Parts 6 and 7 of the Town and Country Planning General Development Order 1995 gave farmers the right to construct or extend agricultural buildings without requiring a formal planning process to be followed providing that certain qualifications were met. This is known as Permitted Development.
Now the good news ends there. Although no planning process is necessary in the full sense of the words the local authority retains the right to "concern" themselves with :
Now fish tanks and cages crops up an awful lot in the legislation but from now on I am just going to exclude all references to fish in order to keep your interest.
Even though the legislation gives you permitted development rights you still have to make an application to the Planning Authority telling them what you are going to build so that they can decide whether or not your project qualifies. This is termed an   "Application for Prior Notification of Proposed Agricultural or Forestry development – proposed building" ( aka 28 Day Notice - more on this later).   Even if it does qualify and cannot therefore be refused in principle, they will still be able to dictate how the building is sited and the materials used.
This is most likely to become intrusive in National Parks and Areas of Outstanding Beauty, in these locations the an "Article 4 Direction" may be imposed.
While permitted development rights exist everywhere it is possible for the Local Authority to use Article 4 to remove those rights and force a full planning application in sensitive locations such as National Parks and AONB's. However it should be noted that you cannot be made to pay a full planning fee! All that can be asked for is the prior notification fee which is a lot lower.
YOU MUST PUT IN A PRIOR NOTIFICATION APPLICATION BEFORE YOU START WORK!
This is a good time to cover an old chestnut: "I am knocking down an old building and replacing it so I don't need planning permission". Wrong. Actually you need planning permission to significantly alter an existing building and it doesn't get much more significant than total demolition. Technically you cannot tear down a building without planning permission anyway. OK lets move on...
So does my project qualify as permitted development?
Is it sited on agricultural land for the purposes of agriculture as a business? This is a prerequisite under the legislation and technically horses and hobby farming do not qualify but implementation of this rule varies widely. However your first answer will always be treated as final so prudence would suggest that you do not try to submit a prior notification for "pony stables" but more for hay and straw storage. Whether it is a business does not depend on the turnover but on whether you can show proper accounts of the trading, profitable or otherwise.
Is the holding greater than 5 hectares (12.3553 acres) AND is the parcel on which the building will be sited over 1 hectare (2.47acres)? If not then it does not qualify HOWEVER while the site must be 1 hectare the rest of the holding does not have to be in one block but can be in seperate parcels and you don't have to own all of the land: rented land is counted to the total. So if you don't have enough go out and rent some.
If your site is less than 1 hectare but bigger than 0.4 hectare you may have rights to extend existing buildings up to 10% but not to erect new agricultural buildings.
Are you anywhere near an AERODROME. The building or structure must be less than 12 metres high (39'.4") BUT less than 3 metres (10') if within 3 km of an aerodrome and unlikely as it may seem this has caught many people out.
Is the building for housing livestock (or slurry) AND sited within 400m of any non agricultural building ie someones house (obviously not your own farmhouse duh.) If so full planning is required. DO I need to say anything about how you describe the purpose of the building in your application?I thought not.
So far we have covered the factors which will include or exclude you from permitted development rights, lets move on to how big a building you can have under permitted development rules.
Your proposed agricultural building can cover a ground area up to 465 square metres (5005 sq ft) including all structures excluding fences so canopies are included in this total contrary to popular belief. Its not my fault that I am right and your mate in the pub is wrong about canopies.
You can only apply for up to 465sq metres every two years. So if you build 265 square metres this year you can extend it by a further 200 sq metres next year but no more. However the following year you can apply for a further 465 sq metres. Just a note that if you are lucky enough to own more than one registered holding then the 465m limit is per holding.
Hardstanding areas are potentially an bit of an issue here: the legislation is very clear in that if a hardstanding is used for livestock handling then it is included in the 465 sq m however REGARDLESS of purpose it is used to calculate total build area for the two year cycle. This means that if you build a 265 sq metre building with a 100 sq metre concrete yard area for general operational use then the following year you can build only up to 100 sq metres. It also means that if you submit plans for a 465 sq metre building with 100 sq metre feed or handling yard then you will be outside the qualification size. So the best course is to submit a prior notification for everything you want to build in the next two years in one go and DO NOT include livestock handling hardstanding areas in your notification.
Now then - lets deal with those among us with smartarse tendencies:- No you can't build a mansion with tennis courts, stick a few sheep in it and tell the council its an agricultural building. It will go horribly wrong, you will lose in Court and you will be down hundreds of thousands of pounds and every one will secretly laugh at you. Forget about it - as far as the Courts are concerned an agricultural building looks like an agricultural building. It does not look like a five bedroom executive residence with a goat in it.   Believe me I can show you at least two gorgeous houses with the windows boarded up because no-one can live there legally.
The next disappointment to any "under the radar" property developers out there: you should be aware of the local authority's ace in the hole to control unwanted development on the pretext of agricultural use:-
"Subject to the completion of parliamentary procedures, if a building or extension erected under specified agricultural permitted development rights on or after 1 April 1997 permanently ceases to be used for agriculture within ten years of its substantial completion, and planning permission has not been granted authorising development for purposes other than agriculture within three years of the permanent cessation of its agricultural use, and there is no outstanding appeal, the building or extension must be removed unless the local planning authority has otherwise agreed."
This means if you got planning for agricultural use then you can't immediately rent the building out for use as a workshop under the new class M change of use regulations, you have to use it for agriculture for 10 years. Please note that the default position is demolition if the Council do not agree to change of use unless you have got 10 years use under your belt. which is dangerous in the extreme for the Machiavellian amongst you.
To summarise: If your building falls under the permitted development rules then you just submit a brief description and plan of your intentions, on the official Prior Notification form, with the inevitable small fee. Here's a tip - DO NO POST your application, either do it online or take it into the council offices and get a dated receipt!
Now just wait 28 days for the council to tell you whether they agree. If they don't agree, or you don't qualify, then you have to follow a more expensive and drawn out planning process but it is all entirely do-able especially if you get some professional help .
What happens if they don't respond in the 28 days? well you can just go right ahead and build but make sure you stick to what you submitted plans for.
What is far more likely is that they wait 21 days and then ask you for more time. They cannot insist on this but if you say no then they may just turn the whole thing down - you will then have to challenge and you will win, but only after a few years of foot dragging. If you are asked for more time a good strategy is ring them, insist on speaking to the person who has written to you and make sure they understand how important this all is and how good and sympathetic you are being by allowing them more time.
Finally if you are trying to establish a completely new farm on a parcel of land then jolly good luck to you. I can tell you that is possible but not easy. I would point you in the direction of David Acreman's website and book "From Field to Farm". We are all used to getting information for free these days and Mr Acreman stands out in requiring you to buy his book up front but if you need the information then the price is dirt cheap!
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