What are the biggest renovation mistakes to avoid?

Five mistakes account for ~70% of project regret: (1) skipping the permit — voids insurance, blocks resale, and triggers fines + rip-out 10–15% of the time; (2) under-budgeting contingency — never start without 10–25% beyond contract; (3) selecting on lowest bid alone — the 30%-below-others contractor finishes 40% over budget via change orders; (4) buying materials before measurement — fixtures, tile, and counter cuts go wrong if ordered before demo finishes; (5) DIY-ing under-floor plumbing or electrical above 120V — these are the failure modes that show up at resale inspection and force costly rip-and-replace.