How do you estimate the cost of a home addition?
Measure the proposed footprint, enter the number of equal-area floors and obtain comparable construction quotes. The worksheet separates base construction from excluded work, design fees, permits, temporary accommodation, tax and contingency. An optional fixed-rate loan estimate is displayed separately from construction cost.
How is total addition floor area calculated?
Area equals footprint length × width × equal-area floor count. Use the quote’s stated area basis, which may differ between gross and usable floor area. Unequal floors or irregular footprints need separate calculations. The calculator does not assume the structure or foundation can support another floor.
What does shell versus turnkey scope change?
The selected scope labels the quote; it does not add a hidden price multiplier. A shell quote may exclude finishes and services, while a turnkey quote may include them, but the actual contract controls. Add only work excluded from that base quote. Verify the same inclusions before comparing low and high rates.
Construction range = footprint × equal floors × entered area quote + separately scoped construction extras; budget adds design fee, permits, holding, tax and contingency
What does a twenty-by-fifteen-foot addition example show?
A single 20 × 15 ft floor is 300 sq ft. At illustrative entered construction quotes of 150–200 per sq ft, base construction is 45,000–60,000. Separate foundation, tie-in, services, design and permit items are not included unless already covered by the quote.
How are excluded construction items and fees added?
Separate construction totals can include site work, structural tie-in, HVAC, electrical, plumbing and finishes. Design percentage and construction tax each apply to base construction plus those extras. Permit or survey totals and temporary accommodation are then added, before the selected contingency is applied.
How should temporary accommodation be included?
Enter months and monthly cost together. The product is a separate project subtotal. It does not model delays or predict build duration. If accommodation is not needed, leave both blank. For a phased project, use a realistic quoted schedule rather than treating the calculator as a scheduling service.
What does the fixed-rate loan estimate include?
Financing uses the entered principal, nominal annual interest rate and whole-year term for equal monthly principal-and-interest payments. At zero nominal annual interest rate, payment is principal divided by months. At positive nominal annual interest rate, payment uses the standard fixed-rate amortization formula. Fees, insurance, draw schedules and rate changes are excluded; this is payment arithmetic, not a financing offer.
Use the contractual nominal interest rate rather than an APR that includes fees. Monthly payment = principal × r/[1 − (1+r)−n], where r is the monthly rate and n is payment count. The zero-rate case uses principal/n.
Which input units and assumptions should you check?
Use the labeled units and measured project values. Optional fields remain blank unless their data are available; paired fields must be entered together. A scoped quote worksheet, not a structural or permit design. Equal-floor area and entered local quotes determine the range; existing-house repairs and exclusions must be identified separately.
| Input | Unit basis | When used |
|---|---|---|
| Addition footprint length | ft / m | all modes |
| Addition footprint width | ft / m | all modes |
| Number of equal-area floors | as labeled | all modes |
| Lower construction quote per selected area unit | as labeled | all modes |
| Upper construction quote per selected area unit | as labeled | all modes |
| Site work / foundation not in base quote (optional) | as labeled | all modes |
| Structural tie-in not in base quote (optional) | as labeled | all modes |
| HVAC not in base quote (optional) | as labeled | all modes |
| Electrical not in base quote (optional) | as labeled | all modes |
| Plumbing not in base quote (optional) | as labeled | all modes |
| Finishes not in base quote (optional) | as labeled | all modes |
| Design fee on construction subtotal (%) (optional) | as labeled | all modes |
| Total permit / survey quote (optional) | as labeled | all modes |
| Temporary accommodation months (optional) | as labeled | all modes |
| Monthly temporary accommodation cost (optional) | as labeled | all modes |
| Tax on construction subtotal (%) (optional) | as labeled | all modes |
| Contingency (%) (optional) | as labeled | all modes |
| Loan principal (optional) | as labeled | all modes |
| Loan nominal annual interest rate (%) | as labeled | finance:yes |
| Loan term (whole years) | as labeled | finance:yes |
How can you compare and save different scenarios?
Calculate and save a baseline to comparison, change one input and calculate again. Review the itemized results before transferring values. Copy or share the result, download the input and result sheet, or print the worksheet. Clear changes removes the current inputs and results, and refresh starts clean.
Which related calculators help with the next step?
Use Concrete Slab Cost Calculator and Deck Cost Calculator. Check that the measurement units and scope match before using a result as an input elsewhere.
