How We Calculate Your Estimate
Full transparency on every number. Here's the exact formula, data sources, and update process behind your estimate.
The formula
// Step 1: Base cost
base_mid = square_footage × base_cost_per_sqft[addition_type]
// Step 2: Apply quality and regional adjustments
adjusted = base_mid × quality_multiplier × regional_multiplier
// Step 3: Add features
mid = adjusted + sum(selected_feature_costs × regional_multiplier)
// Step 4: Calculate range
low = mid × 0.85
high = mid × 1.20
All values round to the nearest $500. The range (85%–120% of midpoint) reflects real-world contractor quote variance. It's not a hedge — it's honest. Bids on identical projects regularly vary by 20–30%.
Base costs per square foot (2026)
Source: RS Means Building Construction Cost Data 2025, calibrated to 2026 BLS PPI. Assumes mid-range quality, national average labor market.
| Addition Type | Base/sq ft (mid) | Example: 300 sq ft |
|---|---|---|
| Bedroom | $115 | $34,500 |
| Bathroom | $195 | $58,500 |
| Sunroom | $85 | $25,500 |
| Garage | $65 | $19,500 |
| Second Story | $175 | $52,500 |
| Kitchen Extension | $210 | $63,000 |
| Family Room | $120 | $36,000 |
| Custom | $130 | $39,000 |
Regional multipliers
Source: RS Means City Cost Index 2025. Your ZIP code maps to one of seven regions. We use the 3-digit prefix to determine state, then state to region.
| Region | States | Multiplier | Difference |
|---|---|---|---|
| Pacific Coast | CA, WA, OR, AK, HI | 1.35× | +35% vs national |
| Northeast | MA, CT, NY, ME, NH, VT, RI | 1.28× | +28% vs national |
| Mid-Atlantic | NY, NJ, PA, MD, VA, DC | 1.18× | +18% vs national |
| Mountain West | CO, UT, AZ, NV, ID, MT, WY | 1.02× | +2% vs national |
| Midwest | IL, OH, MI, IN, WI, MN, IA | 0.97× | -3% vs national |
| Southeast | FL, GA, NC, SC, TN, AL, MS | 0.92× | -8% vs national |
| South Central | TX, OK, LA | 0.88× | -12% vs national |
Unknown or missing ZIP defaults to national average (1.00×).
Quality multipliers
Source: NAHB Cost of Constructing a Home 2024 finish-tier analysis.
| Tier | Multiplier | Typical finishes |
|---|---|---|
| Budget | 0.70× | Vinyl flooring, stock cabinets, standard fixtures |
| Mid-Range | 1.00× | LVP or hardwood, semi-custom cabinets, quality fixtures |
| High-End | 1.40× | Solid hardwood, custom cabinetry, designer fixtures |
| Luxury | 2.00× | Marble, fully custom millwork, smart home systems |
Update schedule
We update every cost figure four times per year: January, April, July, and October. Each update cross-references the BLS Producer Price Index for construction materials against our RS Means base. If materials moved more than 3% since the last update, we adjust.
The current data reflects 2026 Q1 pricing. Next update: April 2026.
Known limitations
- 1.Metro areas within a state can vary 15–25% from the state average. We use state-level regional assignments, so dense urban markets may be understated.
- 2.Site-specific conditions (soil type, slope, existing foundation integrity) can add significant cost that no calculator can predict without a site visit.
- 3.Specialty labor shortages in your market may push specific trade costs above our model. Always get quotes.
- 4.Permit fees are rough estimates. Your actual municipality fee schedule may differ significantly.