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 story for the construction timeline calculator, 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. When the electrical line gets that big, drop into the electrical rewire cost calculator to tighten the scope-promotion logic and the AFCI 210.12 cascade specifics before overriding the electrical line on this calc.
- 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 construction timeline calculator 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-calculator 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 you'd use to decide walk, bid, or kill before calling contractors. Not a contractor bid. What 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. In GC-managed mode, GC overhead and profit also gets a line. Where the numbers come from: Planning ranges anchored on Chicago-area residential cost data and cross-checked against the national cost-guide aggregators, with operator flipper experience overlaid on the high-risk lines. Methodology section below the FAQ lists the specific anchors.
Why is the output a range instead of a single number?
Because every honest gut-rehab estimate is a range. A single number on a feasibility decision is false confidence. How the bands work: Each line carries a confidence label. High-confidence lines (appliances, permits) run a tighter ±10% band. Rough-confidence lines (kitchen/bath finishes, exterior) run a wider ±32% band. What widens the bands further: Active Rehab Leak Check flags raise the per-line band on every line they affect. What the total represents: The dollar amount you'd actually need liquid plus 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 based on what you can see at the walk-through. The seven flags: • Pre-1980 home — older-home asbestos-material and dump-fee heuristic. (Lead-based paint is a separate pre-1978 issue.) • Visible water damage. • Galvanized plumbing. • Knob-and-tube wiring. • Foundation cracks. • Bad subfloor visible. • Strict permit jurisdiction. What each flag does: Documented per-line sensitivities. Example: knob-and-tube raises the electrical line by 35% and adds to demo and insulation. Foundation cracks raise foundation by 50% and add to framing. Where this surfaces: The Contingency Drivers list under the budget summary shows exactly which flags moved the band — the methodology is transparent, not a black box.
What's the difference between GC-managed and Owner-builder mode?
GC-managed adds 10-15% overhead-and-profit on hard costs (12.5% mid). Owner-builder drops that line entirely but adds calendar time. The dollar delta: 12.5% mid in GC mode is the Chicago-metro residential general-contractor norm. The calendar delta: Owner-built single-family construction averages 15.2 months vs 12.1 months for contractor-built — a 1.26× slowdown per the most recent US Census Survey of Construction data. Where each delta shows up: This calc surfaces the dollar delta. The construction timeline calculator surfaces the calendar delta. The 16.7% number people quote: That comes from a national builder survey and is calculated on sale price (includes builder profit on resale), not the construction markup. It's a different number for a different question.
How accurate is this calculator's output?
This is a planning range, not a contractor bid. Intended for the walk / bid / kill decision before you pay for inspections and contractor walk-throughs. The accuracy ceiling: 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. How to tighten the result: Override individual lines by dropping into the detail calcs — concrete yardage for foundation, tile calc for paint/flooring/tile, topsoil/mulch/deck/gravel for exterior, electrical-rewire and plumbing-repipe for those trades. When to leave the calc and call contractors: When the property pencils at the high band. Otherwise walk before paying for bids.
Why doesn't the calculator have a ZIP code lookup?
Because ZIP precision implies an accuracy the underlying data doesn't support. The location-tier dropdown captures most of the regional variation honestly. The three tiers: • Low cost (Midwest exurb) — 0.85× multiplier. • Mid (Chicago metro) — 1.0× multiplier. • High (coastal high-cost-of-living) — 1.25× multiplier. Why not a black-box ZIP adjustment: Aggregator calcs that lead with ZIP precision pull from a hidden adjustment factor, not transparent methodology. The multipliers above are published in the methodology section, so you can check the math.
What I’d do next
- Estimate the calendar, not just the budget
Cost and timeline are two halves of the same decision. The construction timeline calculator estimates door-open from permit through punch list — owner-builder slowdown included.
- Tighten the electrical number — knob-and-tube or aluminum
If the electrical leak-check flag fired, drop into the rewire calc for scope-promotion logic + the AFCI cascade specifics before overriding the electrical line here.
- Read what's behind these numbers
Planning-range methodology, source anchors, and which numbers are calibrated vs which are operator estimates.
By James Wu. Planning-range methodology and per-calculator 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.