The answer
IRC Section R105 legally mandates building permits for structural changes, moving walls, mechanical/HVAC alterations, electrical modifications, and plumbing additions. Minor cosmetic work (painting, surface flooring, like-for-like cabinet replacements) is exempt under IRC R105.2. Consequences of unpermitted work: immediate stop-work orders, double/triple permit penalties, mandatory demolition of finished drywall/tile for inspection, denial of insurance claims (fires/leaks), disclosure liabilities at resale, and contractor license violations (CSLB BPC 7110/7090).
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.