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The quick and simple answer is: you usually don't need a permit if the shed is 120 square feet or less. In California, most jurisdictions have adopted the 120 square foot rule. If your shed area is larger than 120 square feet, it needs a building permit.

A building permit must be obtained before you erect, construct, enlarge, alter, move, repair, improve, convert, or demolish any building or structure, including decks over 30 inches high, patio covers, sheds over 120 square feet, fences over 7 feet high, and retaining walls more than 4 feet in height measured from the ...

One-story detached accessory buildings used as tool and storage sheds, playhouses, and similar uses, provided the floor area does not exceed 120 square feet, do not require a Building Permit (per California Building Code 105.2 "Work Exempt from Permit").

Do I need a permit to build a shed? A San Jose building permit is required for sheds and other accessory buildings if they are 120 square feet or greater.

Projects that change the configuration of walls, pipes, wiring, roofs, or major appliances typically DO require at least one type of building permit. Projects that DO NOT need a building permit are simple surface (cosmetic) projects; minor repairs and replacements; and minor site improvements or small structures.