A 20×20 wood carport is not automatically permit-exempt; local codes decide the foundation, setbacks, and structural limits before any lumber is cut.
The most common reason a 20×20 carport project stalls is that the builder picked a plan before checking local code. Before you start figuring out how to build a 20×20 wood carport, call or visit your local building department: it is a 400-square-foot accessory structure, and almost every county treats that size as a real building with permit requirements. The sequence that works is to check your jurisdiction’s prescriptive rules, confirm your site stays inside those limits, submit the right paperwork, then build.
Does A 20×20 Carport Need A Permit?
Yes, in nearly every case. There is no national “free size,” and the only question is which permit path your county applies.
Pierce County, Washington requires a building permit for every carport, no matter the size, and its predesigned carport guide only covers projects that stay inside its listed limits. Chesterfield County, Virginia exempts one-story detached accessory structures from 0 to 256 square feet from a permit, but a 400-square-foot carport falls well above that, and any carport placed in a setback area needs a permit regardless of size. Mountain Home, Idaho requires permits for accessory structures over 120 square feet and also requires one for any carport in a setback area.
Assume a permit until your local building department tells you otherwise.
What Prescriptive Limits Apply?
Your county’s adopted code sets the limits, and two published county examples show how much they vary.
Pierce County’s predesigned carport guide covers a 20×20 carport only when it is open on at least two sides (IRC R309.2), keeps eaves and overhangs to 12 inches, keeps posts to 10 feet above grade, and stays at least 5 feet from property lines. It applies at wind speeds up to 110 mph, seismic zone D, and a 30 psf ground snow load. The county’s material rules require #2 grade or better lumber, 2,500 psi concrete, and hot-dipped galvanized or stainless hardware installed per manufacturer specs.
| Requirement | Pierce County, WA | Chesterfield County, VA |
|---|---|---|
| Permit threshold | All carports | Over 256 sq. ft.; any size in setbacks |
| 20×20 coverage | Included in guide | Built to code with permit |
| Foundation | 2,500 psi concrete | 12 in. below final grade |
| Eave or overhang | 12 in. max overhang | Eave under 10 ft. for 12 in. footing |
| Post height | 10 ft. max from grade | Follows adopted code |
| Design loads | 110 mph wind, seismic D, 30 psf snow | Adopted code |
| Property line | 5 ft. minimum | Zoning setback applies |
Chesterfield County treats a 20-foot by 20-foot light-frame carport as a 257-to-600-square-foot accessory structure that must be built to code with a permanent foundation at least 12 inches below final grade when the eave height is under 10 feet. If the structure crosses 600 square feet or the eave rises above 10 feet, the footing depth grows to 18 inches of continuous footing. Chesterfield also requires 18-inch continuous footings for non-light-frame structures over 400 square feet.
The pattern to see is simple: permit rules and footings vary widely, but the question is always whether your county has a prescriptive guide that covers your exact footprint. If it does, stay inside it. If it does not, plan on engineered drawings. If your snow, wind, seismic, post height, or setback conditions exceed the county’s prescriptive limits, you move out of the free guide and into engineered plans from a registered engineer. That is the point where most unpermitted builds get caught.
Get The Permit Before You Start Digging
The fastest path is to pull your county’s predesigned carport guide, verify every site condition fits, and hand the guide back in as your plan set. That is the entire trick: prescriptive plans are only free when you stay inside their boxes.
- Draw a site plan showing property lines, all existing structures, the new carport, septic if applicable, and setback distances.
- Complete the application — Pierce County, for example, asks for its Residential Construction Application and Submittal Standard Checklist along with the completed guide itself as the plans.
- Add a fire flow worksheet when the carport sits closer than 10 feet to any other structure.
- Submit the package and wait for the permit before ordering materials or pouring concrete.
Treating a 20×20 carport like a garden shed is the most common mistake. Shed rules often exempt small structures or use different footing requirements, and carports have their own classification under IRC R309.2. If your site exceeds any limit, stop and hire a Washington State registered engineer; in Pierce County, construction beyond the guide requires engineered calculations.
Once the application is approved, you’ll receive the permit with the county’s conditions attached. That approval is the signal to start building; the approval itself is where most 20×20 projects fail, so start with the county’s free guide instead of a plan you bought online. If you would rather buy a kit than stick-build, our tested 20 x 20 carport roundup compares the kits that match this footprint.
FAQs
Do I need engineered plans for a 20×20 carport?
Only if your project falls outside your county’s prescriptive limits. When the eave, post height, snow, wind, seismic, or setback numbers exceed what the published guide allows, most counties require calculations from a registered engineer. Staying inside the published limits is what keeps the predesigned guide valid.
Why does a carport get treated differently from a shed?
Carports are open accessory structures with their own IRC classification, and counties often set separate permit thresholds and footing rules for them. A 400-square-foot carport usually falls into a different code category than a small shed, so shed exemptions rarely apply. Check the carport-specific guide instead of assuming shed rules cover it.
What happens if I build without a permit?
You risk a stop-work order, fines, and having to move or tear down a structure that violates setbacks or code. Most counties require inspection records before a carport can be used, and an unpermitted build can complicate a future home sale. The permit is inexpensive compared with those outcomes.
References & Sources
- Pierce County, Washington. “Predesigned Project Guide – Carport.” Defines the prescriptive 20×20 carport limits, materials, and application requirements.
- Chesterfield County, Virginia. “Requirements for Sheds, Carports and Garages.” Sets permit thresholds and footing depths for accessory structures.
- Mountain Home, Idaho. Mountain Home City Code, Section 9-9-2. Requires permits for accessory structures over 120 square feet and for carports in setbacks.