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Building Code Reference

Got a CMC 304.1 correction?

Appliance access and clearance

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CMC 304.1 is the California Mechanical Code section that requires every appliance to be located so it can be accessed for repair or replacement, with working space of at least 30 inches in depth, width, and height unless otherwise specified. Attic and under-floor installs follow CMC 304.4. Clearances to combustibles come from the appliance listing and installation instructions under CMC 303.10, and the plan-checker is verifying that your drawings show both. Tight utility closets, under-stair installations, and attic furnace platforms are the most common spots where this gets flagged.

How to fix it

  1. Pull the appliance nameplate listing and installation manual

    Before you revise the drawings, get the make and model of every fuel-burning appliance on the plan and download the manufacturer's installation instructions. The listed clearances on the nameplate are what CMC 303.10 actually requires (not a generic table), and CMC 904.3.1 says the same for listed furnaces and boilers. For most residential gas furnaces the sides are 0-1 inch, the rear 0-1 inch, the top 1-6 inches, and the front service area 30 inches — but those numbers vary by model, and the plan-checker will flag any discrepancy between what's on the drawings and what the nameplate says.

  2. Add a clearance schedule to the mechanical plan sheet

    Create a clearance schedule (a simple table works) on the M-series or floor plan sheet that lists each appliance by tag, model number, and its required clearances from the nameplate: top, sides, rear, front (service clearance). Reference it in a note: 'All appliances installed per manufacturer's listed clearances per CMC 303.10 and 304.1.' This gives the plan-checker a single place to verify compliance without hunting through the drawings.

  3. Dimension the clearances on the installation detail

    For any appliance in a closet, alcove, or tight mechanical room, draw a plan-view detail at 1/4"=1' or larger showing the appliance outline and the required clearance envelope as a dashed zone. Dimension each side explicitly. The 30-inch front service clearance is the one that most commonly fails — show that the door swing (or fully open door) does not intrude into it. If it does, note the door type change (swing-out, bi-fold, or sliding) right on the detail.

  4. Verify and show attic or crawl-space access dimensions

    If the appliance is in an attic or crawl space, confirm the access meets CMC 304.4: an opening and passageway at least 22" x 30" and larger than the largest appliance component, solid flooring at least 24" wide, a passageway no longer than 20 feet where under 6 feet high, and a level 30" x 30" platform at the service side. Show the access hatch on the plan with dimensions and a note indicating its location. The path from the hatch to the appliance must be passable — a furnace at the far end of a narrow attic with no walking surface is a service access failure even if the clearance envelope around the unit itself is correct.

  5. Address combustion air if in a confined space

    Appliances in closets or small mechanical rooms that fall below the volume threshold for unconfined space need combustion air openings per CMC 701. If your enclosure is under roughly 50 cubic feet per 1,000 BTU of total appliance input, show the combustion air openings (size, location, and whether they're high/low) on the plan. Plan-checkers often cite CMC 304.1 and CMC 701 together on confined-space installations, so fixing access and clearance without addressing combustion air just generates a second correction.

  6. Note the gas connection type on the detail

    Show whether the gas connection is rigid or flex, and confirm the flex connector length doesn't exceed the limit for the appliance type. A flex connector crowded against a wall inside the listed rear clearance is a double failure. Calling out the connector type and length on the detail heads off a separate gas-connection comment that often appears alongside the access and clearance correction.

  7. Resubmit with a response letter keyed to each comment

    When you resubmit, include a written response to each plan-check comment. For the access and clearance comment, cite the sheet and detail number where you added the clearance schedule and dimension detail. Plan-checkers have dozens of projects open. A one-page response letter that says "CMC 304.1: see M-1 clearance schedule and Detail 3/M-2 for dimensioned clearance envelope" gets you through the second round faster than a clean redline with no narrative.

Take the CMC 304.1 fix steps with you

All 7 steps in one email, so you have them on site instead of on this page.

Why you got flagged

Plan-checkers flag CMC 304.1 when the drawings don't show that the space around an appliance is sufficient for both the required clearance to combustibles and the service technician who'll work on it over the life of the equipment.

The most common trigger is a furnace or water heater crammed into a closet with no clearance dimensions called out on the plan. The code requires the clearances from the appliance listing (CMC 303.10) plus the 30-inch working space (CMC 304.1), and if those numbers aren't on the drawings, the plan-checker can't confirm compliance.

Attic and crawl-space installations get flagged for inadequate access. Even if the clearances are technically fine once you're inside, the route to the appliance has to meet CMC 304.4: an access opening at least 22 by 30 inches, a solid floor at least 24 inches wide, and a level 30-by-30-inch platform at the service side. A plan that shows the furnace location but skips the access hatch dimensions will get kicked back.

Closet installations also fail when the door opens into the required service clearance. If a furnace is 4 inches from the door and the code requires 30 inches of working clearance in front, that door needs to swing out or be a bi-fold that doesn't eat into the front clearance. Plans that show a standard interior door without noting the clearance arithmetic are a common source of corrections.

The official code

2025 California Mechanical Code (Title 24, Part 4) Section 303.10 Clearance to Combustible Materials and Section 304.0 Accessibility for Service, via California Building Standards Commission

Read the source →

Common questions

What's the minimum clearance required by CMC 304.1?
Two numbers matter. For service, CMC 304.1 requires working space of at least 30 inches in depth, width, and height unless otherwise specified. For clearance to combustibles, CMC 303.10 sends you to the appliance listing and the manufacturer's installation instructions, printed on the nameplate. Those vary by model; many residential gas furnaces list 0 to 1 inch on the sides and rear and a few inches at the top. The 30-inch working space is the one that most frequently causes closet installations to fail plan check, because the door swing eats into it.
Does CMC 304.1 apply to tankless water heaters and heat pump units?
Yes. CMC 304.1 applies to all appliances, which includes tankless water heaters, heat pumps, and mini-split air handlers. Tankless water heaters mounted on a wall in a closet still need the front service clearance shown on the plan, even though they're compact. Heat pump water heaters are larger and typically need more surrounding air space than a conventional water heater — the installation instructions will specify both the clearances and any minimum room volume requirements for the heat-exchange cycle to work.
My furnace is in the attic — do the clearances still apply?
Yes, clearances apply in attics the same as anywhere else. Attic furnace installations get flagged for two separate reasons. First, the clearance envelope around the unit itself still has to match the nameplate. Second, CMC 304.4 governs the route: an opening and passageway larger than the largest appliance component and at least 22 by 30 inches, solid flooring at least 24 inches wide, a passage no longer than 20 feet where it is under 6 feet high, a level 30-by-30-inch platform at the service side, and a light and 120 V receptacle near the unit. If the attic plan doesn't show the access hatch with dimensions and a clear service path, expect a correction.
Can I get a variance or exception for a tight closet installation?
There's no blanket variance for clearances to combustibles. They are life-safety requirements tied to the appliance's listing, and reduced clearances must comply with the listing and CMC Table 303.10.1. The simplest route is often to change the appliance. If a standard gas furnace won't fit with proper clearances, a listed zero-clearance or near-zero-clearance appliance might. These are engineered and tested at tighter dimensions, and the nameplate will reflect the reduced clearances. Your mechanical engineer or equipment supplier can identify models with smaller clearance requirements. Don't just show reduced clearances on the plan without changing to a model that's listed for them — that's a failed inspection waiting to happen.
What if the previous owner installed the furnace with no clearances and I'm just doing a remodel?
If your permit scope touches the mechanical system (new equipment, gas line work, HVAC replacement), the plan-checker will apply the current CMC 303.10 and 304.1 requirements to any appliance shown on the plans. Existing non-compliant installations that aren't part of the permit scope typically don't trigger a correction — but the moment you draw them on your plans, they're in scope. If you're replacing an existing furnace in a tight closet and can't meet clearances with a standard unit, address it as part of the replacement by selecting a model with a smaller clearance requirement or reconfiguring the enclosure.
Does CMC 304.1 apply to ADU projects?
Yes. ADU projects that include new mechanical equipment (a furnace, water heater, or mini-split) are fully subject to CMC 304.1. ADU corrections letters cite this section frequently because ADU unit sizes push designers to put appliances in closets and tight mechanical rooms that don't leave room for the required service clearances. Tankless water heaters are popular in ADUs precisely because they're compact, but they still need the front service clearance shown on the plan. Budget for the clearance detail on any ADU permit that includes mechanical equipment.

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