Farm reservoirs and the Reservoirs Act threshold
The volume above natural ground level decides whether a farm reservoir carries registration and panel engineer duties for life. How planning, abstraction and waste rules interact with it.
Farm reservoir planning involves three separate regimes, and the one that decides the design is not the planning system. A reservoir designed to hold more than 25,000 cubic metres of water above the natural level of the surrounding land is a large raised reservoir under the Reservoirs Act 1975, which brings registration, panel engineer supervision and a permanent inspection regime. Below that figure the Act does not apply. So the volume you choose sits on one side or the other of a line that changes the cost of owning the thing for the next forty years.
Provisions in the Flood and Water Management Act 2010 would lower the threshold in England to 10,000 cubic metres, and they have not been brought into force. Check the current position before you size a reservoir close to either figure.
Whether a reservoir needs planning permission
Forming a reservoir is an engineering operation, so it is development. The question is whether it is permitted development.
On an agricultural unit of 5 hectares or more, Class A of Part 6 of Schedule 2 to the General Permitted Development Order 2015 permits excavations and engineering operations reasonably necessary for the purposes of agriculture within the unit. An irrigation reservoir serving the holding can fall within that, subject to the limits in the class, and where the excavation covers more than half a hectare an application for a determination on siting is required.
Three things take a reservoir outside the right. Water supplied off the unit or sold, because then it is not for agriculture within the unit. A unit of less than 5 hectares, where Class B does not permit new excavations of this kind. And any of the general limits being exceeded, including the standoff from a trunk or classified road and the restrictions that apply on designated land.
Where the right is not available, a full planning application follows. The principle is usually accepted on functional need grounds, and the argument is about landscape, hydrology and construction effects, so a Planning Statement setting out the water balance and the alternatives does most of the work.
One consequence of needing permission rather than relying on Part 6 is that the biodiversity gain condition applies unless an exemption is claimed. On an arable field the metric baseline is usually low, so the gain is achievable, but it has to be planned and the plan approved before construction starts.
The capacity threshold and what follows
| Capacity above natural ground level | Position under the Reservoirs Act 1975 |
|---|---|
| More than 25,000 m³ | Large raised reservoir: registration and the full regime |
| At or below 25,000 m³ | Outside the Act, though common law liability remains |
Measure capacity as the volume held above the natural level of any part of the surrounding land, which is not the same as the total volume of water. A reservoir dug into a slope can hold far more water than its raised volume, and that distinction is where the design work lies for anyone wanting the storage without the regime.
Do not design to 24,900 cubic metres and stop thinking about it. The Act catches a reservoir capable of holding more than the threshold, so freeboard, spillway level and the possibility of raising the embankment later all matter to the assessment. Get the engineer to state the raised capacity explicitly in the design record.
Registration, inspection and the duties of an undertaker
Where the Act applies, the owner or operator is the undertaker and carries continuing statutory duties. In England the enforcement authority is the Environment Agency.
The duties in outline are these. Register the reservoir. Appoint a qualified civil engineer from the relevant panel to supervise construction and certify it, and a supervising engineer to keep the reservoir under surveillance. Arrange inspections by an inspecting engineer at intervals not exceeding those the Act sets, and carry out any measures in the interests of safety that the engineer directs. Keep records, and where the reservoir is designated as high-risk, prepare a flood plan.
None of this is nominal. Panel engineer fees, inspection costs and any works directed in the interests of safety are real and recurring, and they transfer with the land. A purchaser's solicitor should be asking about them.
Abstraction and impounding as separate regimes
Filling the reservoir is regulated separately from building it. Abstraction of water above a small daily quantity needs an abstraction licence from the Environment Agency, and most winter storage schemes are licensed on the basis that water is taken at high flows and stored for summer use.
Two points that catch farms out. Licence availability is catchment-dependent, and in a catchment where no water is available for further licensing, a reservoir with no means of filling it is an expensive hole. Establish the licensing position before the planning application, not after.
And building a structure across a watercourse to raise or store water is impounding, which needs its own licence. A reservoir filled by pumping from a borehole or a river under an abstraction licence is a different proposition from one formed by damming a stream, and the second is much harder to consent.
Landscape and hydrological evidence
Applications turn on two evidence bases.
The hydrological case shows the water balance: irrigated area, crop demand, the licensed abstraction quantity and period, the storage volume, and the residual risk in a dry year. Show the arithmetic. Officers and the Agency both read it, and a scheme where the storage far exceeds the demonstrated demand invites the question of whether water will be sold.
The landscape case addresses the embankment, which is the part people underestimate. A raised reservoir in flat arable country is a new landform visible for a considerable distance, and the mitigation is shaping, grading and planting rather than screening after the event. Where the site is prominent or within a designated landscape, a Landscape and Visual Impact Assessment is proportionate, and the earthwork profile should be designed with the landscape architect rather than handed to them.
Large excavations also disturb archaeology. Expect a request for an Archaeological Desk-Based Assessment, and in a sensitive area a field evaluation or a watching brief secured by condition. Councils expect a suitably qualified author for this work and professional review is strongly advised.
Soil handling and the fate of excavated material
A reservoir generates a large volume of material, and what happens to it is a planning matter, a waste matter and an agricultural one.
Deal with it in a soil management approach set out with the application. Strip topsoil and subsoil separately, store them in shallow bunds to preserve structure, and reinstate them in the right order on the embankment and any regraded ground. Handling soil wet destroys it, so the method statement should tie earthworks to ground conditions rather than to a programme date.
Material taken off the excavation footprint may be waste in law, and moving or using it can need an environmental permit or an exemption unless it is being used in a way the regulator recognises. Establish the position before the contractor arrives, because retrospective advice on soil placement is expensive. Where the land is also affected by mineral or waste safeguarding, the picture gets more complicated, as our note on mineral and waste safeguarding on farmland explains.
Conditions councils commonly impose
Expect most of these, and offer them rather than resisting:
- A landscape and planting scheme with a maintenance period.
- Restriction of the reservoir to agricultural use on the holding, preventing water sales.
- A construction management and traffic routing arrangement, because reservoir construction means many lorry movements on rural lanes.
- Soil handling in accordance with an approved method statement.
- Archaeological work before or during construction.
- Details of the spillway, overflow and any discharge, to be agreed.
- Fencing or other safety measures where public access is nearby.
One honest uncertainty: the threshold reduction in the 2010 Act has been pending for years and could be brought into force, which would pull many existing farm reservoirs into the regime. If you are sizing a new one between the two figures, take engineering advice on what compliance would cost if the lower figure came into effect.
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Frequently asked questions
What makes a reservoir a large raised reservoir?
Under the Reservoirs Act 1975 it is one designed to hold, or capable of holding, more than 25,000 cubic metres of water above the natural level of any part of the surrounding land. Provisions in the Flood and Water Management Act 2010 would reduce that figure in England but have not been brought into force.
Can a farm reservoir be permitted development?
On an agricultural unit of 5 hectares or more, Class A of Part 6 permits excavations and engineering operations reasonably necessary for agriculture within the unit, which can include an irrigation reservoir. An application for a determination on siting is needed where the excavation exceeds half a hectare, and water supplied off the unit takes it outside the right.
Do I need a licence to fill it?
Abstraction above a small daily quantity needs an abstraction licence from the Environment Agency, and most winter storage schemes are licensed on the basis of taking water at high flows. Damming a watercourse to store water is impounding and needs its own licence, which is harder to obtain.
How is capacity measured?
As the volume held above the natural level of any part of the surrounding land, not the total volume of water. A reservoir dug into a slope can hold much more than its raised volume. The Act catches a reservoir capable of holding more than the threshold, so freeboard and spillway level matter to the assessment.
What ongoing duties come with the Act?
Registration, appointment of a panel engineer for construction and certification, a supervising engineer keeping the reservoir under surveillance, periodic inspections by an inspecting engineer, any safety measures directed, record keeping, and a flood plan where the reservoir is designated high-risk. The costs recur and transfer with the land.
Does biodiversity net gain apply?
Where the reservoir needs planning permission rather than falling within permitted development, the biodiversity gain condition applies unless an exemption is claimed. On arable land the metric baseline is usually low, so the gain is achievable, but the gain plan must be approved before construction starts.
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.