House flipping looks deceptively simple on reality television: buy cheap, renovate fast, sell high. The reality involves dozens of cost categories, financing complexity, market timing, and execution risk. Understanding every line item before you make an offer is the difference between professional flippers who build wealth systematically and weekend investors who hand their profit to contractors and lenders.

The 70% Rule and Its Limitations

The 70% rule is the industry's most widely used quick-underwriting tool: Maximum Allowable Offer = (ARV × 70%) − Renovation Costs. The 30% buffer is designed to cover selling costs of approximately 8% in agent commissions and closing costs, holding costs of 3–5% over a typical 6-month project, financing costs of 3–5% for hard money interest and origination fees, and a target profit margin of 15%. In expensive coastal markets with compressed margins, many investors adjust to 75%; in slower or riskier markets, some require 65% or lower. The 70% rule is a quick filter, not a final answer. Sophisticated flippers build detailed cost models — accounting for their specific hard money terms, local insurance and property tax rates, and target returns — to determine their actual MAO for each deal. Use the rule to screen opportunities quickly and avoid spending due diligence time on deals that cannot pencil out; always follow up with a full itemized cost model before committing to any offer.

The Real Cost Categories New Investors Miss

New investors typically account for the purchase price and renovation budget but systematically underestimate four additional cost categories that can consume $25,000 to $30,000 on a deal that looked like it had $60,000 of profit. Financing costs on a $120,000 hard money loan at 12% interest over 6 months equal $7,200 in interest alone, plus 2 origination points at closing equal $2,400, totaling $9,600 before a single nail is driven. Holding costs — property taxes, insurance, utilities, and any HOA fees — add another $3,000 to $6,000 over a 6-month project. Selling costs, including real estate agent commissions and the seller's portion of closing costs, on a $220,000 sale at 7.5% equal $16,500. Unexpected repair items — foundation issues discovered after demo, plumbing that fails inspection, permit delays requiring scope changes — are the norm, not the exception. Budgeting a 15–20% contingency on your renovation estimate is a professional standard, not conservative pessimism.

ROI vs Annualized ROI

Beginning investors measure success in dollar profit or simple ROI, which ignores the most important variable: time. Professional flippers measure annualized ROI because it enables fair comparison across deals of different durations and against passive investments. A $20,000 profit on $100,000 invested is 20% ROI regardless of how long the flip took. If the flip completed in 6 months, the annualized ROI equals ((1 + 0.20)^(12/6) − 1) = 44%. If it took 12 months, annualized ROI equals 20%. The 6-month deal is objectively superior — same dollar profit, but capital was tied up for half the time, allowing a second deal to begin immediately. For context, the S&P 500 averages roughly 10% annually. A flip producing 20% annualized ROI is generating twice the market rate of return, which is typically the minimum threshold professional flippers use to justify the execution risk, active management demands, and capital concentration of the business relative to simply investing in index funds.

Renovation Budget Reality

Accurate renovation budgeting separates profitable flippers from break-even operators. Cosmetic-only flips — paint, flooring, fixtures, landscape cleanup — typically run $15 to $30 per square foot and carry lower cost overrun risk since the scope is limited. Mid-level flips that include kitchen and bathroom renovations run $30 to $60 per square foot, with greater risk from tile lead times, cabinet delays, and subcontractor scheduling conflicts. Full gut rehabs involving structural changes, mechanical system replacements, and complete finish-out run $60 to $150 per square foot. On any project, experienced investors add a 10–20% contingency to every contractor estimate, obtain multiple bids for work exceeding $5,000, require written fixed-price contracts before committing, and perform thorough pre-purchase inspections including plumbing scope, electrical panel evaluation, and foundation assessment. Renovation overruns are the most common cause of missed profit projections on flip deals at every experience level — from first-time flippers to investors with 50 completed projects — making contingency budgeting a non-negotiable professional standard rather than an optional safety margin.