Many US jurisdictions stop requiring a building permit at 200 square feet — roughly a 10 by 20 footprint — but Seattle's line is less than 120 and Fairfax County's is over 256. What size shed you need a permit for is set by your city or county, so treat 200 as a common figure rather than your answer.
And an exemption from the building permit is narrower than it sounds. It almost never exempts you from zoning, and it rarely exempts electrical work.
Six real thresholds, and the conditions attached
Published exemptions, each checked on October 17, 2026. Read the conditions, not just the number.
| Jurisdiction | Threshold | Conditions published alongside it | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Seattle, WA | less than 120 sq ft of projected roof area | Seattle's wording is exact. Its exemption covers a one-story detached accessory building "if the projected roof area is less than 120 square feet and the building foundation is only a slab on the ground", and it does not apply in environmentally critical areas | | Austin, TX | 200 sq ft | One story, detached, up to 15 feet tall, no plumbing, and outside flood hazard areas | | Portland, OR | 200 sq ft | Non-habitable, one story, detached, 15-foot height limit measured to the average height of the highest roof surface | | Maricopa County, AZ | 200 sq ft | Single story, detached, non-habitable, and no plumbing, electrical, or mechanical services or equipment | | Denver, CO | 200 sq ft of projected roof area | One story, detached, non-conditioned, 8-foot top plate, 14-foot maximum height. Two lights and two receptacles allowed with an electrical permit | | Fairfax County, VA | over 256 sq ft needs a permit | Fairfax states that building permits are required "for detached accessory structures that are over 256 square feet in area", and for anything attached to an existing structure regardless of size |
What the pattern actually tells you
Four of the six land on 200 square feet, which is why that number circulates. The other two sit on either side of it: Seattle stops at less than 120, and Fairfax County does not require a permit until over 256. That is more than a two-fold spread between the tightest and the loosest, which is the point — the common figure is a pattern, not a rule, and it is not even the midpoint of the range.
The three ways people misread these thresholds
It is a building permit exemption, not a general one. Portland states that zoning setbacks and lot coverage still apply. Austin's exemption is void inside a flood hazard area. Your shed can be permit-exempt and still illegally placed, and only your local authority can tell you which rules remain.
The conditions do real work. Maricopa's exemption disappears the moment there is electrical, plumbing, or mechanical equipment. Denver allows two lights and two receptacles, but only with a separate electrical permit. Running power to an exempt shed usually needs its own permit even where the structure does not.
How the area is measured varies. Denver measures projected roof area, not floor area, which on an overhanging roof is the larger number. Portland measures height to the average height of the highest roof surface rather than to the peak. Two sheds of the same nominal size can land on opposite sides of a threshold.
What the threshold means for common footprints
Pure arithmetic, so you can see where the usual sizes sit:
| Nominal footprint | Floor area | 120 sq ft line (Seattle) | 200 sq ft line | 256 sq ft line (Fairfax) | | ----------------- | ---------- | ------------------------------------------- | -------------- | ------------------------ | | 8 × 10 | 80 sq ft | exempt | exempt | exempt | | 10 × 10 | 100 sq ft | exempt | exempt | exempt | | 10 × 12 | 120 sq ft | not exempt — 120 is not less than 120 | exempt | exempt | | 12 × 16 | 192 sq ft | not exempt | just exempt | exempt | | 16 × 20 | 320 sq ft | not exempt | not exempt | not exempt |
A 10 × 12 is exactly 120 square feet, and Seattle's exemption reads "less than 120 square feet" — so it falls outside by a single square foot of wording. That is why the exact phrasing in your own code matters more than the number: "less than 120", "120 or less" and "not exceeding 120" do not mean the same thing. And nominal size is not measured size — where a jurisdiction measures projected roof area, an overhang can carry a 192-square-foot shed past 200.
How to find your own threshold
- Identify the authority for your parcel. City if incorporated, county if not. The county is who you call for an unincorporated address even with a city mailing name.
- Search for "work exempt from permit" or "accessory structure" on that authority's building department site. Most publish a list, and "accessory structure" is the term the code itself uses.
- Read the conditions, not only the square footage. Height, habitability, plumbing, electrical, and foundation type all appear.
- Ask about zoning separately. Setback and lot coverage are a different department's answer in many places — shed distance from the house and property line covers that side.
- Ask about electrical separately if the shed will have power.
Questions people ask at this step
Is a 10 × 12 shed under the permit threshold?
At 120 square feet it is comfortably under a 200-square-foot line and under Fairfax County's 256. In Seattle it is not exempt, because that exemption reads "less than 120 square feet" and 120 is not less than 120. So the answer flips on wording, not only on the number, and it also depends on how your authority measures — floor area and projected roof area give different results for the same shed. Read your own threshold's exact phrasing before treating 10 × 12 as safe.
Does a shed on skids need a permit?
Foundation type appears in some exemptions and not others. Seattle's exemption requires a foundation that is only a slab on the ground. Elsewhere portability is not mentioned at all, and a structure is judged on size and use. "It is not permanent" is a common assumption rather than a published rule, so confirm it locally.
What else still applies
Do I need a permit to run power to a shed?
Usually yes, and usually separately. Maricopa's structural exemption does not apply once there is electrical equipment, and Denver's allowance for two lights and two receptacles is explicitly conditioned on holding an electrical permit. Treat power as its own application.
Does being under the threshold mean I can build anywhere on the lot?
No. Portland states plainly that zoning setbacks and lot coverage still apply to exempt structures. The exemption is about whether the building is reviewed, not about where it may stand.
Carry this into the rest of the decision
The permit threshold and the setback are two different limits that both shrink what you can build. Shed distance from the house and property line covers the distance side, will a shed fit in my yard checks the footprint against your measured space, and choose a practical shed size works back from what has to fit inside.