General purpose farm buildings
The shed with no fixed job, which is exactly why the floor is hard to specify. It will end up doing something you have not thought of yet — and the base is the one part you cannot change afterwards.
- Typical slab
- 175 mm
- Sub-base
- 150 mm
- Concrete, 12 × 20 m
- 45.5 m³
- Design rule
- Worst case
A middle spec. Go to 200 mm if machinery is likely.
Compacted Type 1, spread around 190 mm loose.
At 175 mm. Around 52 m³ at 200 mm.
Spec for the most demanding use it might see, not the first one.
The building that absorbs everything
Every farm has one. It goes up for fodder, then the trailer lives in it over winter, then it holds fertiliser, then somebody puts the sprayer in the end bay. Nobody planned any of that, and on most holdings it ends up the hardest-worked building on the yard.
That is a problem for the floor, because a floor is a decision you make once. Everything else about a steel frame building can be changed later — cladding, doors, dividing walls, even the span if you are prepared to extend. The slab is under the building for its whole life.
Spec for the worst case, not the first case
The rule that saves the most money over the life of the building is to specify the floor against the most demanding thing it might realistically hold, rather than the thing you are putting in it this year.
175mm with mesh is a fair middle specification for genuinely mixed use. If there is any real chance of tractors and loaded trailers turning inside, take it to 200mm and treat it as a machinery store floor.
6.5 m³
The difference between 175 mm and 200 mm on a 12 × 20 m building. That is the entire cost of not having to make the decision correctly first time.
Put the two figures in front of the person paying for it and the decision usually makes itself. Change the slab thickness in the calculator below to see both.
Form the falls while you can
The same logic applies to drainage. If the building ever holds stock, or gets washed out, or stores anything you would not want sitting in standing water, it needs a fall and somewhere for the water to go.
Forming a fall towards a door while the slab is being laid costs almost nothing. Cutting a channel into a finished floor, with a building over it, is a different job entirely. If you are unsure whether you will need it, put the fall in.
Planning follows the use, not the shed
Permitted development under Part 6 covers buildings reasonably necessary for the purposes of agriculture on the agricultural unit. A genuine general purpose farm building normally satisfies that comfortably.
Where it gets awkward is the flexibility that makes the building useful in the first place. If it ends up storing plant for a contracting business, or goods for something unconnected with the holding, that is a change in use rather than a change in the building — and the test attaches to the use. Worth being straight about at the prior approval stage.
The usual conditions apply otherwise: Class A on units of five hectares or more, Class B on units of 0.4 to under five hectares, a prior approval determination on siting, design and external appearance decided within 28 days, a floor area limit that was amended in 2018 and again in May 2024, and restrictions on protected land.
A summary, not planning advice. The legislation is Part 6 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended. Talk to Cornwall Council about your own site.
Work out the base for your building
Set to a general purpose specification at 175mm. Move it to 200mm to price the machinery option before you decide.
What this does not work out: the pad foundations
The concrete pads the stanchions sit on are sized by a structural engineer, not by a formula. They change with the span, the eaves height, the wind loading on your particular site and what the ground will bear. Anyone quoting you a pad size without a design has guessed it.
Want this priced properly?
Send us the job and we'll get you engineered drawings and a quote for the frame — with the pad sizes this tool can't work out. The figures above go with your enquiry, so you don't have to type them again.
Every enquiry gets a reply the same day, and your figures are sent with it. We are not a steel building manufacturer — we pass your details to a steel building supplier so they can price the job and contact you. How we handle your details.
Common questions
- What is a general purpose building?
- A steel frame shed put up without a single fixed job — somewhere to store fodder this year, machinery next year, and fertiliser the year after. Most farms have one, and on many farms it is the building that gets used hardest because it absorbs whatever there is no other space for.
- How do you spec a floor for a use you have not decided?
- Against the most demanding use it might realistically see, not the first one you have in mind. 175mm with mesh is a reasonable middle specification, but if there is any real prospect of tractors and loaded trailers turning inside it, take it to 200mm. The extra concrete on a 12 by 20 metre building is roughly six or seven cubic metres. Relaying a floor with the building already over it costs many times that.
- Should I put drainage in even if I do not need it yet?
- It is worth thinking about. If the building ever holds stock or gets washed down, it will need falls and somewhere for the water to go, and those are far cheaper to form while the slab is being laid than to cut into it afterwards. A fall to a door and a channel outside costs very little at the pour and saves a great deal if the use changes.
- Does a general purpose building qualify for permitted development?
- It has to be reasonably necessary for the purposes of agriculture on that agricultural unit — which a genuine farm building normally is. Where it gets harder is if the building is used for something not connected to the holding, such as storage for another business. That is worth being straight about at the prior approval stage rather than later, because the condition attaches to the use rather than the structure.