15 = 460 ft² allowance area, then 460 × $20 = $9,200. These are different quantity bases—not a second charge for extra stories.
Inputs explained — what changes the answer?
- Area and stories: floor-based costs follow total floor area; foundation/site costs follow the footprint. Dimensions mode multiplies length × width × stories.
- Finish level: the displayed factor affects only checked lines. A kitchen also needs plumbing, cabinetry and electrical scope; the finish multiplier alone does not create those items.
- Foundation and structural tie-in: choose the actual scope and replace placeholder rates. Existing foundations are not automatically adequate for added loads.
- Ceiling and roof geometry: these are editable allowance factors, not a detailed materials takeoff. Set a quoted lump sum when that better matches the job.
- Region: label the project and supply local rates or a manual factor. No region silently changes the result.
- Low / typical / high: enter a defensible range for each scope. The model sums corresponding bands; the result is not a probability interval.
Addition example library
| Example type | Total ft² | Low | Typical | High |
|---|---|---|---|---|
| Bump-out | 120 | $33,717 | $60,104 | $99,556 |
| Bedroom | 400 | $76,915 | $128,133 | $206,710 |
| Family / living room | 400 | $76,915 | $128,133 | $206,710 |
| Home office | 400 | $76,915 | $128,133 | $206,710 |
| Bathroom | 100 | $39,436 | $70,737 | $121,092 |
| Kitchen expansion | 200 | $63,479 | $111,999 | $189,571 |
| Primary suite | 600 | $121,299 | $201,664 | $326,608 |
| 3-season sunroom | 240 | $52,571 | $90,521 | $148,086 |
| Conditioned / 4-season sunroom | 240 | $57,755 | $100,255 | $163,692 |
| Attached garage | 400 | $55,888 | $92,426 | $149,578 |
| Garage conversion | 400 | $39,462 | $74,307 | $130,376 |
| ADU / in-law suite — INCOMPLETE known-cost subtotal | 600 | $140,078 | $238,033 | $389,427 |
| Over-garage addition | 400 | $89,321 | $157,625 | $266,064 |
| Second-story addition | 800 | $143,411 | $244,249 | $402,546 |
| Custom conditioned space | 400 | $76,915 | $128,133 | $206,710 |
The ADU example needs a manually priced utility-connection allowance; its displayed subtotal is incomplete. Different example sizes mean this is not an apples-to-apples type comparison. Use the scenario feature with the same size and matching scope for that comparison. Small wet-room or unusual structural work can fall outside the examples.
Formula & methodology
- Total area = entered area, or length × width × stories. Footprint = total area ÷ equal-sized stories. Roof allowance area = footprint × roof factor.
- Each cost line = its quantity × selected band’s rate × any enabled local, finish and ceiling-height factors.
- Direct construction = sum of included construction lines. Soft costs = sum of included design, fee and other soft-cost lines.
- OH&P = direct construction × markup %. Tax = manually entered taxable amount × tax %.
- Contingency = (direct + OH&P + soft costs + tax) × contingency %. Total = that subtotal + contingency.
- Cost per floor area = complete project total ÷ total added/converted floor area.
OH&P here is a markup on cost, not profit margin on selling price. No fixed materials/labor split, automatic financing cost, home-value gain or inflation adjustment is applied.
Common budgeting mistakes
- Entering total two-story floor area and then multiplying it by two again.
- Adding contractor markup to a quote that already includes it, or charging permit inspections twice.
- Using a bathroom plumbing allowance without including the vanity, tile/waterproofing or other required finishes somewhere in the budget.
- Using new-build square-foot rates as a guaranteed small-addition quote. Mobilization, existing-building repairs and structural connections can dominate.
- Leaving utility connections, soil work, surveys or hazardous-material work excluded without verifying they are unnecessary.
- Changing currency labels or broad region and assuming the prices were automatically converted or localized.
Assumptions, exclusions & sources
Use this as a planning worksheet, not a contractor quote, appraisal, tax calculation, permit determination or structural design. Foundation capacity, beams, soil conditions, zoning, energy rules and service capacity need project-specific review. All-in quotes should replace overlapping lines, not be added on top.
Sources reviewed 2026-09-02. The references explain context and where to obtain better project-specific data; they do not supply or endorse this calculator’s numerical example rates.
- NAHB: Cost of Constructing a Home in 2024 (2025 publication) — New-home survey context only; not used as addition prices or a local cost index.
- City of Portland: building permit estimates — Example of obtaining project-specific permit and trade fees. Not a national fee schedule.
- NAR / NARI: Remodeling Impact — Project-scope and cost-recovery context. No resale return is predicted by this calculator.
No geographic rate dataset was available to substantiate state/ZIP precision. User-entered dated quotes should replace the illustrative values. No land purchase, financing, resale uplift, temporary accommodation, future property tax or unentered work is included.