Got asked this week about the right foundation for a 6m x 4m garden shed by a mate setting up his backyard workshop in Brisbane. The foundation question is the one that gets glossed over more often than it should and gets people into trouble more than any other shed-building decision. Here are the realistic options, with the trade-offs.

Concrete slab.

The concrete slab is the gold-standard foundation for a shed you actually plan to use. The advantages are real — the slab is flat, the slab stays put, the slab keeps moisture from rising into the floor, the slab provides a stable surface for heavy workshop equipment, the slab makes a clean and easy-to-sweep floor surface. The shed sitting on a properly-poured slab will outlast the shed sitting on most other foundations.

The disadvantages are the cost, the planning, and the permanence. The concrete slab for a 6m x 4m shed runs into a meaningful four-figure cost in 2026 — material, formwork, reinforcement, labour, and finish. The job needs to be planned with the shed kit dimensions clearly understood. The slab is genuinely permanent — once you have poured a slab in your backyard, that footprint is committed for many years.

The slab specification I usually recommend for a workshop shed: 100mm thickness on a compacted base, with reinforcement mesh, set on a damp-proof membrane. The concrete should be 25 MPa or stronger. The finish should be a wood-float finish or a power-trowel finish depending on the use. The slab should extend at least 50mm beyond the shed footprint to allow proper rain runoff away from the walls.

The slab needs to be allowed to cure properly before the shed goes up. Seven days minimum for the structural strength to be reasonable. Twenty-eight days for the full design strength to be reached. The shed can usually go up at the seven-day mark but the workshop equipment should wait.

Concrete pier and bearer system.

The concrete pier system uses individual concrete piers at the corners and at intermediate points along the bearer line. The bearers — treated pine, hardwood, or steel — span between the piers. The shed floor sits on the bearers.

This option is meaningfully cheaper than a full slab and is appropriate for backyards where the ground is sloped or where the soil conditions make a slab impractical. The piers are easier to install on a sloped site than a slab.

The disadvantages: the air gap under the floor needs to be screened to keep pests out. The flooring needs to be substantial enough to be stable — typically 90mm or thicker hardwood decking or a structural plywood subfloor. The level of the shed floor is harder to get exactly right with the pier-and-bearer approach than with a slab.

For a workshop shed that needs heavy equipment, the pier-and-bearer system can work if the piers are well-spaced and well-built. For a garden storage shed without heavy load requirements, this is often the right choice.

Crushed rock and pavers.

The crushed rock base with pavers as the floor surface is a reasonable budget option for a shed that will store garden equipment, bikes, or similar light loads. The technique is to excavate the footprint, lay a layer of compacted road base, top with concrete pavers, and build the shed on top.

The advantages: cheap, relatively quick, and the shed can be moved or removed in future without leaving a permanent foundation. The drainage is naturally good — water just soaks through the gaps in the pavers.

The disadvantages: not as flat as a slab, not as stable for heavy equipment, the rodents and insects can come through the gaps if the perimeter is not properly sealed, and the pavers can settle and become uneven over time.

For most genuinely useful shed installations, this option is a budget compromise rather than a recommendation. For a small garden shed where the budget is constrained, it can be acceptable.

Treated timber bearers on stumps.

The traditional Australian backyard shed often sat on creosote-treated timber stumps with hardwood bearers spanning them. The technique is well-understood and works but the modern timber treatment is not as durable as the older creosote was, and the stump life is shorter than people expect.

For a temporary shed or for a sloped backyard where a slab is genuinely impractical, this is workable. For a long-term workshop shed, the concrete pier-and-bearer system is the better version of the same idea.

Galvanised steel stirrups on concrete pads.

The hybrid approach uses small concrete pads cast at each bearer point with a galvanised steel stirrup embedded in the concrete. The bearer slots into the stirrup and is bolted through. The bearer is held off the concrete by the stirrup design so the timber stays dry and decay-free.

This is a really good middle option for sheds where a full slab is overkill but where durability is wanted. The cost is between the crushed-rock option and the full slab. The installation is more involved than the pier system but the durability is excellent.

Site preparation matters more than the foundation choice.

Whichever foundation you choose, the site preparation makes the difference between a foundation that lasts and one that fails.

Drainage. The site should drain water away from the shed location. The slope, the surface runoff paths, the surrounding garden — all should direct water away from the foundation. The shed sitting in a slight depression that collects rainwater is a shed with a moisture problem for its entire life.

Compaction. The base under any foundation needs to be properly compacted. The soft fill that has not been compacted will settle over time and your foundation will move with it.

Removal of organic material. Any topsoil, garden mulch, or organic material under the foundation should be removed. The organic material will decompose, settle, and create a void under your foundation.

Termite considerations. In termite-active areas, the foundation design should include termite barriers and the timber elements should be properly treated. The shed that is termite-attractive is a problem for the whole property, not just the shed itself.

Council permits.

The permit question varies state by state and council by council. For most backyard sheds under a certain size — typically 9 square metres in most jurisdictions — no permit is required if the shed meets certain setback and height conditions. The shed over the small-shed threshold typically requires a permit. Check with your local council before committing to the project — the permit cost is usually small but the cost of building without a permit when one was required can be significant.

The realistic budget for a properly-foundationed 6m x 4m workshop shed in 2026 — including the foundation work, the shed kit, the assembly, and the basic finish — sits in the upper four to mid five figures for most installations. The foundation is typically 15–25% of the total project cost. Cutting the foundation budget to save on the project total is usually the wrong place to save money — the foundation is the part of the project that determines how long the shed lasts and how usable it is over its life.