How I’d actually use this on a flip
On a Chicago-metro deal, I run this calc twice. First pass before I ever see the property: square footage from the listing, standard quality, mid location, moderate complexity, no flags raised. That gives me a clean-property budget. If that number is already past what the comps support, I don’t book the showing. The math saved me a Saturday.
Second pass is during the walkthrough. Phone in hand, working through the basement and the panel and the bathrooms with the seven leak-check flags as a checklist. A 1920s Chicago bungalow with original knob-and-tube branch wiring and galvanized supply lines flips three flags and rewrites the bottom number by twenty to forty thousand dollars before I’ve even looked at the kitchen. That’s the difference between an offer that pencils and an offer that doesn’t — and the math is doing the work my gut would otherwise miss in the rush of trying to get the listing before the next call.
The Owner-builder toggle is for the deals where I’m running the subs myself. It drops the GC overhead line — but I keep the contingency wide because owner-builder mistakes show up in change orders, not in the original quote. The Census 2024 numbers anchor the 1.26x calendar slowdown; that’s a timeline calc story (Cycle 2 Week 2), but the cost calc is the half of the decision that owner-builders skip when they think they’re saving money on markup. They aren’t — they’re trading dollars for calendar.
Rehab Leak Check — where the budget actually breaks
Five gotchas the calc surfaces through the leak-check flags, with the per-line sensitivities the engine actually applies:
- Pre-1980 home, no other flags.The pre-1980 flag is a broad older-home heuristic — asbestos-material risk in floor tile, mastic, popcorn ceilings, and pipe lagging — and that drives the engine’s 25% bump on demo plus 5–15% across MEP and paint/flooring. Separated disposal of suspect materials runs $40–80/cu yd higher than standard C&D in most Chicago-suburb landfills. The protocol depends on testing the actual material, not just the year built. Lead-based paint and the EPA RRP rule are a SEPARATE pre-1978 trigger — different threshold, different certification (RRP-certified contractor for any disturbance of painted surfaces). If a property has original painted surfaces, test before disturbing them regardless of what the leak-check flag says about year.
- Knob-and-tube wiring.Always a full rewire and a 200A service upgrade — partials get red-tagged at the rough-in inspection on every Chicago-metro jurisdiction I’ve worked in. Engine raises electrical by 35%. The trap is when a homeowner insurance carrier spots K&T after closing and gives you 30 days to remediate or lose coverage; that timeline collides with the drywall hang gate and the project slides four to six weeks.
- Visible water damage.The dollar number is the small part. The pattern is: wherever the visible damage is, the framing behind it is wetter than you think, and the insulation is mold-incubated. Engine raises framing by 20% and insulation/drywall by 15% — but the contingency widens further because mold remediation isn’t a calc-friendly line item. If the inspector finds active moisture, walk away from the deal until you have a moisture-damage scope priced separately.
- Foundation cracks.Engine raises the foundation line by 50%. Cosmetic settling cracks (hairline, vertical, no offset) document and continue. Active settlement (offset of more than 1/8", doors that won’t close, fresh sheetrock cracks radiating from the corner) is a structural-engineer call before you bid — not a contingency line. The calc band widens, but a real settlement issue is a walk-away signal, not a budgeting problem.
- Strict permit jurisdiction.Engine raises permits by 40% and design by 25%. The dollar number isn’t the issue; the calendar slip from the third plan revision is. Some Chicago-metro jurisdictions move plans through in two weeks; others take twelve. The cost calc widens the band when the flag is set — but the timeline calc (Cycle 2 Week 2) is where this flag actually earns its keep. For now, treat a strict-jurisdiction flag as a heads-up that the construction-loan term may need to be longer.
Methodology
Every dollar number on this page traces to one of three layers: SiteworkMath planning ranges (anchored on HomeAdvisor / Fixr / Angi cross-checks plus flipper experience), Assembly Service IL Chicago regional data point, and Census 2024 build-duration anchors for the owner-builder slowdown. 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 × (1 + Σ leak-flag deltas)= engine mid. Override (if set) replaces the engine mid; band still derives from confidence. - Quality multiplier:standard 1.0× / mid 1.3× / high 1.7×. Per-line response factor (full / partial / none) determines how much of the multiplier applies — finishes scale full; demo and structural 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, +1.5% per active leak-check flag, capped at 22%. Band: low ±2%, high +3%.
- 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).
- Confidence band: high ±10% / medium ±18% / rough ±32%. Each line additionally widens by ±4% per active flag that affects it.
- Engine logic:
lib/sitework/projectcost/projectcost.tswith line catalog inline-items.tsand types intypes.ts. Tested inprojectcost.test.ts.
Frequently asked
What does this gut rehab cost calculator actually estimate?
A planning-range feasibility budget for a residential gut rehab — the dollar number an operator uses to decide walk, bid, or kill before calling contractors. Not a contractor bid. The engine sums 14 line items (design, permits, demo, foundation, framing, envelope, electrical, plumbing, HVAC, insulation/drywall, paint/flooring/tile, kitchen/bath, exterior, appliances) plus computed contingency and (in GC mode) overhead. Numbers are SiteworkMath planning ranges anchored on Assembly Service IL Chicago data, HomeAdvisor / Fixr / Angi cross-checks, and flipper experience.
Why is the output a range instead of a single number?
Because every honest gut-rehab estimate is a range, and a single number on a feasibility decision is false confidence. Each line carries a confidence label (high / medium / rough); high-confidence lines like appliances run a tighter ±10% band, and rough-confidence lines like kitchen-and-bath finishes run a wider ±32% band. Active Rehab Leak Check flags widen the bands further on the lines they affect. The total is what you'd actually need liquid + reserve for, not a marketing-grade single dollar figure.
What is the Rehab Leak Check?
Seven property-specific risk-flag inputs that modulate the contingency band and raise specific line items: pre-1980 home (broad older-home asbestos-material and dump-fee heuristic — lead-based paint and EPA RRP are a separate pre-1978 trigger), visible water damage, galvanized plumbing, knob-and-tube wiring, foundation cracks, bad subfloor visible, and strict permit jurisdiction. Each flag carries documented per-line sensitivities — knob-and-tube raises electrical by 35% and adds to demo and insulation; foundation cracks raise foundation by 50% and add to framing. The Contingency Drivers list under the budget summary shows exactly which flags moved the band, so the methodology is transparent rather than a black box.
What's the difference between GC-managed and Owner-builder mode?
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 cost calc shows the dollar delta; the timeline calc (shipping in Cycle 2 Week 2) will surface the calendar delta. NAHB's 16.7% figure is on SALE PRICE, a different calculation that includes builder profit on resale, not the construction markup.
How accurate is this calculator's output?
This is a planning range, not a contractor bid. Intended use: to decide whether a property pencils before paying for inspections and a contractor walk-through. Per the methodology, no calculator can be more accurate than the line items the operator hasn't observed yet — that's exactly what the Rehab Leak Check exists to surface as inputs and what the contingency band exists to protect against. Tighten by overriding individual lines after dropping into a detail calc (concrete yardage for foundation; tile calc for paint/flooring/tile; topsoil/mulch/deck for exterior).
Why doesn't the calculator have a ZIP code lookup?
The location-tier dropdown — low cost (Midwest exurb) / mid (Chicago metro) / high (coastal HCOL) — captures most of the regional variation without claiming ZIP-precision the underlying data doesn't support. ZIP-precision aggregator calculators imply a level of localized accuracy that comes from a black-box adjustment factor, not transparent methodology. SiteworkMath publishes the multipliers (low 0.85x, mid 1.0x, high 1.25x) and the Assembly Service IL Chicago anchor in the methodology section so operators can check the math.
What I’d do next
- Refine the foundation line — concrete yardage calc
If foundation work is in scope, drop into the concrete calc to tighten the slab / footing / pier numbers, then come back and override the foundation line.
- Refine the paint/flooring/tile line — tile calc
Tile is the visible-finish bottleneck in most kitchens and baths. The tile calc surfaces waste-factor adjusted box counts; override paint/flooring/tile after.
- Read the planning-range methodology
What’s anchored, what’s a planning range, and how the Assembly Service IL data point and NAHB sanity-check fit in.
Also in this cluster
By James Wu. Planning-range methodology and per-cluster sourcing tiers in methodology. Regional anchor: Assembly Service IL Chicago published gut-rehab line-item percentages and $100–300/sqft band. 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. 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.