Agricultural units: what Part 6 means by a unit

Part 6 of the Town and Country Planning (General Permitted Development) (England) Order 2015 gives agricultural permitted development rights to the unit, not to the field. The agricultural unit definition for permitted development is agricultural land occupied as a unit for the purposes of agriculture, together with any farmhouse or farmworker's dwelling on it occupied for farming that land. Occupation is the test. Ownership is not.

That single definition decides which class you are in. Units of five hectares or more get Class A, which permits new buildings reasonably necessary for agriculture. Units below five hectares get Class B, which permits extensions and works to what already exists and no new buildings at all. The gap between the two is wide enough that farmers buying extra land to cross the line is a familiar story in rural planning offices.

The definition that controls the rights

Three terms do the work, and they stack.

Agriculture takes its meaning from section 336 of the Town and Country Planning Act 1990, which brings in horticulture, fruit and seed growing, dairy farming, the breeding and keeping of livestock, the use of land as grazing, meadow, osier land, market gardens and nursery grounds, and woodland use ancillary to farming. Keeping horses for riding, training or livery falls outside it, which is why equestrian buildings so often need a planning application while a hay barn does not.

Agricultural land, for Part 6, is land in use for agriculture immediately before the development, used for the purposes of a trade or business, and it excludes any dwellinghouse and its garden. So the garden of the farmhouse is not part of the agricultural land even though it sits in the middle of the farm.

The agricultural unit is then the agricultural land occupied as a unit for agriculture. A tenant farmer occupying 40 hectares across three landlords has one unit of 40 hectares. An owner of 12 hectares who has let 8 of them on a farm business tenancy occupies 4, and is in Class B territory. The planning unit is a different question again, and the two concepts get muddled in correspondence more often than they should.

The five hectare line and how land is counted

Measure the unit, not the parcel where the building is going. Five hectares is 12.36 acres, and holdings sit near that line often enough that the measurement needs to be done properly on a plan rather than taken from a sale particular.

Count the agricultural land in your occupation on the day before the development is carried out. Exclude the farmhouse and its garden. Exclude land you own but have let to someone else. Include land you rent in, including seasonal grass keep where the occupation is real and documented. Exclude woodland unless its use is ancillary to the farming of the rest.

A grazing licence raises the hardest question. Where the licensee has exclusive occupation of the field for most of the year, that field is part of their unit rather than yours. Where you keep the grass, take the hay cut and simply allow stock on for a few weeks, the occupation stays with you. Get the agreement out and read what it actually says before relying on the area.

Separate parcels, and whether they form one unit

Non-adjoining parcels can form a single unit when they are farmed together as one enterprise. Shared labour, shared machinery, one set of accounts, stock moving between blocks and a single holding number all point that way. Parcels run as distinct businesses, with separate stock and separate accounts, point the other way even where one person owns both.

Class A then adds its own constraint on where you can build. Development is not permitted on a separate parcel of land forming part of the unit which is less than one hectare in area. A farm of 50 hectares with an isolated half hectare paddock three miles away cannot use Class A on that paddock, however large the rest of the holding is. Authorities check this on an aerial image in about a minute, and it catches people out on fragmented holdings.

Class A carries the rest of the familiar limits: a ground area cap of 1,000 m² for the building, taking into account other buildings and works on the unit within 90 metres put up in the preceding two years, a height limit of 12 metres and 3 metres within 3 kilometres of an aerodrome, no part within 25 metres of the metalled part of a trunk or classified road, and restrictions on livestock buildings and slurry storage within 400 metres of a protected building. Dwellings are excluded throughout.

The trade or business test

This is where small holdings fail. Part 6 needs the land to be in agricultural use for the purposes of a trade or business, and a few sheep kept for pleasure on three hectares is not a trade or business however genuine the husbandry.

There is no statutory profit test and no minimum turnover. What authorities look for is the ordinary indicia of a business being run on the land. The building also has to be reasonably necessary for the purposes of agriculture within that unit, which is a functional test about the enterprise you actually have, not the one you plan: the same phrase appears in the barn conversion rules and the arguments about what counts as reasonably necessary run along similar lines.

A 400 m² machinery store on a four hectare holding with two ponies invites a visit from an enforcement officer. The same store on a 40 hectare arable unit does not.

New holdings and the restrictions that come with them

Buying bare land and immediately claiming Class A rights is the most common mistake. The land must already be in agricultural use for a trade or business before the permitted development is carried out, so a newly acquired field with no established enterprise supports nothing until the farming is real and documented.

Dividing an existing farm has the opposite effect: it can create two units, each below five hectares, each losing Class A. Where a holding is being restructured for succession or sale, work out the permitted development consequences before the transfers are signed.

Class A buildings also come with a condition that outlasts the construction. Where the building is erected under Class A and the use of that building for agriculture permanently ceases within ten years of substantial completion, and planning permission has not been granted for another use, the building must be removed and the land restored. That condition is the reason authorities take an interest in barns that acquire roller shutter doors and a van rental sign.

The larger developments listed in Class A need prior notification. You apply for a determination as to whether the authority's prior approval is needed for siting, design and external appearance, then wait 28 days or for the response. The authority cannot refuse the building in principle through that route, which is the central difference between prior notification and a planning application.

Evidence of agricultural use and intensity

Assemble this before you build rather than after an enforcement enquiry.

• The holding number issued for the land and the records held against it
• Farm business accounts, and the trading history behind them
• Livestock movement records, flock or herd registers and medicine books
• Invoices for seed, feed, fertiliser, contracting and veterinary work
• Tenancy agreements, grazing licences and grass keep agreements with dates and areas
• A plan showing every parcel in occupation with its area in hectares

The same evidence base answers the question when the issue is a conversion rather than a new building, where proving a building was in agricultural use is the first obstacle.

Where the unit falls below the threshold

Class B still gives something on units under five hectares: extensions and alterations to existing agricultural buildings within size limits, replacement plant and machinery, hard surfaces, and works for accommodating agricultural plant. New buildings are not available. And Part 6 gives nothing at all on the smallest holdings, because Class B does not apply where the unit is less than 0.4 hectares, so read the current text of Class B before relying on anything there.

Below the line the route is a planning application. On a small holding that means a Planning Statement explaining the enterprise, the functional need for the building, its siting and its landscape effect, usually with a design and access case if the site is in a National Landscape or a conservation area. National Park authorities and the Broads operate modified rights, so check the Order's text for designated land rather than assuming the national position.

A lawful development certificate under section 192 is the only formal way to confirm that proposed works are lawful without permission, and on holdings near either threshold it costs less than removing a building.

Enter your postcode on PlanScope to see which reports a farm building or diversification scheme is likely to need.

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.