Ductwork permits hinge on scope. A straight replacement of a failed duct run with identical materials and routing, with no change to the HVAC system's capacity or configuration, is often exempt. New ductwork, ductwork that changes system airflow, modifications to duct size or material, or work that affects code-required ventilation or exhaust systems almost always requires a permit. The IRC R105 and your local building code govern which HVAC work is permittable versus exempt — and the line varies slightly between jurisdictions. If you're adding a new zone, upgrading from flex duct to rigid, rerouting ducts, or modifying return-air or exhaust pathways, assume you need a permit and call your local building department before you start. A 2-minute confirmation call costs nothing and saves weeks of work if you guess wrong.
When ductwork requires a permit
The threshold for ductwork permits is straightforward: any modification to an HVAC system that changes its operation, capacity, routing, or code compliance status requires a permit. This includes new ductwork runs, replacement of duct material (e.g., upgrading from flex to rigid, or switching to insulated duct), any change to duct diameter or cross-section, modifications to return-air or exhaust ducts, and work affecting any duct that serves a code-mandated ventilation or exhaust function. If the scope involves more than a like-for-like swap — same path, same material, same size — expect to file.
Like-for-like ductwork replacement is the main exemption. This means: identical duct material (flex duct gets replaced with flex duct; rigid galvanized steel gets replaced with rigid galvanized steel), the same diameter and routing, no change to the system's supply, return, or exhaust configuration, and no modification to dampers, registers, or control logic. If you're pulling out a 6-inch flex duct run and installing a new 6-inch flex duct run in the exact same path, most jurisdictions exempt this from permit — it's routine maintenance. But the moment you decide to upsize to 7 inches, switch to insulated duct, or relocate the run, you've crossed into permit territory.
New ductwork, extensions, or zoning additions always require a permit. If you're installing ductwork to serve a new room (a bedroom addition, finished basement, new bathroom), a new HVAC zone (e.g., adding a second thermostat to separate upstairs from downstairs), or expanding an existing system to serve previously unductworked areas, you need a permit. These projects involve mechanical plans, load calculations, and code compliance checks that the building department must review.
Ductwork affecting exhaust or ventilation systems triggers a mandatory permit. Kitchen exhaust ductwork, bathroom exhaust ducting, clothes-dryer venting, combustion-gas venting, and any duct serving a code-required ventilation function (like ASHRAE 62.2 or 62.1 compliance) always requires permitting and inspection. These systems have specific code paths — duct sizing, damper placement, termination locations, backflow prevention — that inspectors verify against IRC M1501, M1502, M1605, or equivalent local amendments.
The two key questions to ask yourself: (1) Does the scope change the system's operation, capacity, or layout compared to today? (2) Does it touch any exhaust, ventilation, or combustion-gas duct? If either answer is yes, file for a permit. Most building departments will rubber-stamp a like-for-like replacement over-the-counter if you describe the scope clearly — 'removing 40 feet of failed 6-inch flex return duct and installing 40 feet of new 6-inch flex return duct, same routing' — without triggering a formal plan review. But if you're unsure, call before you order materials.
Duct material choice matters for code compliance but doesn't by itself require a permit; the scope does. Galvanized steel, rigid fiberglass-lined duct, insulated flex duct, and uninsulated flex duct are all code-compliant in the right application — but all require a permit if they're new or modified. The material choice affects insulation R-value, air-leakage rates, and fire-rating requirements, all of which the building department reviews.
How ductwork permits vary by state and climate zone
The IRC (International Residential Code) is the baseline, adopted by most states with local amendments. States in cold climates (Minnesota, Wisconsin, Michigan, upstate New York, Colorado) often require tighter insulation specifications for ductwork in unconditioned spaces (attics, crawlspaces, basements). Minnesota Statutes Section 326B.401 and Wisconsin's adoption of the 2015 IRC with amendments both mandate R-6 duct insulation as a minimum for supply ducts in unconditioned spaces — higher than the baseline IRC R5. If you're running ductwork through an attic in a cold-climate state, expect the building department to flag undersized or missing insulation during the rough inspection.
Hot and humid climates (Florida, Louisiana, Texas, parts of Georgia and North Carolina) emphasize moisture control and air-sealing. The Florida Building Code (8th Edition, most current adoption) and the Texas Building Code both stress duct sealing, mastic-and-tape or aeroseal systems, and condensation prevention in high-humidity attics. If you're installing new supply or return ductwork in Florida, the permit application will likely require a duct-leakage estimate or a commitment to test and seal at final inspection. California's Title 24 energy code goes further: new ducting must be tested for leakage (HVAC System Quality Installation standard) and sealed if it exceeds 15% leakage at 25 pascals. New ductwork permits in California routinely include a duct-leakage test requirement.
High-altitude states (Colorado above 5,000 feet, Utah, New Mexico) adjust duct sizing and air-handler capacity per the IRC's altitude tables. If you're installing new ductwork in Denver or Salt Lake City, the mechanical plans must show altitude-adjusted CFM (cubic feet per minute) calculations. The building department will review these during plan check to ensure adequate airflow at altitude. Undersized ductwork at altitude is a common permit rejection because the designer forgot to apply the altitude derating factor.
Seismic zones (California, Pacific Northwest, parts of the Mountain West) require ductwork to be supported and braced per the International Building Code Section 2307 or equivalent local amendments. Ducts in seismic design categories C, D, or E must be secured with resilient hangers at intervals (typically 4 feet for rectangular, 6 feet for round), and flexible connectors are required at equipment connections to isolate vibration. A permit application for new ductwork in a seismic zone must include hanger specifications and spacing; inspectors will call out missing or undersized support during rough-in.
Common scenarios
Replacing a damaged flex return duct run with identical material and routing
You have 40 feet of 8-inch return ductwork in your attic that has separated at a seam and is leaking conditioned air. You want to pull it out and install 40 feet of new 8-inch flex duct in the same path. This is like-for-like replacement: same material, size, and routing, no system change. Most jurisdictions exempt this from permit — it's routine maintenance. However, confirm with your local building department first, because some municipalities require a 'maintenance permit' or a simple notification (especially if the ductwork is in a jurisdiction that tracks all HVAC work). In states like California or Florida with strict energy-code tracking, even like-for-like replacement may trigger a notification or minor permit. Before you start: call the building department, describe the work exactly, and ask if a permit is required. If they say no, get it in writing or note the date and name of the inspector who cleared it.
Installing new ductwork to serve a finished basement bedroom with its own zone control
You're finishing a basement, adding a bedroom and bathroom, and want to extend your main HVAC system with new supply and return ducts, plus a second thermostat for zone control. This is new ductwork, a system extension, and a change to the HVAC configuration — all trigger-points for a permit. You will file a mechanical permit, provide ductwork plans (supply and return routing, duct sizes, register locations, thermostat placement, and any dampers or zoning controls), and the building department will review the design against IRC M1601–M1602 (sizing, insulation, sealing). Depending on your climate zone, you may need to provide load calculations (Manual J or equivalent) to justify the duct sizing. The inspector will perform a rough-in inspection before you close walls/ceilings, then a final inspection after system startup. Expect 2–4 weeks for plan review, plus 1–2 weeks after filing before the rough-in inspection window opens. Cost: typically $150–$350, depending on the jurisdiction's fee structure (usually 1.5–2% of the estimated HVAC cost). If your basement is in a state with strict energy codes (California, Minnesota, Massachusetts), the permit may also include a requirement for duct-leakage testing or duct sealing certification.
Upgrading supply ducts from uninsulated flex to insulated, R-6 flex duct in an attic
You're replacing existing uninsulated 6-inch flex supply ductwork with insulated R-6 flex ductwork in your attic. Even though the routing and duct size remain the same, the material change (uninsulated to insulated) is a modification that affects the system's thermal performance and code compliance. Most building departments will issue a permit because the change triggers a code review: you're upgrading to meet current energy-code insulation minimums (IRC R6.2 or local equivalent). File a mechanical permit, provide before-and-after photos or sketches showing the routing and duct type, and note the R-value of the new duct. The building department may approve this over-the-counter (no formal plan review) if the routing is unchanged and the insulation upgrade is straightforward. Expect a $50–$125 permit fee and a rough-in inspection to confirm the duct is insulated and sealed per spec. This work is especially common in cold-climate states (Minnesota, Wisconsin) where older homes have undersized attic insulation on ducts. Once you file, the inspector may also flag missing duct sealing or recommend mastic sealing if the existing ducts are already leaky — so budget for that conversation.
Installing new kitchen exhaust ductwork and venting a range hood to the exterior
You're installing a new range hood and running dedicated exhaust ductwork from the hood through the wall to the exterior. This is new ductwork serving a code-mandated exhaust function, which always requires a permit. You will file a mechanical permit (or possibly a mechanical/electrical combo if there's also a hardwired switch or electrical outlet for the hood). The permit application must include: duct routing from hood to termination, duct diameter (typically 6 or 7 inches for a standard range hood), the type of damper (normally open, gravity, or motorized), and the termination detail (wall vent cap with louvers, soffit vent, or roof penetration). The building department will review this against IRC M1502 (exhaust systems) and verify: duct sizing is adequate for the hood's CFM rating, the duct run includes no horizontal sections that slope backward (causing grease backup), the damper is appropriate for the application, and the termination is compliant (not into a soffit that can recirculate exhaust into the home). Expect 1–2 weeks for plan review, a rough-in inspection after the ductwork is installed but before drywall closes it in, and a final inspection after the hood is connected and operational. Permit cost: $100–$250 depending on jurisdiction. If your home is in a state with strict energy codes (California, Massachusetts), the permit may require a sealed duct with insulation if any portion runs through an unconditioned space.
Relocating and resizing an existing return-air duct to fit a new basement layout
Your basement renovation requires relocating the return-air duct to a new wall and increasing its size from 14x8 inches to 16x10 inches to accommodate the layout and improve air return capacity. This is a modification: the duct is being resized and rerouted, which changes the system's operation and requires a permit. File a mechanical permit with plans showing the old and new duct routing, the new dimensions, and how it connects to the air handler. The building department will review the new duct sizing against the home's HVAC load and make sure the return-air pathway is not restricted. If the new duct passes through an unconditioned space (attic, crawlspace), insulation will be required per the local energy code. Expect 2–3 weeks for plan review and one rough-in inspection. Permit cost: $100–$250. If the ductwork crosses a code-required fire-rating boundary (e.g., between a garage and living space), the inspector may flag this and require a fire-damper installation, so ask during plan review.
What documents you'll need and who can pull the permit
| Document | What it is | Where to get it |
|---|---|---|
| Mechanical permit application | The building department's standard form for HVAC work. It will ask for the project address, scope of work, estimated HVAC cost, and contractor license number (if applicable). | The local building department's website, or in person at the permit counter. Many jurisdictions now offer fillable PDFs or online portals (e.g., Madison's eTraKit system, Denver's ePermitting portal). Check your department's website first. |
| Ductwork plans or routing diagram | For simple like-for-like replacement, a sketch or photo showing the old duct location may suffice. For new or modified ductwork, provide a plan showing supply and return duct routing (marked with duct size, material, and insulation R-value), register and grille locations, dampers, and equipment connections. Include a floor plan and any relevant cross-sections. For exhaust ductwork, show the path from source to termination, duct size, damper type, and exterior termination detail. | You can draw this yourself (a sketch on graph paper, or a CAD drawing from an HVAC contractor). For complex systems or code-required load calculations, hire a mechanical engineer or HVAC designer. Many jurisdictions accept hand-drawn schematics for straightforward work. |
| Equipment cut sheets and specifications | For new or modified systems, include the HVAC unit nameplate data (BTU capacity, CFM, model number), duct material specs (galvanized steel, flex duct R-value, etc.), damper types, and any specialty items (duct sealing materials, insulation R-value). | From the equipment manufacturer or distributor. Cut sheets are usually available as PDFs from the vendor's website or on the equipment itself. |
| Existing HVAC system information | A photo or nameplate data from your current air handler or furnace, showing model, capacity, and CFM rating. This helps the building department understand the scope of the modification. | Take a photo of the equipment nameplate in your basement, attic, or closet where the HVAC unit is located. |
| Load calculation or sizing justification (if required) | For new ductwork serving expanded square footage or a new zone, some building departments require proof that the duct sizing is adequate for the space being served. This is typically a Manual J load calculation or a simplified sizing chart from the HVAC contractor. Required in California, Massachusetts, parts of Minnesota, and some other energy-code-strict jurisdictions. | Your HVAC contractor should provide this. If they don't, it's a red flag — most reputable contractors include load calcs with their ductwork designs. |
Who can pull: Homeowners can file the permit themselves and pull the permit without a contractor license in most jurisdictions — ductwork is not always licensed trade work in residential settings. However, the actual ductwork installation usually must be done by a licensed HVAC contractor if your state requires HVAC licensing (which most do). Confirm your state's requirements: if you're in a state like California, Minnesota, or Florida, an HVAC contractor license is typically mandatory for ductwork work, even if the homeowner files the permit. If you're in a jurisdiction that doesn't mandate HVAC licensing (some rural areas), the homeowner can self-perform the work, but must still obtain the permit and pass inspections. For exhaust ductwork serving appliances (range hoods, dryers), check if your state classifies this as plumbing, mechanical, or general work — in some states, a licensed plumber or HVAC tech is required; in others, it's open to the homeowner if they're doing their own home.
Why ductwork permits get rejected (and how to fix them)
- Application incomplete or filed under the wrong permit type
Ductwork work should be filed under a 'mechanical permit' or 'HVAC permit.' Don't file under electrical or plumbing. Include every required field on the application: project address, work description (new supply ductwork serving finished basement, kitchen exhaust ductwork, etc.), estimated HVAC cost, and contractor info if applicable. Incomplete applications get kicked back with a form letter. Take 5 minutes to fill every box. - Routing or sizing plan missing critical details
Supply plan sketches should show duct diameter (e.g., '6-inch round flex duct'), material (galvanized, flex, fiberglass-lined), insulation R-value if in an unconditioned space, register locations, supply outlet sizes, and how the duct connects to the air handler. Return-air plans should show the path from grilles back to the return plenum and any filters or dampers. Exhaust plans must show the complete path from source (hood, bathroom fan, etc.) to exterior termination, duct size, damper type, and damper location. Inspectors reject vague sketches ('existing layout') with a request for detail. Pro tip: use the HVAC contractor's design drawings if you have them; they're usually detailed enough. - Duct material or sizing doesn't match current code
If you're installing new supply ductwork in an unconditioned space, the insulation must meet the local code minimum (IRC R6 in many states, R6 minimum in Minnesota and Wisconsin, California Title 24 may require sealed ducts and leakage testing). If your plans show uninsulated duct in an attic, they'll be rejected. For sizing, the building department may reject undersized ducts if a load calculation shows the CFM is insufficient. If you're in a high-altitude location (Denver, Salt Lake City), duct sizing must account for altitude derating; missing this is a common rejection. Fix: confirm the code edition and climate zone requirements with the building department before you finalize plans, or have the HVAC contractor pull current code spec sheets. - Missing dampers or backflow prevention on exhaust runs
Bathroom exhaust ductwork must include a backdraft damper (either in the duct or built into the grille) to prevent outside air from flowing back into the home. Kitchen range hoods must include a damper to prevent backdrafting when the hood is off. Gravity dampers or motorized dampers are both acceptable, but they must be specified on the plan. If your exhaust ductwork plan omits the damper, it will be rejected. Resubmit with damper location and type clearly marked. - No evidence of contractor license or improper permit applicant
In states that mandate HVAC licensing (California, Minnesota, Florida, etc.), the permit application must list a licensed HVAC contractor. If you're a homeowner doing DIY work, confirm whether your state allows this for ductwork; if not, you must contract with a licensed HVAC company to pull the permit and do the work (even if you fund it). If you file as a homeowner in a licensing-mandatory state, the permit will be rejected unless you're in an owner-occupied residence and state law allows homeowner-exemptions. Call your state's licensing board before filing if you're unsure. - Electrical subpermit not filed for motorized dampers or hardwired range-hood circuits
If your new exhaust ductwork includes a motorized damper, or if you're wiring a hardwired range hood with a dedicated 240V circuit, an electrical permit may be required separately. The building department may flag this during plan review and ask for proof of electrical permitting. File both mechanical and electrical permits before starting work, or note on the mechanical application that electrical will be filed separately. Coordinate the rough-in inspection timing with both departments if both are required.
Ductwork permit costs and fees
Ductwork permit fees vary by jurisdiction and are typically calculated as a percentage of the estimated HVAC work cost (not the total construction cost). Most building departments charge 1.5–2% of the mechanical work valuation, with a minimum fee of $50–$75 and a maximum of $300–$500 for high-value systems. For example, a $5,000 ductwork installation might incur a $75–$100 permit fee; a $20,000 system extension might trigger a $250–$400 fee. Some jurisdictions charge a flat fee ($50–$150) regardless of system cost. Plan review, inspection, and reinspection fees are usually bundled into the base permit fee, but a few jurisdictions charge separate plan-review fees (add $50–$150 if applicable). Use the building department's fee schedule (usually online) or call to get a quote before filing — it takes 30 seconds and prevents surprises.
| Line item | Amount | Notes |
|---|---|---|
| Mechanical permit (base) | $50–$150 | Flat fee in many jurisdictions; in others, 1.5–2% of HVAC work valuation up to a cap |
| Plan review fee (if separate) | $50–$150 | Added if the jurisdiction charges separate plan-review fees; bundled into base fee in most places |
| Rough-in and final inspection | Included in permit fee | Usually two inspections: after duct is installed (rough-in), after system is operational (final). Reinspections for failures add $25–$75 each. |
| Electrical subpermit (if motorized dampers or hardwired hood) | $50–$150 | Separate electrical permit if dampers or hood circuits are hardwired; same process and fee as standard electrical permit |
| Duct-leakage test or sealing certification (if required) | $100–$300 | California, Massachusetts, and some other energy-code-strict jurisdictions may require HVAC contractor to test ductwork for leakage (HVAC Systems Quality Installation standard) and provide a test report. Not all jurisdictions require this; confirm with your building department. |
Common questions
Can I do ductwork myself, or do I need a licensed contractor?
It depends on your state. Most states require a licensed HVAC contractor to install residential ductwork because it's classified as mechanical work. California, Minnesota, Florida, and most major states mandate HVAC licensing. Some rural or less-regulated areas may allow homeowner DIY, but even then, you must pull the permit and pass inspections. Best practice: check your state's licensing board and ask your local building department. If a license is required and you do the work yourself, the permit may be denied or the system will fail final inspection. If you're uncertain, hire a licensed contractor — the cost of a permit denial or rework is far higher than the contractor's bid.
How long does it take to get a ductwork permit approved?
Timeline varies by jurisdiction and complexity. Simple like-for-like replacements or minor duct relocations (low risk, clear scope) can be approved over-the-counter in a day or two. New ductwork, system extensions, or projects requiring load calculations typically take 1–3 weeks for plan review. High-demand jurisdictions (California, Colorado metro areas) may stretch to 3–4 weeks. After approval, the rough-in inspection window usually opens within a week; schedule it when the ductwork is installed but before you close any walls or ceilings. Plan for 2–4 weeks total from filing to rough-in inspection, plus 1–2 weeks after rough-in to final inspection.
What is the difference between a rough-in inspection and a final inspection?
A rough-in (or intermediate) inspection happens after the ductwork is installed but before you close walls, ceilings, or floors. The inspector verifies: duct sizing and routing match the approved plans, insulation is present and properly attached in unconditioned spaces, dampers are installed, sealing is adequate, and connections are secure. Do not close walls or spray foam over ducts until the rough-in is approved. A final inspection happens after the system is fully operational: the inspector verifies the system starts and runs, all registers and grilles are open and functioning, exhaust dampers work, and the overall installation is complete and safe. You must call for both inspections; don't assume the inspector will come twice automatically.
Do I need a permit to replace old flex ductwork with new flex ductwork in the same location?
Probably not, but confirm first. If the old duct is 8 inches and you're installing 8 inches, same routing, same material, with no change to the system — this is typically exempt as routine maintenance. However, some jurisdictions (especially in California and other strict energy-code states) require notification or a simple maintenance permit even for like-for-like swaps. Call your building department and describe the work: 'removing 50 feet of failed 8-inch flex duct and installing new 8-inch flex duct, same location.' If they say no permit needed, ask if they want a site photo after the work. If they say yes, it's usually a $50–$100 over-the-counter permit with no plan review.
What happens if I install ductwork without a permit?
If you're caught, the building department will issue a stop-work order and require you to obtain a permit, submit to full inspection, and potentially pay penalties or removal/remediation costs. If the ductwork is undersized, unsealed, uninsulated in an unconditioned space, or otherwise non-compliant, you'll be forced to fix or remove it — expensive remediation. If you later sell the home, a buyer's inspector may flag unpermitted HVAC work and require a retroactive inspection or removal. Insurance may deny claims related to unpermitted work. If there's a fire or injury traced to the unpermitted ductwork (e.g., a backdrafting exhaust system), liability is on you. The permit is cheap ($50–$200); the consequences of skipping it are not.
Do I need a separate electrical permit for a hardwired range hood or motorized ductwork damper?
Yes, if the hood or damper is hardwired. A hardwired range hood (connected to a dedicated 240V circuit or 120V circuit) requires an electrical permit and inspection by a licensed electrician. A motorized ductwork damper with an electrical control may also require a separate electrical permit depending on the circuit. File both mechanical (for the ductwork) and electrical (for the wiring) permits before starting work. The building department will coordinate the inspections, or you may need to schedule them separately. If the range hood is plug-in (available outlet, no hardwire), no electrical permit is needed for the hood itself — only the mechanical permit for the ductwork.
What is duct sealing, and why might my permit require it?
Duct sealing is the process of air-sealing leaks in the ductwork joints, connections, and dampers using mastic (a putty-like sealant), fiberglass-mesh tape, or aeroseal (a pressurized sealant sprayed from inside). New ductwork is assumed to leak slightly at seams and connections; sealing reduces leakage by 50–90% depending on the method. California Title 24 and some other energy codes require new ductwork to be tested for leakage (using a blower-door or duct-blaster machine) and sealed if leakage exceeds a threshold (typically 10–15% at 25 pascals). If your permit is in a strict energy-code jurisdiction, expect the permit to require either: (1) duct sealing with a third-party certification, or (2) a duct-leakage test with a report showing compliance. Your HVAC contractor should include this in their bid; if not, ask.
Can I run ductwork through an attic without insulation?
No, not in most jurisdictions. The IRC and nearly all state amendments require that any supply ductwork in an unconditioned space (attic, crawlspace, garage) be insulated to a minimum R-value, typically R-5 or R-6. Some cold-climate states (Minnesota, Wisconsin) mandate R-6 minimum. Uninsulated ductwork in an attic loses heating or cooling to the unconditioned air, wastes energy, and violates code. If you're installing new supply ducts in an attic, specify insulated flex duct (R-5 or R-6) or rigid fiberglass-lined duct. Return ducts in unconditioned spaces may have slightly lower insulation requirements in some jurisdictions, but check locally — you're safer insulating both supply and return. The building department will reject plans showing uninsulated supply duct in an unconditioned space.
What is a damper, and why is it required on exhaust ducts?
A damper is a one-way flapper valve that opens when airflow moves in one direction (out of the home via an exhaust fan) and closes when airflow reverses (outside air trying to backdraft into the home). Bathroom exhaust, kitchen range hoods, and clothes dryers all produce exhaust that must not be allowed to backdraft into the living space — moisture, odor, or combustion gases can re-enter the home. Building codes require a damper in the duct or integrated into the grille. Gravity dampers (simple flaps) are common; motorized dampers add a solenoid that opens/closes with the fan. If your exhaust ductwork plan omits a damper, it will be rejected. Install it in the duct near the exterior termination or in the wall-vent cap; don't rely on gravity dampers in ducts that run horizontal or upward, as they may stick.
What if I discover the proposed ductwork conflicts with existing structure or utilities?
Stop and flag it during plan review, not mid-installation. If your plan shows a new supply run that would hit a floor joist, existing plumbing, or an electrical conduit, contact the building department and your HVAC contractor immediately. You may need to reroute the duct, which means re-filing plans or getting a written modification (a 'permit amendment' or 'change order'). Rerouting is faster than demolition and remediation. If you discover the conflict after installation, you'll be asked to remove and reroute the duct at your cost and time. Before finalization of plans, walk the path where ductwork will run and look for obstacles — a 15-minute site walk saves weeks.
Ready to file for your ductwork permit?
Start by calling your local building department (or visiting their website) to confirm permit requirements for your specific scope. Have a clear description ready: 'I'm installing new supply and return ductwork to serve a finished basement; the ductwork is 6-inch flex insulated duct; the system will be connected to the existing air handler.' A quick call takes 3 minutes and gives you a definitive yes or no — and often a fee estimate. If a permit is required, gather your plans (or hire an HVAC contractor to provide them), complete the application, and submit it in person or online depending on the department's portal. Then schedule the rough-in inspection as soon as the ductwork is installed. If you're hiring a contractor, ask them to pull the permit and handle plan review — many licensed HVAC companies do this as part of their service and have templates that speed approval.
Related permit guides
Other guides in the HVAC & energy category: