Flood Risk & Drainage Requirements for Boston Borough Council

Boston Borough Council is the local planning authority for Boston and the surrounding fen villages, and few places in England put flood risk and drainage under a harder spotlight. The town sits at the tidal limit of the River Witham, on some of the lowest ground in the country, behind the Boston Barrier and miles of raised Haven banks. On almost any application here the council will expect a flood risk assessment, a surface-water drainage strategy, or both, and it takes advice from Lincolnshire County Council as the Lead Local Flood Authority before it decides.

We prepare flood risk assessments and drainage strategies for development across Boston Borough for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and sustainable drainage (SuDS) design. A flood risk assessment, a sequential or exception test and a drainage strategy are three different reports: one shows the site is safe, one shows whether the development belongs there, and one shows where the surface water goes.

If you want to talk a Boston site through or need a quote, contact one of our experienced flood risk consultants on 01293 214444 or at enquiries@unda.co.uk.

Policy 4 of the South East Lincolnshire Local Plan blocks development within 50 metres of the toe of the raised Witham Haven banks. It is a defence-specific rule that catches applicants who assume national flood policy is the whole story.

Flood risk and drainage in Boston: at a glance

• The local planning authority is Boston Borough Council. It validates and decides planning applications, and refuses them where the flood risk or drainage evidence falls short.
• The Lead Local Flood Authority is Lincolnshire County Council. It advises on surface-water drainage and SuDS on major development, and sets the discharge standards your scheme must meet.
• Other bodies are involved. The Environment Agency leads on the tidal Haven, the River Witham and the Boston Barrier; the Witham Fourth District and Black Sluice Internal Drainage Boards manage the pumped fen drainage; Anglian Water handles foul and public sewers.
• The development plan is the South East Lincolnshire Local Plan 2011–2036, adopted March 2019 and shared with South Holland District Council.
• Policy 4 does most of the work. It folds the sequential test, the site-specific flood risk assessment, SuDS and the Haven banks buffer into one policy.
• The evidence base is the 2017 Strategic Flood Risk Assessment, which maps the breach and overtopping hazard behind the defences.

A review of the Local Plan is under way, and under Greater Lincolnshire local government reorganisation Boston Borough Council may be replaced by a new unitary authority later this decade. The requirements set out below reflect the adopted plan and the current arrangements, and the substance of them will carry across to any successor council.

Who decides flood risk and drainage in Boston?

Boston is a two-tier area, and the single most common mistake applicants make is assuming one body handles everything. It does not, so knowing who to ask what, early, saves weeks.

• Boston Borough Council — the local planning authority. It receives, validates and determines applications, and it is the body that refuses a scheme where the flood or drainage evidence is missing. Planning policy is set jointly with South Holland District Council through the South East Lincolnshire Joint Strategic Planning Committee.
• Lincolnshire County Council — the Lead Local Flood Authority. On major development it is a statutory consultee on surface-water drainage, and an LLFA objection is one of the most common reasons a Boston drainage strategy stalls. As highway authority it also handles highway drainage. Our guide to the role the LLFA plays explains its weight.
• The Environment Agency. It leads on the tidal Haven, the River Witham as a main river, and the Boston Barrier, and its defences and their residual risk are a material consideration near the town. See the Environment Agency's planning role.
• The Internal Drainage Boards. The Witham Fourth District IDB and the Black Sluice IDB manage the ordinary watercourses and pumped drainage that keep the fens dry. Any discharge to a board's watercourse needs the board's consent, separate from planning permission, so they matter for drainage. See why IDBs matter for planning.
• Anglian Water. The sewerage undertaker for foul flows and the public surface-water sewer confirms whether the network can serve a development. Our note on water authorities sets out the split.

Is a flood risk assessment or drainage strategy required for a Boston application?

Because so much of Boston sits in Flood Zone 3, the flood risk assessment is the norm rather than the exception, and a surface-water drainage strategy is expected on almost anything that increases hard surfacing. Here is when each is typically triggered.

If you are not sure which reports your site needs, we will tell you free of charge. Our guides on when a flood risk assessment is required, flood zones 1, 2 and 3 and the right vulnerability classification are a good starting point, and getting the scope right first time is how you avoid an invalidated application.

Boston's Local Plan flood and drainage policies

This is where Boston becomes specific. The adopted South East Lincolnshire Local Plan 2011–2036 sets the local rules, and one policy carries most of the load.

Policy 4 — Approach to Flood Risk

Policy 4 permits development in an area at risk of flooding only where the sequential test is passed, a site-specific flood risk assessment covers risk from all sources including climate change, and the development is made safe for its lifetime without increasing risk elsewhere. The sequential test is applied across a Borough- or District-wide search within the defined settlement boundaries, and sites already allocated in the Local Plan, minor development and most changes of use are exempt. Three parts of the policy catch applicants out, and all three are specific to Boston.

• The Haven banks buffer. No development is permitted within 50 metres of the toe of the raised Witham Haven flood-defence banks (Appendix 10 of the plan), safeguarding access to maintain and raise the defences. On riverside sites this can quietly sterilise a strip of the plot.
• Danger hazard zones. The plan classifies land as "danger for some", "danger for most" and "danger for all" using the Strategic Flood Risk Assessment. Ground-floor residential accommodation is resisted in the higher hazard categories, and where a change of use is allowed, unrestricted access to a habitable room above the flood level and an evacuation plan are required.
• SuDS and drainage built in. Policy 4 requires SuDS unless they are demonstrably unfeasible, confirmation of how they will be maintained for the lifetime of the development, adequate foul-water treatment, and safeguarded access for maintaining drainage infrastructure. Surface water connects to the public sewer only in exceptional circumstances where no feasible alternative exists.

Policy 4 also requires development in all flood zones, and any site over one hectare in Flood Zone 1, to show surface water can be managed without increasing flood risk to third parties. In practice it rolls flood risk and drainage into a single test, which is why a Boston submission is rarely a flood risk assessment on its own. Where a more vulnerable use is proposed in a higher-risk zone, the sequential and exception tests come into play, and our note on how those tests work explains the difference.

Boston's drainage and SuDS requirements (Lincolnshire LLFA)

Drainage is the half of a Boston application that most often trips people up, because the ground works against you. The fens are flat, low and largely pump-drained, infiltration rarely works in the silt and clay, and Lincolnshire County Council as Lead Local Flood Authority sets clear expectations before it will support a scheme.

The starting point is the SuDS hierarchy: reuse or store runoff first, then infiltrate, then discharge to a watercourse at a controlled rate, with the public sewer only as a last resort. In much of Boston, soakaways are not viable because the ground does not drain and groundwater sits high, so most schemes attenuate runoff and discharge slowly to an ordinary watercourse or an IDB drain, which brings the relevant drainage board into the conversation.

The common reasons an LLFA objection lands in Lincolnshire are familiar: a discharge rate that is not properly justified, attenuation that ignores climate change, infiltration assumed without a soakage test, or no maintenance plan. We design drainage strategies against Lincolnshire's standards from the outset, using the same runoff calculations the LLFA will check. Our free geology and ground conditions map and BRE 365 infiltration rate calculator give an early read on whether the ground will take water, the drainage calculators help size the scheme, and our guides to the 2025 national SuDS standards, the SuDS proforma and whether you need infiltration testing set out what the LLFA will want. When a scheme reaches the detailed stage, we prepare the information to discharge drainage conditions, and a preliminary drainage design flags issues at outline stage.

The South East Lincolnshire Strategic Flood Risk Assessment

Boston's flood risk cannot be read off the national Flood Map for Planning alone, because the town is defended. The Strategic Flood Risk Assessment fills that gap.

The South East Lincolnshire Strategic Flood Risk Assessment (2017) maps flood depths, velocities and the "danger" hazard categories that Policy 4 relies on, and it models breach and overtopping scenarios behind the Haven banks and the Boston Barrier — the residual risk that remains even with the defences in place. For an applicant it does two jobs. It underpins the sequential test, so sites allocated in the Local Plan are treated as having passed it but still need a site-specific assessment to show they are safe for their lifetime. It also sets the hazard context a flood risk assessment must engage with. A Boston assessment that ignores the SFRA's breach mapping will not satisfy the council.

Local validation and required documents

Boston Borough Council validates applications against a local list, and flood risk and drainage are frequent reasons an application is held or returned.

In broad terms, expect to need a site-specific flood risk assessment wherever the flood zone or site size triggers it, a surface-water drainage strategy wherever runoff changes, and a sequential test where a more vulnerable use is proposed in a higher-risk zone. Discharge to an IDB watercourse needs board consent alongside the planning application. The quickest way to lose weeks in Boston is to submit a flood risk assessment and assume drainage is covered, or to submit a drainage strategy that has not been run past the LLFA's standards. Our note on the drainage information needed for planning sets out what a validating officer looks for.

How national policy applies in Boston

The local rules sit on top of the national framework, not instead of it.

The National Planning Policy Framework and its Planning Practice Guidance set the risk-based approach: steer development to the lowest-risk land through the sequential test, apply the exception test where necessary, and require a site-specific flood risk assessment in Flood Zones 2 and 3 and on sites of a hectare or more in Flood Zone 1. Climate-change allowances are applied to peak rainfall, river flow and sea level over the development's lifetime, which in a tidal town like Boston materially changes the numbers. Sustainable drainage follows the national standards for SuDS. Boston's Local Plan localises all of this, most visibly by running the sequential test within settlement boundaries and by adding the Haven banks buffer, but the national tests still have to be met. Our NPPF guidance explains how the pieces fit together.

Local flood risk in Boston's area

Boston's flood risk is genuinely multi-source, and the assessment has to read the sources together.

The dominant threat is tidal: the River Witham becomes the tidal Haven as it runs out to The Wash, and a North Sea surge caused the town's worst modern flood on 5 December 2013, when the banks were overtopped and more than 800 properties across 55 streets were flooded. Tidal flooding, fluvial risk from the Witham and its fenland drains, surface water on the flat ground and high groundwater all add to the picture, and much of the surrounding fen stays dry only because it is pumped. The Boston Barrier, operational since December 2020, now protects around 14,000 homes and 800 businesses to a high standard against tidal surge, but a defended town is not a risk-free one, which is why the council leans on breach and residual-risk assessment. The pressures on this landscape are set out in our piece on the limits of fen flood management, and level-for-level compensatory storage is often part of the answer on riverside sites.

For the full picture of how Boston floods — the watercourses, the history and the defences — see our flood risk assessment in Boston area page, and our wider Lincolnshire overview.

How Unda helps with Boston applications

We are experts on this authority's requirements and on the way Lincolnshire's LLFA and the local drainage boards work, and we deal with the Environment Agency, LLFAs and Internal Drainage Boards as a matter of routine.

We are a Crawley consultancy, not a local Boston firm, and we do not pretend otherwise. What we bring is the requirements knowledge: we have prepared flood risk assessments and drainage strategies across Lincolnshire and other heavily defended, low-lying areas. Every report is signed off by a senior flood risk consultant, and we scale the work to the actual risk so you are not paying for scope you do not need. Our chartered consultants (CIWEM members, including Chartered Water and Environmental Managers) prepare the flood risk assessment, design the SuDS scheme in industry-standard software, and stay involved past submission to answer the council's or the LLFA's questions before they turn into objections.

Flood risk and drainage services for every situation in Boston's area

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions, and we prepare all of them alongside the right form of assessment for buyers, lenders and BREEAM.

• Flood risk assessment for planning. The site-specific report for a Boston planning application, prepared to satisfy the council, the Environment Agency and national policy.
• Sustainable drainage strategy and SuDS design. Surface-water schemes built to Lincolnshire's discharge standards.
• Sequential and exception tests. The locational case where a more vulnerable use is proposed in a higher-risk zone.
• Groundwater flood risk assessment. For the high water tables common across the fens, assessed as a source within the flood risk assessment.
• Flood modelling. 1D and 2D modelling where the national mapping and the SFRA are not detailed enough.
• Property purchase and insurance assessments. The same rigour for conveyancing and underwriting, and for BREEAM certification.

You can screen a postcode on our free flood risk map, browse the full flood risk assessment service range or the wider local authorities guides, and every report is overseen by a chartered flood risk consultant.

Flood risk and drainage in Boston: frequently asked questions

Official Boston flood risk and planning sources

The most reliable information comes from the deciding bodies themselves.

• Boston Borough Council planning. Application requirements and the local validation list at boston.gov.uk/planning.
• The Boston Barrier. Background on the tidal defence at Boston Borough Council.
• South East Lincolnshire Local Plan. The adopted plan and Policy 4 at southeastlincslocalplan.org.
• Lincolnshire LLFA. Flood risk management and SuDS guidance at Lincolnshire County Council.
• National flood risk guidance. When an assessment is needed for planning at gov.uk, and live warnings at check-for-flooding.service.gov.uk.