Do You Need a Permit to Replace a Furnace or AC in Modesto?
Updated 2026-10 · Modesto, CA
The short answer
Yes. In California, replacing a furnace, an air conditioner, a heat pump or the indoor coil is an alteration under the state energy code, Title 24 Part 6, and it needs a building permit. In Modesto the permit comes from the City of Modesto's building division; outside city limits, in places such as Salida, Del Rio and the county islands, it comes from Stanislaus County. Ordinary repairs (an igniter, a motor, a capacitor) generally do not need one. We are not a contractor and cannot rule on your particular job, so check with the city if the work is anything beyond a parts swap.
What the energy code adds to a change-out
Duct leakage testing
When new equipment goes onto existing ducts, the ducts generally have to be tested for leakage and sealed if they exceed the allowed limit. There are exceptions, such as very short duct runs outside the conditioned space or ducts already tested and registered as sealed. In a 1970s or 1980s Modesto attic, ducts often fail the first test.
Airflow and refrigerant charge
The new system's airflow and refrigerant charge are checked against what the equipment needs, so it performs at its rated efficiency.
HERS verification
Some of these results have to be confirmed by an independent HERS rater before the permit can be finalled. It is a separate, smaller cost, and a quote should say whether it is included.
Why people skip it, and what it costs them
An unpermitted change-out is cheaper on the day by the permit fee, the duct sealing and the HERS visit. Then the costs arrive.
- No MID rebate: the 2026 catalog requires a copy of the permit with the application.
- No check that the ducts are not pouring cooling into a 130-degree attic.
- A problem when you sell: buyers' inspectors look for a permit on a new system.
- No inspection of the gas, venting and electrical work around the new equipment.
Who pulls the permit
The installing contractor, normally. Their licence (a C-20 from the Contractors State License Board for heating and air work) is on the application. If a company suggests you pull an owner-builder permit for work they are doing, ask why; it moves responsibility for the code onto you.
How to tell a quote includes it
Look for the permit, duct testing and sealing, and HERS verification as line items, or a written statement that they are included. If you cannot find them, ask. A quote that is much cheaper than the others and silent on all three is usually the reason.
Getting replacement quotes? Call and say what you have now. A local heating and air company will quote it with the permit included.
Where to go next
- Furnace and AC replacement in Modesto
- MID rebates and the permit copy they require
- Salida, where permits come from the county
Common questions
Does a new thermostat need a permit?
Generally not on its own. If it is part of a system replacement, it falls under that permit.
How long does the permit add?
Usually little to the install itself. The final inspection and HERS visit happen after the system is running, often within a week or two.
I bought a house with an unpermitted system. What now?
Talk to the City of Modesto building division about permitting it after the fact. Requirements vary, and a contractor can advise on what testing it needs.