A shed looks like the simplest building project you will ever undertake. Four walls, a roof, a slab, done.

Take a composite but thoroughly typical couple on a few acres outside Bendigo. They want a 9 by 12 metre Colorbond shed: a workshop down one end for his welding and woodwork, bay space for the caravan at the other. They find a kit they like online, pay the deposit on a Friday night, and feel very pleased with themselves.

Over the following month they discover that the block sits on reactive clay and the slab needs an engineer’s design. The kit has a twelve week lead time. The council wants a planning permit because of the wall height needed to clear the caravan. And the sparky who was going to run power out there needs a trench that should have been dug before the concrete went down.

None of it is a catastrophe. All of it was avoidable. Sheds are simple buildings with a surprisingly unforgiving sequence, so here is the order things need to happen in, who you need at each point, and where our Bendigo couple could have saved themselves a lot of grief.

Before anyone picks up a tool

Three things determine everything downstream and all three are frequently discovered too late.

Approval. Requirements vary by council and state, and the triggers are usually floor area, height, wall height, setback from boundaries and proximity to easements. Small sheds often fall under exempt development. Larger ones, anything close to a boundary, anything on a sloping site and anything you intend to connect services to typically need approval.

The mistake that costs the most is ordering the kit before confirming approval, because approval conditions frequently change the design. A council requiring a lower wall height or a different setback turns your delivered kit into an expensive problem.

Soil classification. A geotechnical assessment classifies the site under AS 2870, the Australian Standard for residential slabs and footings, and that class determines the slab design. The scale runs from Class A, stable sand and rock with little ground movement, through S, M, H1 and H2 to Class E for extremely reactive clays, with Class P reserved for problem sites such as filled ground or soft soils. A Class A block on the Perth coastal plain and a Class H2 block in central Victoria need very different slabs for the same shed. This is not optional for anything of size, and it is the input the engineer needs.

Services location. Before any excavation, get the underground services located. Water, sewer, gas, power, telecommunications. Before You Dig Australia, the free national referral service formerly known as Dial Before You Dig, will tell you what is under the ground. Hitting a service is not free.

Stage one site preparation and earthworks

Trades: surveyor if boundaries are uncertain, excavator operator, sometimes a retaining wall contractor.

The pad is cut and levelled, fall is established for drainage, and any retaining is built.

Drainage is the item most often skipped and most often regretted. A shed slab sitting in a low point collects water, and water against a slab edge finds its way in. Establish where water goes before the slab is poured, not after you notice the floor is wet every winter.

If the site slopes meaningfully, retaining may be required, and depending on height that can itself need engineering and approval.

Stage two the slab

Trades: concreter, structural engineer for the design, sometimes a pest treatment applicator.

This is the single most consequential stage, because everything else sits on it and nothing about it can be changed afterwards.

What needs to be right:

Dimensions and squareness. A slab that is out of square will not accept the kit. Check the diagonals before the pour, not after.

Edge detail and thickening. Shed kits fix down to the slab and the fixing detail depends on the wind classification and the kit manufacturer’s requirements. Beam and edge thickening needs to match the design.

Levels and fall. Level inside, with fall away from the perimeter outside.

Anything that needs to be under the slab goes in now. Conduit for power and data, plumbing rough-in if you are adding a sink or toilet, and drainage.

This is where our Bendigo couple came unstuck. Nobody mentioned conduit, the concreter poured exactly what was drawn, and the slab looked magnificent for about a week. Then the electrician arrived, walked around it twice, and quoted to trench around the perimeter, bore under the new concrete path and come up through the wall instead of the floor. It cost a second visit, an extra day and a noticeably quieter drive home. A few lengths of conduit before the pour would have cost almost nothing.

Termite protection where required, which depends on jurisdiction and whether the structure is habitable or attached. Subterranean termites, including the destructive Coptotermes species, are active across most of mainland Australia, and AS 3660 sets out the management systems, from physical barriers such as stainless steel mesh and graded stone to chemical soil treatments. A steel framed shed is far less vulnerable than a timber one, but anything timber you store or fit out inside it is not.

Cure time before the kit goes up. Concrete needs time. Erecting a kit on green concrete risks the fixings, and the manufacturer’s warranty will have something to say about it.

Because slab quality determines whether the rest of the build works, this is not the stage to choose on price alone. When comparing concreters in your area, the relevant questions are whether they have poured shed slabs specifically, whether they work from an engineer’s design, and whether they will confirm squareness and edge detail against the kit specification before pouring. A concreter who wants to see the kit drawings before quoting is the one you want.

Stage three the kit and the frame

Trades: shed erector or builder, crane hire for larger structures.

The kit goes up. For a standard domestic shed this is usually a two to four day exercise with a small crew.

Points worth knowing.

Know what you are buying. Larger kits come from specialist manufacturers such as Ranbuild, Fair Dinkum Builds and Stratco, while smaller garden sheds from brands like Absco are sold off the shelf through retailers including Bunnings. Most steel sheds are rolled from BlueScope steel in either a Colorbond pre-painted finish or unpainted Zincalume. Colorbond costs more and looks better. On coastal sites, check the manufacturer’s recommendations for distance from breaking surf, because salt spray shortens the life of standard finishes.

Lead times are long and getting longer. Order early. A twelve week lead time is not unusual and it needs to sit in your programme rather than surprise you.

Wind classification drives the specification. Australian sheds are engineered to a wind classification derived from AS/NZS 1170.2, based on regional wind speed, terrain category, shielding and topography. Most of southern Australia sits in the non-cyclonic Region A, while the tropical north falls into the cyclonic Regions C and D. Cyclonic regions carry substantially more demanding requirements. A kit specified for the wrong classification is both unsafe and non-compliant, and it will fail an inspection.

Check the delivery against the schedule. Missing components discovered on erection day cost a full crew day.

Fixing down is where compliance lives. The connection between the frame and the slab is what resists uplift. It is worth watching that this is done to the engineered detail.

Stage four roofing and stormwater

Trades: roof plumber, gutter installer, sometimes a separate stormwater contractor.

On most kits the roof sheeting comes as part of the package and goes on with the frame. Guttering, downpipes and where the water actually goes is frequently treated as an afterthought, and it should not be.

A shed roof is a large catchment. On a decent sized shed you are collecting a significant volume in a storm, and dumping it at the base of the slab creates exactly the problem the earthworks stage was supposed to prevent.

Connect to stormwater properly, or to a tank, or to a designed soakwell. Do not just run a downpipe onto the ground beside the slab.

Working at height on a shed roof is genuinely dangerous and it is worth engaging people who do it routinely. Comparing Trade Heroes listings or any equivalent directory by category at least gets you businesses that do roofing rather than general trades taking it on.

Stage five services

Trades: electrician, plumber if applicable, data installer.

Power to a shed is licensed electrical work in every Australian state and territory, without exception, and it is more involved than people assume.

Running a submain from the house switchboard to a detached structure has specific requirements under AS/NZS 3000, the Wiring Rules, covering cable sizing, residual current protection, earthing and often a separate distribution board in the shed. Depth and mechanical protection requirements apply to the underground run. This is not a job for an enthusiastic amateur with a spade and a roll of cable, and an unlicensed installation will void your insurance and create a genuine hazard.

If you did not lay conduit under the slab at stage two, this is where you find out how much that decision cost.

Consider capacity rather than just connection. A shed used as a workshop with a welder, a compressor and a dust extractor has a different load profile from one with a light and a power point. Size it for what you will actually do. Our Bendigo couple’s electrician, once he had finished shaking his head about the conduit, made one very good call: he sized the submain for the welder and a future caravan hookup rather than for the two lights and four power points on the original plan.

Stage six fit-out and finishing

Trades: carpenter, insulation installer, concreter for aprons and paths, painter, door installer.

Insulation if the shed will be occupied for any length of time. An uninsulated steel shed in an Australian summer is unusable.

Roller doors and personnel doors, either supplied with the kit or fitted separately. Worth specifying the door for the actual use, because a door that gets used twenty times a day is a different product from one used twice a week.

Concrete apron at the entry, which prevents the slab edge eroding and stops mud being tracked in.

Racking, benches and storage, which sounds trivial but affects where the power points should have gone.

Budget and where it actually goes

A useful corrective for anyone budgeting from the kit price alone: the kit is frequently less than half the total.

Site preparation and earthworks vary enormously with the site. A flat, clear, accessible block is inexpensive. A sloping site needing cut and fill, retaining and improved access can rival the cost of the shed.

The slab is a substantial line and is driven by the engineered design rather than by the area alone. A reactive clay site requiring a stiffened raft costs considerably more than the same footprint on sand.

The kit is the visible number and the one people anchor on.

Erection if you are not doing it yourself, plus crane hire for larger spans.

Services which people routinely omit from the budget entirely. A submain run to a detached structure, a distribution board and a reasonable number of circuits is not a trivial cost, and it is the most commonly forgotten item.

Stormwater, aprons and access which also tend to be discovered rather than planned.

A reasonable planning assumption is to treat the kit price as roughly a third to a half of the delivered cost for a typical domestic shed with power. Sites with difficulty, or specifications with insulation, lining and multiple doors, move well beyond that.

Common mistakes that cost the most

Ordering the kit before approval is confirmed. Approval conditions change designs. A delivered kit that no longer suits is an expensive problem with no easy resolution.

No conduit under the slab. The single most common regret. Conduit is inexpensive at pour time and the alternative is trenching, cutting or surface-mounting later.

Slab out of square or wrong dimensions. Kits have no tolerance for this. Check diagonals before the pour and check them against the manufacturer’s setout, not the plan.

Stormwater as an afterthought. A large roof produces a large volume and it has to go somewhere designed.

Ignoring wind classification. The specification is derived from the site, not chosen from a catalogue. A kit rated for the wrong classification will not pass inspection and is genuinely unsafe.

Undersizing. Almost nobody regrets building a larger shed. A great many people regret the opposite, and extending later is disproportionately expensive.

The sequence in short

Approval and soil classification, then services located, then earthworks and drainage, then conduit and slab, then kit erection, then roof plumbing and stormwater, then electrical, then fit-out.

The three errors that cause the most pain are ordering the kit before approval is confirmed, pouring the slab without the conduit in, and leaving stormwater until the end.

Specifying for what you will actually use it for

The single decision that most affects satisfaction is one made before any trade is involved: what the shed is actually for.

Storage only. The simplest case. Standard kit, basic power, no insulation, roller door sized for whatever needs to go in.

Workshop. Changes almost everything. You need real electrical capacity rather than a single circuit, and enough circuits that a welder and a dust extractor are not on the same one. Lighting needs to be genuinely bright and positioned over benches rather than centrally. Insulation moves from optional to necessary, because an uninsulated steel shed is unusable for most of an Australian summer. Ventilation matters.

Vehicle storage. Door height and width become the governing constraint, and people consistently undersize both. Measure the actual vehicle including roof racks and allow real tolerance for approach angle.

Occupied space of any kind. Changes the regulatory picture significantly. Habitable structures face different requirements around insulation, ventilation, egress, and in many jurisdictions require full development approval rather than exempt development. Do not plan a habitable conversion informally.

The cost difference between a storage shed and a properly serviced workshop of the same footprint is substantial, and the upgrade is far cheaper to build in than to retrofit.

A note on doing it yourself

Plenty of people build their own sheds and plenty of them turn out well.

The parts you can reasonably do: site clearing, some of the earthworks if you can hire the gear, assembly of the kit with enough hands, insulation, fit-out, painting and internal joinery.

The parts you cannot: electrical and plumbing work, which is licensed everywhere. Asbestos removal if you are demolishing something old first. Anything requiring engineering certification.

And the part where self-building most often goes wrong is the slab. It looks like the simplest stage and it is the least forgiving, because errors are permanent and everything above depends on it. If you are going to spend money on one trade, spend it there.

As for the Bendigo couple, the shed got finished about six weeks later than planned and a little over budget, which by shed standards counts as a success. The first cold evening he spent out there with the radio on and the welder running, the conduit saga had already become a story told at the pub rather than a grievance. The next shed, he says, gets the conduit first.