Farm development on units of less than five hectares
On an agricultural unit between 0.4 and five hectares, Class B permits extensions, hard surfaces, private ways and plant, but never the erection of a new building.
Part 6 Class B of Schedule 2 to the General Permitted Development Order 2015 applies to agricultural units of not less than 0.4 hectares but less than five hectares, and it is far narrower than the right that larger farms enjoy. The headline point: Class B does not allow you to put up a new agricultural building. Smallholders arrive at that sentence with some surprise, usually after the steel frame has been ordered.
Units of five hectares and above fall under Class A, which does permit new buildings within stated limits. Below 0.4 hectares there is no agricultural permitted development at all, and everything needs an application.
What Class B covers and the size band it applies to
The unit, not the field, sets the threshold. An agricultural unit is the land occupied as a unit for the purposes of agriculture, which can include several detached parcels in the same occupation. Add them up before deciding which class applies.
Within the 0.4 to five hectare band, Class B permits a defined list of works carried out on agricultural land comprised in the unit and reasonably necessary for the purposes of agriculture within that unit:
- The extension or alteration of an existing agricultural building.
- The installation of additional or replacement plant or machinery.
- The provision, rearrangement or replacement of a private way.
- The provision, rearrangement or replacement of a hard surface.
- The deposit of waste material produced on the unit.
- Certain operations connected with fish farming.
Every item on that list is qualified by conditions and exclusions set out in the Order itself, and the ground area and height caps in Part 6 were amended in 2024. Read the current consolidated text rather than a summary written before those changes.
The buildings Class B does not permit
There is no power under Class B to erect a building. An extension to something that already exists, yes. A new hay barn, a new stable block, a new machinery store on bare land, no.
The distinction between an extension and a new building is one of fact. A structure attached to an existing building by a corridor, or added to one end of it, is usually an extension. A separate structure standing a few metres away and sharing only a yard is a new building however the drawings label it. Councils and inspectors look at physical and functional integration rather than at the application form.
Class B also excludes dwellings entirely. Agricultural workers' dwellings need a full application and an agricultural need assessment, and no part of Part 6 provides a route to residential use. Conversion of an existing agricultural building to a house is a different right altogether, and our note on Class Q barn conversions after the 2024 amendments sets out how that one works.
Extensions and alterations to an existing building
The Order limits the resulting building by ground area and by height, and reduces the height allowance within three kilometres of an aerodrome perimeter. It also restricts works in the vicinity of protected buildings where livestock accommodation or slurry storage is involved, using a distance measured from the building in question.
| Question to settle first | Why it matters |
|---|---|
| Is the unit between 0.4 and five hectares? | Below 0.4 there is no right; at five or more, Class A applies instead |
| Does an agricultural building already exist? | Class B permits extension and alteration only |
| Does the resulting building stay within the Part 6 caps? | Exceeding either the ground area or height limit removes the right |
| Is any part within a protected distance of a dwelling? | Livestock and slurry provisions restrict development near protected buildings |
| Is the site in a National Park, National Landscape or conservation area? | Article 2(3) land carries extra restrictions and, in some cases, removes the right |
Cumulation catches people out. The Order measures the resulting building against other works carried out nearby within a defined recent period, so a sequence of modest extensions can exhaust the allowance without any single one appearing to.
Hard surfaces, private ways and waste
These three rights do useful work on a smallholding and are often overlooked in favour of the building that is not permitted anyway.
A private way can be provided, rearranged or replaced where it is reasonably necessary for agriculture on the unit. That covers a new access track to a field gate, and it is the right that most often needs a prior notification because of its siting and means of construction.
Hard surfacing is permitted on the same basis, which is how a muddy gateway becomes a usable yard apron lawfully. Waste deposit is narrow: material produced on the unit, subject to restrictions, and it does not authorise importing spoil from elsewhere. Importing inert waste to raise levels is a separate operation needing permission and usually an environmental permit or exemption.
The prior notification step
Class B is conditional, not automatic. For the works that require it, the developer must first apply to the local planning authority for a determination as to whether prior approval will be required for the siting, design and external appearance of the building, or for the siting and means of construction of a private way.
The authority then has 28 days from receipt to tell you whether approval is required. Begin before that period expires, or before the authority responds, and the work is not permitted development at all. The process is procedural rather than a merits test, but it is a real hurdle, and our explanation of how the prior approval procedure actually works covers the traps that apply across every class.
Send drawings, a location plan with the whole unit outlined, and a short statement of agricultural need. A notification that leaves the officer guessing will simply attract a request for approval.
Proving the unit is agricultural
Agriculture is defined in section 336 of the Town and Country Planning Act 1990, and the definition excludes some things people assume are included. Grazing is agriculture. Keeping and breeding horses for riding is not, so a stable block on a paddock is almost never permitted development under Part 6.
Evidence that persuades an officer includes a holding number, business records, stocking numbers, sales receipts, a herd or flock register, and a straightforward account of what the building will be used for. The reasonably necessary test is doing real work here. A building must be reasonably necessary for agriculture on that unit, so a machinery store sized for a contracting business run from a five-acre holding will fail.
Where small holdings come unstuck
Four recurring problems, in rough order of frequency.
- The unit is under 0.4 hectares, so no right exists and the work is unauthorised from the first day.
- The proposal is a new building dressed up as an extension.
- The use is equestrian or hobby keeping rather than agriculture.
- Work starts before the 28-day notification period has run.
The remedy in each case is an application, which is also the moment a Planning Statement setting out the agricultural need earns its keep. A lawful development certificate is the only formal way to confirm that works already carried out were lawful, and buyers' solicitors increasingly ask for one on smallholdings.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Enter your postcode on PlanScope to see what a farm building proposal is likely to need.
Frequently asked questions
Can I build a new barn on a smallholding under permitted development?
Not under Part 6 Class B. On a unit of between 0.4 and five hectares the right covers extension or alteration of an existing agricultural building, plant and machinery, private ways, hard surfaces and limited waste deposit. A new building on bare land needs planning permission, whatever its size or purpose.
What counts as the agricultural unit for the size test?
The land occupied as a unit for the purposes of agriculture, which can include several detached parcels in the same occupation. Add them together before deciding whether Class A or Class B applies. Land occupied separately, or held only under a short grazing licence, may not count, so check the occupation arrangements carefully.
Do stables for horses qualify?
Usually not. Section 336 of the Town and Country Planning Act 1990 treats grazing as agriculture but excludes the keeping and breeding of horses for riding. A field shelter for grazing stock may qualify, while a stable block serving a livery or private riding use will need planning permission in its own right.
How long does the prior notification take?
The local planning authority has 28 days from receipt of a valid notification to say whether prior approval is required. Starting work before that period expires, or before the authority has responded, takes the development outside the permitted development right entirely and leaves it open to enforcement action.
What happens if I have already built something without permission?
Apply for a lawful development certificate if you believe the work was permitted development, or a retrospective planning application if it was not. Enforcement time limits exist but should not be relied on as a plan. Purchasers and lenders increasingly require a certificate before completing on a smallholding.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.