Do I need a permit to replace a furnace, AC or heat pump?
Usually needs a permit
Mechanical replacement typically requires a permit, including straight like-for-like swaps.
What changes the answer
- Like-for-like replacement — mechanical permit typically required
- Changing fuel type or adding refrigerant lines — permit required, plus electrical in most cases
- Load calculation and duct sizing are increasingly enforced on replacement, not just new build
These thresholds come from the model code and are commonly amended locally, which is usually where people get caught.
Why it matters
Combustion appliance venting is a carbon monoxide issue, and unpermitted HVAC is a common finding in pre-sale inspections.
Confirm the requirement with the authority that would issue the permit before deciding.
The version of this people actually ask
Do I need a permit to replace a furnace with the same size unit?
Typically yes. Replacement of fixed heating equipment is not on the exemption list, and matching the old unit's capacity does not change that — partly because the old unit was frequently the wrong size to begin with. Venting is the reason inspectors care: a high-efficiency furnace vents differently from what it replaces, and orphaning a water heater on a now-oversized chimney is a documented way to produce a carbon monoxide problem in an appliance nobody touched.
Do I need a permit to replace just the outdoor AC condenser?
Usually yes, because it is a refrigerant system alteration and an electrical connection even when the indoor equipment stays. This is also where the refrigerant transition bites: matching a new outdoor unit to an old indoor coil is often not permitted by the manufacturer's listing, and an unlisted mismatch is a code problem as well as an efficiency one.
Do I need a permit to replace a thermostat?
No. Low-voltage control wiring is explicitly outside what the code regulates here, and a thermostat swap is one of the genuinely clear no-permit items in the whole mechanical category.
Do I need a permit to add or move ductwork?
Modifying duct is regulated work in most jurisdictions even where no equipment changes, and it is more consequential than it looks. Duct sizing determines whether the system can actually move the air it was sized for, sealing determines how much of it reaches the room, and duct run through unconditioned space has insulation requirements under the energy code.
This is the typical rule, not your answer
This is the typical rule in the United States and not a decision about your property — permitting is run by more than 22,000 separate authorities and any of them can amend the code it adopts, so confirm with your local building department before you start. Put a ZIP in to see what applies in that state, and how to reach the office that decides.
Not in the IRC R105.2 exemptions; mechanical permits required under IRC Chapters 13–24.
Other projects
- Build a deckA permit is usually required once a deck is more than 200 square feet, more than 30 inches above grade, serves an exit door, or is attached to the house.
- Put up a fenceFences under 7 feet are typically exempt from a building permit — but height limits, corner-visibility rules and pool-barrier requirements are set locally and catch people constantly.
- Add a shed or small outbuildingOne-story detached sheds are typically exempt up to 200 square feet under the residential code — but the number people quote is 120, and both are right depending on which code your jurisdiction adopted.
- Replace a water heaterReplacing a water heater almost always requires a permit, even though it is routinely done without one.
- Remove or open up an interior wallIf the wall is load-bearing, a permit and engineered drawings are required. If you are not certain whether it is load-bearing, assume it is.
- Upgrade or replace an electrical panelPanel and service work always requires a permit, and in most places must be done by a licensed electrician.