What Size Shed Do I Need? A Practical Sizing Guide
Almost nobody wishes they had built smaller. After more than 12,000 buildings, the regret we hear most often is about the size someone talked themselves out of. This guide walks through how to work out the footprint you actually need, and where the thresholds sit that are worth knowing before you decide.
Start with what goes in, then add the space around it
The usual mistake is measuring the objects and stopping there. A mower is 2 by 5 feet, so a 6 by 8 shed sounds generous. In practice you also need to open the mower’s handle, walk past it to reach the shelf behind, and get the wheelbarrow out without moving three other things first.
A reliable rule: measure the footprint of everything going in, then double it. That second half is circulation, and it is the difference between a building you use and a building you dread opening.
Sizes we build and what they hold
Small: 32 to 80 square feet
4×8, 6×8, 8×8, 8×10. Yard tools, a mower, bikes, garden supplies. The 4×8 at $3,040 is genuinely useful for clearing the side return, but it is storage only. Anything you want to stand and work inside should start at 8×8.
Medium: 96 to 144 square feet
8×12, 10×10, 8×14, 10×12, 12×12. This is where most projects land. A 10×10 is the standard one-person office. A 10×12 handles garden storage plus a working corner. A 12×12 fits a home office with a second chair, or a gym with cardio and free weights. Every size here stays under the 200 square foot permit exemption used by most local jurisdictions.
Large: 160 to 240 square feet
8×20, 10×16, 12×14, 12×16, 10×20, 12×18, 12×20. Serious workshops, gyms with a rack, studios, and buildings split into two rooms. A 12×16 at 192 square feet is the largest common size that still fits under a 200 square foot exemption. Above that, expect to permit.
Three thresholds worth knowing
- 120 square feet. Washington’s statewide minimum permit exemption, and Seattle’s actual limit. Relevant if you are inside Seattle city limits.
- 200 square feet. The exemption most local jurisdictions use, including unincorporated Snohomish and King County. 12×16 is the largest standard size underneath it.
- 12 feet. Gables over 12 feet need manufactured trusses at additional cost, and side walls over 12 feet need a spreader element such as a loft or a 4×4 to stop the walls spreading.
These are size thresholds, not permission. See our permit guide for what the exemptions actually cover, because occupied and heated buildings are treated differently from storage.
Height is the dimension people forget
Standard wall height is 7 feet 4 1/2 inches, and it can be upgraded or reduced. It rarely matters for storage and matters enormously for a gym, where overhead pressing and pull-up bars need clearance, or a workshop where you are standing sheet goods on end. Roof style affects this too: the Rancher’s 6:12 gable and the Craftsman’s 12:12 both add height at the peak, while the Forester’s single slope gives you one tall wall.
Use the vertical space
A storage loft is the best value addition we sell, because it uses volume that is otherwise wasted. In Snohomish County a loft in a permit-exempt structure is capped at one third of the footprint, which is usually enough for seasonal items, offcuts and things you touch twice a year.
Getting it in and out
Standard is a single out-swing door at 36, 42 or 48 inches. That is fine for tools and boxes. It is not fine for an assembled squat rack, a treadmill, a ride-on mower or a table saw. Double shed doors at 36 or 42 inches each cost little at build time and are almost impossible to regret. Add a composite ramp if anything will be wheeled.
Check the lot before the building
Two constraints bind before your wish list does. Setbacks dictate how close you can get to property lines, and lot coverage limits the combined footprint of every structure on the parcel including the house, garage and raised decks. In Seattle’s neighbourhood residential zones that ceiling is roughly 50 to 60 percent. Work out the buildable rectangle first, then size within it.
Also think about access on installation day. We arrive with the building and a crew, and we need a route to the pad.
The honest recommendation
If you are choosing between two adjacent sizes and the larger one fits your lot, your budget and your permit position, take the larger one. The incremental cost per square foot falls as buildings get bigger, and the cost of replacing a building that turned out too small is the whole thing over again.
The cheapest way to test a layout is our free 3D Shed Builder, where you can set the size, style, doors and windows and see the result before speaking to anyone.
Sizing Questions
What is the most popular shed size?
Sizes between 100 and 200 square feet account for most of what we build. 10×12 and 12×16 are particularly common because they are large enough to be genuinely useful while staying under the 200 square foot permit exemption that most local jurisdictions apply to storage structures.
What size shed fits a riding mower?
Plan for at least 8×10, and specify double doors. A riding mower needs turning space and a wide enough opening, and a single 36 inch door will make every trip awkward. If the shed also holds tools and garden supplies, 10×12 is a more comfortable choice.
How big should a backyard office be?
10×10 at 100 square feet suits one person with a desk, chair and storage. Go to 10×12 or 12×12 if you want a second chair for clients, and 12×14 or larger for a treatment room, a recording space or two people working. Remember that an occupied, heated office is treated differently from a storage shed for permitting.
Can you build larger than 12×20?
Yes. The published price list runs to 12×20 at 240 square feet, but we build beyond it, including a recent 410 square foot workshop and storage building. Larger structures need a permit and often benefit from architect involvement, both of which we can help arrange.
Does a bigger shed cost proportionally more?
No, it gets cheaper per square foot. A 4×8 Rancher works out around $95 per square foot, while a 12×20 Rancher is closer to $57. Fixed costs such as delivery, the door, the roof structure and the crew’s time are spread across more floor area. It is one reason going a size up is usually good value.
How much clearance do I need around the building?
Check your local setbacks first, since those are the binding constraint. Beyond that, leave enough room to walk around the building for maintenance and to keep vegetation off the siding. Permit-exempt structures in unincorporated Snohomish County must sit 20 feet from other buildings and property lines, which is considerably more than most people expect.
Try a Size Before You Commit
Build it in 3D and see how it looks, or tell us what is going inside and we will help you size it.
