The Trade-Level Approach
Break the rehab into trades: roof, HVAC, plumbing, electrical, kitchens, baths, flooring, paint, exterior. Get bids per trade. Avoid 'whole-house' GC numbers — they hide markup.
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The 20% Contingency Rule
Add 20% to every estimate. For pre-1978 properties, add another 5% for lead remediation. For pre-1960 properties, plan for at least one major-systems surprise.
Tracking Actuals
Every project teaches you your personal multiplier on contractor estimates. Most novice investors run 30–50% over budget for their first 2–3 projects, then stabilize.