Shed and equipment pads are usually reinforced slabs over a compacted gravel base, while workshops, garages and backyard cottages need thicker slabs and footings. Additions must meet your city's adopted building code, with permits and inspections, and lots on steep slopes may need a geotechnical report. Washington requires contractors to register with L&I, and 811 must be contacted before digging.
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Sheds, backyard offices, detached garages, carports, hot tubs and additions all need something solid underneath. For a small structure, that's often a simple slab. For an addition, a backyard cottage or anything built as living space, it means footings and foundation work that must meet the building code and pass inspection.
Common slab and foundation projects
- Pads for storage sheds, studios and garden buildings
- Slabs for workshops, detached garages and carports
- Pads for hot tubs, generators and heat pump equipment
- Footings and slabs for additions and backyard cottages
Small structures and pads
A shed or equipment pad is usually a reinforced slab over a compacted gravel base, sometimes with a thickened edge. Size it to the building plus a little extra, keep it level for the structure but able to shed water at the edges, and confirm the location meets setbacks from property lines, easements and any septic system. Cities around Puget Sound set their own thresholds for when an accessory building or its slab needs a permit, and HOAs may add rules, so check before pouring.
Workshops, garages and backyard cottages
A slab that will carry vehicles, tools or a building used as living space generally needs more thickness and reinforcement than a shed pad, plus footings to carry the walls. If you'll heat or finish the space, ask about a vapor retarder and insulation under the slab, and about where plumbing, side sewer connections and electrical conduit need to be set before the pour.
Additions and structural work
Foundations for additions must follow the building code adopted by your city, which usually means a permit, footings sized for the soil and the load, inspections before concrete is placed, and sometimes engineered drawings. In Washington, construction contractors must register with the Department of Labor & Industries, which requires a bond and liability insurance. Specialty contractors can only work in their registered specialty and can't hire subcontractors, so the company running an addition is often a general contractor with a concrete crew under it. Make sure everyone's role is clear and check registrations on L&I's site.
Puget Sound soil and site factors
- Glacial till: dense till can be good bearing soil, but it sheds water sideways. Footing drains and grading that moves water away from the new slab are important.
- Fill and soft soils: footings should bear on firm native soil. Loose fill, organic soil and soft valley soils may need to be removed or the design adjusted by an engineer.
- Slopes and critical areas: in Seattle and other cities, mapped steep slopes and landslide-prone areas bring extra review, and a geotechnical report may be required.
- Utilities: Washington's dig law requires a locate request through 811 at least two business days before digging. Private lines, such as power to a shed, aren't marked by the utility.
Questions to ask
- Does this project need a permit, and who will pull it?
- How thick is the slab, and what reinforcement, base and drainage are included?
- Do we need engineered drawings or a geotechnical report?
- Are you registered with L&I as a general or specialty contractor?
Planning a building or addition in the Seattle area? Request a slab or foundation contractor and include the structure's size and use.
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Frequently asked questions
Do I need a permit for a shed slab in the Seattle area?
It depends on the city and the size of the building. Each city sets its own thresholds for accessory structures, and setbacks, critical areas and HOA rules also apply. Check with your local permit office before pouring.
Does a concrete contractor need a license in Washington?
Washington requires construction contractors to register with the Department of Labor & Industries, which includes a bond and general liability insurance. You can check a contractor's registration on L&I's website before you sign.
How thick should a shed slab be?
Shed pads are commonly about four inches thick over a compacted gravel base, often with thickened edges. Workshops, vehicle storage and buildings used as living space usually call for more thickness, reinforcement and footings.
Sources and further reading
- Washington State Department of Labor & Industries: Register as a contractor
- Washington Utilities and Transportation Commission: Call before you dig
- Seattle Department of Transportation: Property owners' responsibilities
- Streets Illustrated (Seattle right-of-way manual): Construction permits
- Seattle Department of Construction and Inspections: Environmentally Critical Areas code
- Seattle Public Utilities
- City of Bellevue: Right-of-way permits
- City of Redmond: Right-of-way use permit
- City of Tacoma: Sidewalks
- City of Renton Municipal Code: Critical areas regulations
- Washington State University Extension
- USDA NRCS Web Soil Survey
- American Concrete Institute (ACI)
- Portland Cement Association (PCA)