General purpose farm buildings
The shed with no fixed job, put up to take whatever comes. Tell us roughly what you need and you get engineered drawings and a price — plus an honest figure for the base, which is the one part of it you cannot change afterwards.
- Common span
- 12–20 m
- Pick for the most demanding use you can see, not the first one.
- Typical slab
- 175 mm
- A middle spec. Go to 200 mm if machinery is likely.
- Sub-base
- 150 mm
- Compacted Type 1, spread around 190 mm loose.
- Design rule
- Worst case
- Spec for the most demanding use it might see, not the first one.
What you are actually buying
The building that gets put up for one job and ends up doing four. Storage this year, stock next winter, a workshop in the corner after that. Because the use moves, the sensible approach is to leave the frame able to take whatever comes — which mostly means a bit more height, a door in the right place, and a floor specced for the heaviest thing you can imagine standing in it.
Span for the worst case
You cannot widen it later without taking the side off. Pick the span that works for the most demanding use you can see coming, not the one you have in mind today.
Length is the easy change
Bays at 6 metre centres, and extending along the length is the least disruptive way to grow — provided the gable end you would extend from is not the one you built the concrete apron and the tank against.
Height buys you options
Extra eaves height costs a little more steel and cladding and keeps every future use open: tipping, stacking, lifting, housing. It is the cheapest flexibility on the quote.
What you get asked to decide
- Doors at both gables, so something can go straight through rather than be reversed out
- Sheeted, part-clad or open on one side, depending on what winters in it
- A dividing wall or panel run, so one end can be dirty and the other clean
- A lean-to now or the room to add one later on the side you leave clear
- Supplied for your own team to erect, or supplied and erected
There is one more choice underneath all of those: whether the frame is cold rolled or hot rolled. It changes the price, the lead time, whether you can put it up yourself, and what goes in the ground.
How it goes from here
Five steps, in this order. The fourth is the one nobody prices for you up front, which is why there is a calculator further down this page. The full steel building groundworks calculator for Cornwall and Devon explains every figure it gives you.
Step 1
Tell us roughly what you need
Span, length and what it is for. Approximate is fine — nobody expects you to have decided, and it means the first reply can be useful rather than a list of questions.
Step 2
Drawings and a price for the frame
The manufacturer prepares engineered drawings and quotes the building. That includes the pad foundation sizes, which no calculator can work out for you.
Step 3
Settle the planning position
Prior approval on a qualifying agricultural holding, or a full application. Worth establishing before steel is ordered rather than after.
Step 4
Get the base in
Site strip, level, stone and slab, ready and cured before the erectors arrive. This is the part you can price on this page today.
Step 5
Delivery and erection
Delivered for your own team to put up, or supplied and erected. Both are normal; the difference is worth asking about at quote stage, not later.
Rough dimensions are enough to start. You get a reply the same day.
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.
Part 6 of the General Permitted Development Order splits at five hectares, and the two halves are not the same right with different numbers on them.
On a unit of five hectares or more, Class A permits the erection, extension or alteration of a building that is reasonably necessary for agriculture on that unit. The ground area of the building cannot exceed 1,500 square metres. That figure rose on 21 May 2024 — it was 1,000 square metres before then and 465 square metres before April 2018, so an older guide will understate it. A separate cap of 1,000 square metres applies to structures for accommodating livestock and to plant arising from engineering operations.
On a unit of at least 0.4 but under five hectares, Class B does not let you put up a new building at all. This is the part most often got wrong. Class B permits the extension or alteration of an agricultural building you already have — along with plant and machinery, private ways, hard surfaces and services — but there is no erection right anywhere in it. The extension is capped at 1,250 square metres of ground area and at a 25 per cent increase in the original building's cubic content, with no increase in height. If you have four hectares and no building to extend, a new shed is a full planning application.
Where Class A or B does apply, you are not applying for planning permission. You apply to the local planning authority for a determination as to whether prior approval is required for the siting, design and external appearance — currently £249 — and they have 28 days. If they do not respond inside 28 days you may proceed, provided the work genuinely is permitted development in the first place.
Two exclusions catch people. No building or structure for accommodating livestock or storing slurry or sewage sludge is permitted within 400 metres of the curtilage of a protected building — broadly a dwelling that is not on your own unit. And since 21 May 2024, nothing can be built under Class A or B on land that is, or is within the curtilage of, a scheduled monument.
On protected land the rights are not usually removed, which is a common misunderstanding. What changes inside a National Park is that the prior approval procedure applies more widely — to every extension or alteration rather than only to significant ones. In Cornwall and Devon that means Dartmoor and Exmoor. Cornwall's National Landscape is a different designation and does not take Class A or B away, though an Article 4 direction or a condition on an earlier permission can, and both are worth checking on your own holding.
A summary, not planning advice, and England only. The legislation is Part 6 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended — most recently on 21 May 2024. Figures checked against legislation.gov.uk in September 2026. Confirm your own position with the authority that covers your site.
Work out the base for your building
Set to a general purpose specification at 175mm, which puts a 12 by 20 at around 45.5 cubic metres of concrete. 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 go with it. The building itself is designed and priced by South West Steel Buildings, our building partner, so your details go to them too, and they will be in touch. 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.
- What span and height should a general purpose building be?
- Both should be set by the most demanding use you can see coming rather than by this year’s job. You cannot widen a building later without taking a side off, and you cannot raise the eaves at all — whereas adding length in 6 metre bays is comparatively easy, provided you have not built the concrete apron and the tank against the gable you would extend from. Extra eaves height is the cheapest flexibility on the quote: it costs a little more steel and cladding and keeps tipping, stacking, lifting and housing all on the table.
- 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.
- Can I add a lean-to or extend it later?
- A lean-to is usually the cheapest square metre you will ever buy, and extending along the length is the least disruptive way to grow a steel building. Both are far easier if they were thought about at the design stage: leave one side clear of the tank, the apron and the drainage runs, and tell the supplier at quote stage that a lean-to may follow, because it can change how that side of the frame is designed.
- 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, provided the holding qualifies on size. 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.
- Do I need planning permission for a farm building?
- It depends on the size of the holding, and the answer is not the same for both classes. On an agricultural unit of five hectares or more, Class A of Part 6 of the GPDO permits the erection, extension or alteration of a building reasonably necessary for agriculture on that unit, up to a ground area of 1,500 square metres — raised from 1,000 on 21 May 2024, and 465 before April 2018. On a unit of at least 0.4 but under five hectares, Class B does not permit a new building at all. It allows only the extension or alteration of a building you already have, capped at 1,250 square metres and at a 25 per cent increase in the original cubic content with no increase in height. A new building on a holding that size needs full planning permission. Where Class A or B does apply you make a prior approval application to the local planning authority, currently £249, and they have 28 days to tell you whether prior approval is required.
- Will putting this building up affect a barn conversion later?
- It can, and it is worth knowing before you commit. Erecting or extending an agricultural building under Class A or Class B blocks a Class Q conversion of a building on that established agricultural unit for ten years, and the reverse also applies — if a Class Q conversion has been carried out on the unit in the previous ten years, you cannot use Class A or B to put up or extend an agricultural building. If a barn conversion is anywhere in your thinking, sequence the two rather than discovering the conflict afterwards.
Other buildings we cover
Same approach on each: what the building is, how it gets sized, where you stand on planning, and what the base underneath it comes to.
Machinery stores
Clear span, no posts in the way, and a floor that works harder than most. What it costs to put up and what it costs underneath.
Grain stores
Walls that get pushed from the inside, a floor that takes a loader, and a roof that rains on the crop if you get it wrong.
Livestock buildings
Housing that has to ventilate, drain and be scraped out. Falls, channels and the slurry rules — plus the 400 metre planning trap.
Farm workshops
Room to get a machine in and work all round it, with a floor specced for the jack rather than the tractor.
Hay barns
Open sides, air through it, and the lightest slab on the farm — if it needs one at all.
Stables and arenas
Stable blocks, American barns and indoor arenas. The same steel frame as a farm building — and a completely different planning position.
Garages and workshops
Steel framed garages, home workshops and man caves at the house. The eaves limit that decides whether you need planning permission, and the floor underneath.
Industrial units
Workshops, warehousing, trade counters and light manufacturing. Full planning, full Building Regs, and a yard that often costs more than the shed.
Get a price for your farm building
Rough span and length is enough to start with. Say what you think it will be used for and what it might end up being used for — both change the answer. You get engineered drawings and a price, including the pad foundation sizes.