Do I need a building permit?

The short version: if the work touches structure, changes the building envelope, moves electrical or plumbing, alters what the space is used for, or adds footprint, you need a permit. If it is purely cosmetic, you almost certainly do not. Everything difficult about this question lives in the gap between those two sentences.

The reliable rule

Permitting exists so that someone independent verifies the work will not fall down, burn, flood, leak or electrocute anyone — and so that a record exists for the next owner. Read that way, most edge cases resolve themselves. Replacing a light fixture on an existing circuit changes none of those risks. Adding a circuit does.

Almost always needs a permit

• Structural change of any kind — removing or altering a wall, beam, header, joist or footing. This is permitted essentially everywhere, and load-bearing status is decided by an engineer or plans examiner, not by whether the wall feels solid.
• New footprint — additions, decks, porches, garages, sheds above a size threshold, carports.
• Electrical beyond a like-for-like swap — new circuits, panel upgrades, service changes, moving outlets or switches.
• Plumbing that moves — relocating or adding drain, waste, vent or supply lines. Replacing a toilet or faucet in place is usually exempt.
• Mechanical — furnace, air conditioner, water heater and ductwork, including like-for-like replacements in most jurisdictions, because of combustion and venting requirements.
• Roofing — most jurisdictions permit re-roofing so an inspector can verify underlayment, flashing and the number of existing layers.
• Windows and doors where the opening size changes, or a new opening is cut.
• Demolition, including partial demolition of a significant portion.
• Change of use or occupancy — garage to bedroom, single family to rental, retail to food service. This triggers review against the code for the new use.
• Pools and spas, retaining walls above roughly four feet, and fences above roughly six feet.
• Solar, battery storage and EV chargers.

Usually does not need a permit

• Painting, wallpapering, flooring, carpet, tile over existing substrate.
• Cabinets and countertops, where plumbing and electrical stay where they are.
• Like-for-like fixture replacement in the same location.
• Non-structural trim, doors within existing openings, and interior finishes.
• Landscaping that does not involve retaining walls, drainage changes or ground disturbance above a threshold.
• Appliance replacement that does not alter gas, electrical or venting.

Why the exemption list is the part that varies

The requirements themselves are fairly uniform, because most of the country builds to some edition of the International Codes. What differs is the threshold at which a jurisdiction stops caring: the shed size that triggers a permit, the fence height, the dollar value of repair work that can proceed without one. Those numbers are set locally and can differ between two adjacent jurisdictions.

This is exactly why a directory that gave you a national answer to "is a 10 by 12 shed exempt" would be doing you a disservice. Look up the exemption list for the office that has jurisdiction over your specific property.

What actually happens if you skip it

The consequence people expect — a fine — is usually the smallest of them. The ones that cost real money are these:

• Retroactive permitting costs more. A work-without-permit penalty is commonly a doubling of the permit fee, and in some jurisdictions considerably more.
• Concealed work has to be exposed for inspection. That means opening drywall or excavating to show a footing, at your cost.
• Insurance claims can be denied where a loss traces back to unpermitted work.
• Sales fall through or get repriced. Unpermitted work found during a buyer's inspection is discovered at the worst possible moment, under time pressure, with the buyer holding the leverage.
• Appraisals exclude unpermitted square footage, so a finished basement that was never permitted may add nothing to the valuation.

If work has already been done

Retroactive permitting is a normal process rather than an exceptional one, and offices deal with it constantly. Apply for an after-the-fact permit, expect the penalty, and be ready to expose work for inspection. Where work genuinely cannot be inspected as built, an engineer's letter certifying it is sometimes accepted. Doing this voluntarily is treated very differently from having it discovered.