How I’d actually use this on a new-build
New-build pencils differently from gut rehab. On a gut, the cost surprises hide behind walls. On a new-build, the cost surprises hide in the dirt. I run this calc twice: once before I make an offer on the lot (lot price + this output should fit comfortably inside ARV minus 20% for new-build profit margin), and once after the soils report comes back. The soils report is what tells you whether the foundation line is honest. If the borings come back with poor bearing capacity or expansive clay, override the foundation line up by 30–50% before re-running the math.
The owner-builder toggle is the question that gets asked most on new-build. The math says “save the GC markup,” but the calendar tells the truer story. Census 2024 puts the slowdown at 1.26x — three extra months on a new single-family build, or about 90 days of extra construction-loan interest at residential rates. On a $400K loan at 8.5%, that’s roughly $8,500 in extra interest before you save a single dollar on markup. The cost calc shows the dollar gap between GC and OB; the timeline calc next door shows the calendar gap. Run both before deciding.
On Chicago-metro new-builds I’ve walked, the lines that consistently surprise are site-prep (utility taps that turn out to be 80 feet from where the architect drew them, curb cuts that require a separate permit), foundation (rolling lots where the surveyor’s elevations and the engineer’s footing schedule don’t agree until concrete is being poured), and kitchen-bath finishes (counter-fab is a hard 2-week lead time regardless of how fast the install crew moves — order cabinets the day permits clear or eat the lead time at the wrong end of the project). The operator notes on those three lines reflect exactly that pattern.
Where the new-build budget actually breaks
Five places the new-build budget lies to operators who haven’t built before:
- Skipping the soils report.A $1,500–$2,500 soils investigation in due diligence beats a $25K helical-pier surprise after the foundation forms are up. The site-prep and foundation lines on this calc assume buildable bearing capacity on undisturbed soil — if the lot has fill, expansive clay, or high water table, the engine’s defaults are wrong by tens of thousands of dollars. Don’t close on the lot until the soils report is back and the foundation line has been overridden if needed.
- Utility tap surprises.The architect drew the water tap at 30 feet from the meter. The municipality’s actual main is 80 feet away, on the other side of a curb cut that requires a separate permit. That’s a $5–15K line item nobody quoted. Walk the lot with the utility company’s field map before bidding, not after.
- Permit jurisdiction calendar burn. A typical Chicago-metro new-build clears plans in 5–8 weeks. A strict jurisdiction with structural review departments routinely takes 12–16 weeks plus a third revision cycle. The construction loan starts ticking when the lot closes — not when the build starts. Three extra months of permit review on a $400K loan at 8.5% is $8,500 in dead interest before a single stud goes up. The cost calc widens the band on the permit + design lines when strict-jurisdiction is flagged; the timeline calc shows the calendar impact directly.
- Quality-tier creep on cabinets.Standard tier is IKEA + builder-grade plumbing fixtures. Mid is shaker semi-custom + spec faucets. High is full custom + slab counters. The mid → high jump on a 2000sqft build adds ~$30K without moving the resale comps unless you’re in a specific high-end submarket. Confirm comps support the spread before flipping the quality toggle to high.
- Counter-fab lead time stacked at the end.Counter template + fabrication is a 2-week lead time. On a project where everything else is on schedule, that 2 weeks shows up at the worst time — when you’re trying to call the final inspection. Order cabinets the day permits clear, template the counters the day cabinets land, and you eat the lead time when slack is cheap rather than expensive.
Methodology
Every dollar number on this page traces to one of three layers: NAHB 2024 Cost-of-Constructing-a-Home survey ($162.50/sqft average, used as a per-sqft sanity check), HomeAdvisor / Fixr / Angi cross-checks for line-level baselines, and Chicago-metro builder experience for the operator notes and confidence labels. The full per-cluster sourcing tier in methodology spells out which sources back which kinds of claims. The numbers are planning ranges, not contractor bids — the feasibility caveat is part of the engine output for a reason.
Show the formulas
- Per line:
perSqftMid × sqft × locationMult × complexityMult × qualityMult= engine mid. Override (if set) replaces the engine mid; band still derives from confidence. Rehab-only leak-flag deltas are n/a on new-build because new-construction line items don’t declare them. - Quality multiplier:standard 1.0× (rental / spec build) / mid 1.3× (move-in / owner) / high 1.7× (custom / luxury). Per-line response factor (full / partial / none) determines how much applies — finishes scale full; site-prep and foundation don’t scale.
- Location multiplier: low cost 0.85× (Midwest exurb) / mid 1.0× (Chicago metro anchor) / high 1.25× (HCOL coastal).
- Complexity multiplier: simple 0.92× / moderate 1.0× / complex 1.18×.
- Contingency:12% baseline on hard mid, capped at 22%. New-build doesn’t expose the rehab leak flags — but if a strict-permit-jurisdiction flag fires from the input row, contingency escalates +1.5% per active flag.
- GC overhead + profit:12.5% mid on hard mid (low 10% / high 15%). GC mode only — Owner-builder drops the line. Anchored on Chicago-metro residential GC industry typical, NOT NAHB’s 16.7% sale-price figure (which is a different calculation including builder profit on resale).
- NAHB 2024 anchor: $162.50/sqft average used as a sanity check on the per-sqft total. SiteworkMath default standard 2000sqft mid lands at ~$157/sqft, just below the NAHB average — defensible for a low-end spec build in Chicago metro.
- Engine logic:
lib/sitework/projectcost/projectcost.tswith line catalog inline-items-newbuild.tsand types intypes.ts. Tested inprojectcost.test.ts.
Frequently asked
What does this new construction cost calculator actually estimate?
A planning-range feasibility budget for a residential new-build single-family home — the dollar number an operator uses to decide whether the lot + build pencils before calling builders. Not a contractor bid. The engine sums 14 line items (design, permits, site-prep, foundation, framing, envelope, electrical, plumbing, HVAC, insulation/drywall, paint/flooring/tile, kitchen/bath, exterior/landscaping, appliances) plus computed contingency and (in GC mode) overhead. Numbers are SiteworkMath planning ranges anchored on NAHB 2024 Cost-of-Constructing-a-Home survey, HomeAdvisor / Fixr / Angi cross-checks, and Chicago-metro residential-builder experience.
How does this differ from the gut rehab cost calculator?
Same engine, different line-item baselines and framing. New-build adds a site-prep phase (excavation + utility tap + grading) where gut-rehab has demolition + disposal; new-build always includes foundation work where gut-rehab leaves it as an opt-in flag; and new-build's framing + envelope baselines reflect full-scope work from scratch instead of partial repair. The Rehab Leak Check property-flag panel is hidden on new-build (no existing structure means no buried-wall surprises) — the strict-permit-jurisdiction risk is captured in the existing input row instead. Default 2000-sqft standard / mid / moderate runs about $315K total mid (~$157/sqft), in line with NAHB 2024's $162.50/sqft average.
Why is NAHB used as a sanity check, not a constructive multiplier?
NAHB's annual Cost-of-Constructing-a-Home survey publishes one nationwide average ($162.50/sqft for 2024 — that's an average across surveyed builders, not a Census median) and an 8-stage breakdown of where the dollars land (site work / foundations / framing / exterior finishes / major systems / interior finishes / final steps / other). Those breakdown percentages are useful for sanity-checking a bottom-up estimate, but they're aggregated across regions and quality tiers, so plugging them into a Chicago-metro mid-quality calc as a multiplier would import noise from very different markets. SiteworkMath uses NAHB to confirm the per-sqft total looks sane (~$157 standard / ~$186 mid / ~$216 high at default location), not to derive it. NAHB's separate 16.7% builder profit-on-sale-price figure is a different calculation from SiteworkMath's GC overhead 10–15% on hard costs — see methodology for the disambiguation.
What's the difference between GC-managed and Owner-builder mode on a new-build?
GC-managed adds 10–15% on hard costs as overhead and profit (mid 12.5% — Chicago-metro residential GC industry typical). Owner-builder drops that line entirely. The trade is calendar: per Census 2024, owner-built single-family construction averages 15.2 months vs 12.1 months for contractor-built — a 1.26x slowdown. The construction timeline calculator surfaces the calendar delta directly; this calc only shows the dollar delta. Most operators who think they're saving the markup on a new-build are actually trading dollars for calendar — and the construction loan eats interest on every extra month.
What's NOT included in this calculator's output?
The lot. Site work assumes a buildable lot is already acquired and the title is clean — lot acquisition + closing costs + impact fees + any environmental remediation are scope for a separate budget. Specialty items (pool, accessory dwelling unit, detached garage at high-end finish, custom millwork above the kitchen-bath-finishes baseline) aren't captured by the 14-line catalog at this resolution; bump the kitchen-bath-finishes line via override or step up to a contractor walk-through for those. Contractor profit on the resale (NAHB's 16.7% on sale price) is not the same as GC overhead on construction — that's an investment-margin calc, not a feasibility calc.
How accurate is this calculator's output?
Planning range, not contractor bid. Intended use: to decide whether a lot + build pencils before paying for soils reports, architectural drawings, and a builder walk-through. Per the methodology, no calculator can be more accurate than the soil conditions and utility tap location an operator hasn't observed yet — that's why the contingency band starts at 12% and the operator notes flag specific surprise lines (site-prep, foundation, kitchen-bath-finishes). Tighten by overriding individual lines after a soils report comes back or after a preliminary builder consultation.
What I’d do next
- Pair the cost with the schedule — construction timeline calc
Same project decision has two halves: dollars and calendar. Run the timeline calc with matching inputs to surface the schedule horizon alongside this feasibility budget — and especially before flipping the OB toggle, where the calendar tradeoff is what the dollar savings really cost.
- Refine the foundation line — concrete yardage calc
Foundation is always in scope on new-build. Drop into the concrete calc to size the footings + foundation walls + slab order, then come back and override the foundation line.
- Read the planning-range methodology
What's anchored on NAHB 2024, what's a SiteworkMath calibration, and how the GC overhead vs NAHB sale-price-profit clarification works.
By James Wu. Planning-range methodology and per-cluster sourcing tiers in methodology. Per-sqft sanity-check anchor: NAHB 2024 Cost of Constructing a Home ($162.50/sqft average). Owner-builder slowdown anchored on US Census 2024 Survey of Construction (single-family build durations: contractor 12.1mo / owner-builder 15.2mo → 1.26x). NAHB 16.7% on sale price is referenced in methodology as a sanity check, NOT used as a constructive multiplier — GC overhead anchored on Chicago-metro residential industry typical 10–15% on hard costs. Engine logic in lib/sitework/projectcost/projectcost.ts; new-build catalog in line-items-newbuild.ts. Numbers are SiteworkMath planning ranges, not contractor bids — feasibility framing is part of the output for that reason. Not structural, financial, or legal advice. Full methodology.