Do I Need a Permit to Replace a Furnace in Washington?

Furnace replacement permit requirements. Even same-model swaps need a mechanical permit. What inspectors check.

Yes — even a like-for-like furnace swap requires a mechanical permit.

In Washington

WASHINGTON HAS ONE STATE BUILDING CODE IN FORCE EVERYWHERE, ADOPTED BY REFERENCE: "Except as otherwise provided in this chapter, there shall be in effect in all counties and cities the state building code which shall consist of the following model codes which are hereby adopted by reference:" IN ALL COUNTIES AND CITIES - the model codes named in that section are the code, without any act by your city. AND YOUR LOCAL GOVERNMENT MAY CHANGE IT, BUT ONLY IN ONE DIRECTION: "The governing bodies of counties and cities may amend the codes enumerated in RCW 19.27.031 as amended and adopted by the state building code council as they apply within their respective jurisdictions, but the amendments shall not result in a code that is less than the minimum performance standards and objectives contained in the state building code except as provided in subsection (2) of this section." NOT LESS THAN THE MINIMUM. So a local amendment may add to the state code and may not fall below it. That is the opposite arrangement from states that cap local rules at the model code, and it means a stricter local requirement in Washington is normal rather than suspect. FOR A HOUSE THERE IS A SECOND GATE, AND IT IS THE PART MOST READERS WOULD NEVER GUESS: "Except as provided in subsection (2) of this section, no amendment to a code enumerated in RCW 19.27.031 as amended and adopted by the state building code council that affects single-family or multifamily residential buildings shall be effective unless the amendment is approved by the building code council under RCW 19.27.074 (1)(b)." SHALL BE EFFECTIVE ONLY IF APPROVED BY THE BUILDING CODE COUNCIL. A local amendment touching single-family or multifamily residential buildings is not merely constrained - it does not take effect at all until a state council approves it. If you are told a local residential rule differs from the state code, that approval is the thing to ask about. ONE ADOPTION CARRIES AN EXCLUSION THAT DECIDES WHICH RULES APPLY TO PIPES: "The Uniform Plumbing Code and Uniform Plumbing Code Standards, published by the International Association of Plumbing and Mechanical Officials: PROVIDED, That any provisions of such code affecting sewers or fuel gas piping are not adopted;" THE UNIFORM PLUMBING CODE IS ADOPTED, BUT NOT ITS SEWER OR FUEL-GAS PIPING PROVISIONS. So a fuel-gas connection is not governed by the UPC here, and the applicable rule comes from elsewhere in the enumerated codes - worth confirming with the permitting office rather than assuming the plumbing code covers it. A FURNACE SITS UNDER THE MECHANICAL CODE, AND IN WASHINGTON THE FUEL DECIDES WHICH OTHER STANDARD COMES WITH IT. Cities here adopt the International Mechanical Code and then name SEPARATE standards for gas: Richland adopts the IMC and, alongside it, "Liquefied Petroleum Gas Code (NFPA 58) for LP gas" and "National Fuel Gas Code (NFPA 54) for LP gas installations", with the International Fuel Gas Code "adopted as part of the IMC". Sammamish names the same three instruments in separate sections of its adopting chapter. SO A LIKE-FOR-LIKE GAS FURNACE SWAP CAN ENGAGE THREE DOCUMENTS AT ONCE, and a change of fuel - propane to natural gas, or either to electric - changes which of them applies and usually which permits you need. The mechanical permit is rarely the only one.

Why Even a Swap Needs a Permit

Furnace replacements involve gas connections, venting, and combustion air — all safety-critical systems. Most jurisdictions require a mechanical permit to verify the new furnace is properly vented, has adequate combustion air, and the gas connections are leak-free. This is true even for exact same-model replacements.

High-Efficiency Conversions

Upgrading from a standard-efficiency (80%) to high-efficiency (90%+) furnace changes the venting requirements: standard units use metal B-vent, high-efficiency units use PVC/CPVC. The old metal vent chase may need to be sealed. A condensate drain line is also required for high-efficiency units.

What Inspectors Check

The final inspection covers: gas piping and drip leg, venting type and clearances, combustion air supply (especially in confined spaces), condensate drain (high-efficiency units), thermostat operation, and CO detector presence on every level.

Bundled with AC?

If you're replacing both the furnace and AC unit at the same time, one mechanical permit typically covers both. The inspector will check the AC refrigerant lines, electrical disconnect, and condenser pad in addition to the furnace items.

Yes, always. The permit is $40-$75 and your HVAC contractor handles it. One final inspection. Don't skip it — CO poisoning from bad venting is real.

Permit fees by city in Washington

Verified from each city’s published fee schedule. Where a city has no separate fee listed for this project, open the city page for the current requirements and amount.

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Permit fees by city in Washington
	Permit fee	Source
Bonney Lake	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Des Moines	Set by the City Manager or designeeVerifiedvia Residential building permit	Residential building permit →
Edmonds	Set by council resolution; not in codeVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Issaquah	$33.26 minimum, tiered by valuationVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Kenmore	Set by adopted fee scheduleVerifiedvia Residential building permit	Residential building permit →
Lake Stevens	Set by city council resolutionVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Longview	Set in the master fee scheduleVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Lynnwood	Set by city council fee ordinanceVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Maple Valley	Set by Development Fee ScheduleVerifiedvia Residential building permit	Residential building permit →
Mercer Island	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Moses Lake	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Mountlake Terrace	Set by annually adopted fee scheduleVerifiedvia Residential building permit	Residential building permit →
Pullman	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
Puyallup	Set by city manager executive orderVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Sammamish	See city page for the current Residential Building Permit fee	Residential Building Permit →
SeaTac	Set by City of SeaTac Fee ScheduleVerifiedvia Residential building permit	Residential building permit →
Tumwater	Set by city council resolutionVerifiedvia Residential building permit	Residential building permit →
University Place	Set by city council fee resolutionVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →
Walla Walla	From $30.00, tiered by valuationVerifiedvia Residential building permit	Residential building permit →
Wenatchee	Not published in code; ask the cityVerifiedvia Residential Building Permit (New Construction)	Residential Building Permit (New Construction) →

See the national overview for this permit →