Part 6 Class B: farm buildings on units under 5ha
Class B of Part 6 covers agricultural units of less than 5 hectares, and it does not permit a new building. What it does allow, the size and proximity limits, and how prior notification works.
If your agricultural unit is smaller than 5 hectares, you cannot put up a new farm building under permitted development. That is the single most useful thing to know about Part 6 Class B of Schedule 2 to the General Permitted Development Order 2015. Class A, which does allow new agricultural buildings subject to limits and prior notification, applies only to units of 5 hectares or more. Below that threshold, Class B permits extensions and alterations to buildings you already have, plus a defined list of smaller works, and a new building needs a full planning application.
Smallholders discover this late, usually after buying a steel frame kit. The 5 hectare line is measured across the agricultural unit, which can include separate parcels in the same occupation, so the first job is to establish the area properly.
Where the 5 hectare line falls
An agricultural unit means agricultural land occupied as a unit for the purposes of agriculture, and it can comprise land in more than one parcel provided the land is in the same occupation and functions together. Rented land counts where the tenancy is genuine and the land is farmed with the rest.
Class A applies where the unit is 5 hectares or more. It permits, within limits, the erection, extension or alteration of a building reasonably necessary for the purposes of agriculture within the unit.
Class B applies where the unit is less than 5 hectares. It permits the extension or alteration of an existing agricultural building, the installation of replacement or additional plant or machinery, the provision or replacement of a private way, the creation of a hard surface, the deposit of waste and works for water supply, all subject to their own limits and conditions.
There is also a hybrid trap. Where a holding of 5 hectares or more is broken up, or where a parcel is sold off and the remaining area drops below the threshold, the rights change with the area. Check the current extent of the unit before designing, not the extent at the time you bought.
What Class B actually permits on a smaller unit
Read the list literally, because it is narrower than the summary suggests.
- Extension or alteration of an existing agricultural building, subject to the size and height limits below.
- Provision, rearrangement or replacement of a private way, meaning a farm track rather than an access onto the public highway.
- Creation of a hard surface, which covers yards and standing areas.
- Installation or replacement of plant or machinery, including some tanks and structures.
- Deposit of agricultural waste on the unit, within limits.
- Works for the provision of a water supply or sewerage within the unit.
What is not there: a new building of any size. Also missing is anything for a use that is not agriculture, which is why equestrian buildings for anything beyond grazing fall outside Part 6 entirely. Our guide to when horses become a business sets out where that line sits.
Size, height and proximity limits
The limits on Class B come from the same family as those in Class A, and the ones that bite most often are:
| Limit | Position |
|---|---|
| Ground area | The building as extended must not exceed 1,000 m² |
| Height, general | No part above 12 metres |
| Height, near an aerodrome | No part above 3 metres within 3 kilometres of the perimeter of an aerodrome |
| Distance from a road | No part within 25 metres of the metalled part of a trunk or classified road |
| Livestock and slurry | Not within 400 metres of the curtilage of a protected building |
| Dwellings | No dwelling permitted under any part of Part 6 |
The 400 metre rule catches small units hardest, because a smallholding is often close to houses. A protected building means, broadly, a permanent building normally occupied by people, with defined exclusions, and the distance is measured to its curtilage rather than to its wall.
Class B also caps how much an existing building can be enlarged, measured against the cubic content of the original building. Read the current text of Class B.1 before you draw anything, because the cap is the provision most often misremembered and the Order has been amended repeatedly.
Additional restrictions apply on designated land, including National Parks, the Broads, National Landscapes and conservation areas, and permitted development rights can be withdrawn by an Article 4 direction or by a condition on an earlier permission. A site within a Site of Special Scientific Interest loses several of these rights outright.
The prior notification procedure and the 28 day period
Prior notification is not required for everything. It applies where the works cross the threshold the Order sets for a significant extension or alteration, defined by an increase in the cubic content of the original building beyond a stated percentage, or an increase in its height. Smaller works proceed without notification, subject to the conditions on the right.
Where notification is required, the procedure runs like this:
- Apply to the local planning authority for a determination as to whether prior approval is required for the siting, design and external appearance of the building.
- Provide a written description of the development, the materials to be used, a plan showing the site, and the fee.
- Wait. The authority may notify you that prior approval is not required, may give prior approval, or may refuse it.
- If 28 days pass from receipt of the application without the authority notifying you of its determination, you may proceed in accordance with the details submitted.
- Start within the period specified in the Order, and build exactly what was described.
That fourth step is why accurate submitted details matter. Where the 28 days expire in silence, the details you sent become the approved scheme, and a building that differs from them is unauthorised. Attach the drawing you intend to build from.
Evidencing that the building is reasonably necessary for agriculture
The right applies to development reasonably necessary for the purposes of agriculture within the unit. Officers do test this, particularly on small holdings where a large building looks like storage for something else.
Assemble the evidence with the notification rather than waiting to be asked:
- The county parish holding number for the unit.
- A plan of the holding with areas, tenure and cropping or grazing use marked.
- Stock numbers, or tonnage and machinery to be housed, with the arithmetic that leads to the floor area.
- Accounts or a business plan where the enterprise is new.
- An explanation of why an existing building cannot serve.
A building sized around a stated need is defensible. A building sized around what fits the concrete pad is not, and the same reasoning applies whether you are extending a shed for hay or scaling up a livestock enterprise, which we look at in poultry units, scale and odour.
Conditions attached to the right, and what happens later
Part 6 rights carry conditions that outlast the building work. Development must be carried out in accordance with the approved or submitted details. The building must be used for agriculture, and using it for something else is a breach rather than a change of use you can drift into.
There are also provisions dealing with what happens when agricultural use stops within a period after the building is erected, which can require removal and restoration of the land. If your medium-term plan involves converting the building to something else, understand that a building erected under Part 6 sits awkwardly with later conversion rights, and get advice before relying on that route.
Where the answer is that you need a full planning application, the case is usually straightforward in principle and contested on siting and scale. A Planning Statement setting out the functional need, the alternatives considered and the landscape approach does most of the work, with a Landscape and Visual Impact Assessment where the site is prominent or designated.
One honest uncertainty: authorities differ on how strictly they test agricultural need on very small units, and some treat anything under a few hectares as hobby farming. Where your unit is close to 5 hectares, establishing the area with a measured plan is the least expensive piece of evidence you will produce.
Enter your postcode on PlanScope to see which reports your site is likely to need before you commit to a design.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Can I build a new agricultural building on 4 hectares?
Not under permitted development. Class A of Part 6, which allows new agricultural buildings subject to limits, applies only to units of 5 hectares or more. On a smaller unit Class B permits extensions and alterations to existing buildings plus tracks, hard surfaces, plant and water works, so a new building needs a full planning application.
How is the 5 hectare threshold measured?
Across the agricultural unit, which is land occupied as a unit for agriculture and can include several parcels in the same occupation, including rented land where the tenancy is genuine. Measure the current extent rather than the extent when you bought, because selling off a parcel can change which class applies.
When does Class B need prior notification?
Where the works cross the threshold the Order sets for an extension or alteration, defined by an increase in the cubic content of the original building beyond a stated percentage or an increase in its height. Smaller works proceed without notification, but the conditions attached to the right still apply to them.
What happens if the council does not respond in 28 days?
You may proceed in accordance with the details you submitted. That makes the submitted description, materials and plan the approved scheme, so anything built differently is unauthorised. Submit the drawing you actually intend to build from, with materials stated precisely, rather than an outline sketch.
How close to a house can I put a livestock building?
Part 6 excludes buildings for livestock accommodation or slurry storage within 400 metres of the curtilage of a protected building, broadly a permanent building normally occupied by people. The distance runs to the curtilage rather than the wall, which on a small holding often rules out the obvious location.
Can I convert a Part 6 building to a house later?
Buildings erected under agricultural permitted development sit awkwardly with later residential conversion rights, and the conditions attached to Part 6 include provisions about removal if agricultural use stops within a defined period. Take advice before building something on the assumption it can be converted.
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.