Internal Drainage Boards (IDBs): what they do and why they matter for planning

An internal drainage board (IDB) is an independent public body that manages water levels and land drainage in a low-lying area at particular risk of flooding. England has 112 IDBs covering around 1.2 million hectares, roughly 9.7% of the country, mostly across the Fens, the Somerset Levels, and parts of Yorkshire and Lincolnshire. For anyone developing land in one of these districts, the local IDB is not a background body. It shapes how surface water is managed, what you are allowed to discharge, and whether your drainage design will be accepted.

This guide explains what IDBs are, what they do, how they are funded, and, most importantly for developers and planners, how they influence the planning process.

Bring the internal drainage board on side early and it rarely holds a scheme up. Discover its requirements after submission and it can force a costly redesign.

What is an internal drainage board?

An internal drainage board is a statutory public body responsible for managing water levels within a defined drainage district. IDBs operate the pumps, channels, sluices and embankments that keep low-lying land from flooding, and they regulate any works or discharges affecting the watercourses they maintain. They are one of several Risk Management Authorities recognised under the Flood and Water Management Act 2010, alongside the Environment Agency, lead local flood authorities (LLFAs) and water companies.

Each board covers a discrete internal drainage district: an area where the natural fall of the land is not enough to drain water away, so it has to be managed actively.

Outside its district, an IDB has no role, so the first question on any low-lying site is simply whether the land sits inside one.

What do internal drainage boards do?

IDBs manage water levels and reduce flood risk within their districts. In practice that means operating and maintaining drainage infrastructure, regulating works and discharges that affect their watercourses, and advising on development. Their remit covers flood risk, land drainage, and increasingly the ecology of the water systems they control.

• Water level management. Operating pumping stations, sluices and weirs, and monitoring rainfall, river flows and groundwater to keep levels within safe limits.
• Maintaining watercourses. Dredging, de-silting, vegetation clearance and embankment repair to keep channels flowing.
• Regulating works and discharges. Enforcing land drainage byelaws so that new development does not increase flood risk or overload the system.
• Environmental management. Working with bodies such as Natural England on wetland habitats, fish-friendly pumps and nature-based solutions.
• Working with partners. Coordinating with the Environment Agency, LLFAs, councils and water companies. In the Fens, IDBs are core partners in the Environment Agency's long-term Fens 2100+ strategy.

Are internal drainage boards public bodies?

Yes. IDBs are independent statutory public bodies, not private companies and not part of a council. Each is run by a board of members: some elected by the ratepayers who fund it, others appointed by the local authorities whose areas it covers. Each board is accountable for water level management within its own district. Their powers and duties come from the Land Drainage Act 1991, which consolidated earlier drainage legislation into the framework still in use today.

Because they are Risk Management Authorities, IDBs sit within the same statutory system as the Environment Agency and LLFAs. Planning authorities are expected to take an IDB's advice into account where development affects its district, which is what gives a board real influence over a scheme even though it operates at arm's length from the planning system. For the wider picture, see our guide to who is responsible for managing flood risk in England.

How are internal drainage boards funded?

IDBs are funded mainly by two local charges: drainage rates paid by agricultural landowners and occupiers within the district, and special levies paid by the local authorities whose urban properties benefit from the drainage system. Funding stays local to the area that benefits, rather than drawing on national flood budgets.

That local model has been under strain. The winter storms of 2023 to 2024, Storm Babet in October 2023 and Storm Henk in January 2024, badly damaged IDB pumps, embankments and channels. In response, the government set up a one-off IDB Fund, later expanded into DEFRA's IDB Storm Recovery and Asset Improvement Fund.

By March 2025 the fund had committed £53 million to more than 200 projects and was expected to support 91 of England's 112 boards, taking the total to around £91 million.

The money is repairing pumping stations, rebuilding flood embankments, de-silting ditches and installing remote telemetry (GOV.UK). It shows how much low-lying England still relies on this infrastructure, and how quickly a bad winter can overwhelm it.

A short history of internal drainage boards

IDBs are among the oldest institutions in English water management, with roots in the medieval drainage of marshland for farming. Large-scale drainage of the East Anglian Fens began in the 17th century. The Land Drainage Act 1930 first formalised the patchwork of local drainage bodies into a coordinated system, and the Land Drainage Act 1991 refined their roles and remains the primary legislation today.

Centuries on, the same districts still rely on constant pumping and maintenance, and wetter winters are making that job harder.

Internal drainage boards and the planning process

If your site falls within an internal drainage district, the IDB matters at two distinct points: as a consultee on the planning application, and as the body whose separate consent you may need for drainage works. Getting both right early is usually the difference between a smooth approval and a late, expensive objection.

Unlike the Environment Agency and the LLFA, an IDB is usually consulted on a non-statutory basis, yet where a development affects its watercourse its requirements still shape what the planning authority can approve.

Where a development discharges to, or otherwise affects, an IDB watercourse, the board's requirements on discharge rates and drainage design carry real weight, and an IDB objection is taken seriously. Our explainer on the role of lead local flood authorities sets out how the statutory side of that consultation works.

Do you need land drainage consent from an IDB?

Often, yes, and it is separate from planning permission. Within an IDB district you generally need the board's consent under its byelaws before you can alter a watercourse, discharge surface water into an IDB drain, or build close to a maintained channel. Planning permission does not remove that requirement. The two approvals run in parallel, and consent can be refused even where planning is granted.

• Altering a watercourse. Culverting, diverting or otherwise changing an IDB-maintained channel.
• Discharging surface water. Releasing runoff from a new development into an IDB drain, usually at a capped rate with any increase attenuated on site.
• Building near a channel. Construction close to an IDB watercourse that could affect its flow or maintenance access.

The principle is close to ordinary watercourse consent elsewhere in the country, and it interacts with riparian ownership where a watercourse runs through or alongside a site.

Internal drainage boards, SuDS and surface water

Where a development drains to an IDB system, the board will scrutinise the sustainable drainage (SuDS) proposals closely. It will expect runoff to be limited to agreed rates, volumes to be managed on site, and the scheme to be maintainable for the long term. In 2025 the government published new National Standards for Sustainable Drainage Systems, replacing the earlier non-statutory technical standards, and these now frame how SuDS should be designed and assessed.

The wider policy picture is still moving. The government has so far chosen to strengthen the planning-led approach to SuDS rather than commence Schedule 3 of the Flood and Water Management Act 2010, which would make SuDS approval mandatory through a SuDS Approving Body, the system already running in Wales since 2019. The 2025 Independent Water Commission recommended that England commence Schedule 3.

A home with sustainable drainage can send as little as 13% of rainwater into the sewer, compared with 100% from a home without it.

Whichever way that decision falls, IDBs will remain central to how surface water is controlled in their districts. Our guide to the SuDS hierarchy explains the design principles a board will expect to see applied.

Internal drainage boards, the Environment Agency and LLFAs: who does what

It is easy to confuse the bodies that regulate watercourses, but the split is straightforward. The Environment Agency is responsible for main rivers, the coast and large reservoirs. Ordinary watercourses, the smaller streams, ditches and drains, are regulated by the lead local flood authority, except inside an IDB district, where the IDB takes that role. So on the same development you might deal with the Environment Agency for a main river and the IDB for the ditches and drains its district maintains.

Why early engagement with an IDB matters

Because an IDB's requirements can reshape a drainage strategy, the cheapest time to involve the board is before the layout is fixed. Early engagement lets you confirm the district boundary, agree an acceptable discharge rate, and design SuDS the board will accept, avoiding the redesigns, objections and delays that follow from discovering a constraint after submission.

For sites in the Fens, the Somerset Levels, Yorkshire and Lincolnshire, treating the IDB as an early design partner rather than a late consultee is simply good project management.

Where a site also sits in a critical drainage area, that early conversation matters even more.

Frequently asked questions

If your site sits within an internal drainage district, the local board's requirements need to be built into your drainage design from the start, not bolted on after an objection. Unda prepares surface water drainage strategies and SuDS designs that meet IDB discharge standards, and supports the discharge of drainage planning conditions and land drainage consent process alongside your flood risk assessment for planning. For advice on a specific site, contact Unda for a no-obligation quote.