Environmental · Mandatory · Pre-Construction

Surface Water Drainage Plan

A plan for managing surface water runoff using sustainable drainage systems (SuDS).

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. Environment Act 2021 / NPPG remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	Environment Act 2021 / NPPF (December 2024)
Status	Mandatory — NPPF requires SuDS for ALL developments
Prepared by	Civil Engineer
2025 Standards	National SuDS Standards: water quality + biodiversity + amenity
Statutory consultee	LLFA is statutory consultee for major developments

1. SuDS — The Default Approach for Surface Water Management

Sustainable drainage systems (SuDS) are now the default approach for managing surface water from all new developments in England. The revised National Planning Policy Framework published in December 2024 extends the requirement for SuDS to all developments, not just major schemes. This represents a fundamental shift in planning policy — surface water must be managed sustainably as a standard condition of development, and conventional piped drainage to sewers is no longer acceptable as the primary approach.

The National SuDS Standards, due to come into force in June 2025, will raise the bar significantly. The new standards go beyond hydraulic performance (flow rates and storage volumes) to require SuDS features that deliver measurable benefits for water quality, biodiversity, and amenity. Developers will need to demonstrate that their SuDS designs provide treatment for surface water run-off, create or enhance habitats, and contribute to the quality and usability of the public realm. Simple underground attenuation tanks that provide storage but no water quality treatment, biodiversity, or amenity benefit will no longer meet the standards.

The Lead Local Flood Authority (LLFA) is the statutory consultee for surface water drainage on major planning applications (10 or more dwellings, or sites of 1 hectare or more). The LLFA reviews the drainage strategy and advises the local planning authority on whether the proposals are acceptable. An objection from the LLFA on drainage grounds will usually result in the refusal of planning permission or the imposition of pre-commencement drainage conditions that must be discharged before development can start.

Design SuDS into the layout from day one

Integrate SuDS into the site layout from the earliest design stage — drainage affects building layout, landscaping, and levels.

2. Key Sections of the Plan

A compliant surface water drainage plan must address each of the following areas. The LLFA will assess the strategy against local drainage guidance, the NPPF, and the National SuDS Standards.

Section	What it covers
Site description	A description of the site location, topography, geology, ground conditions, existing drainage arrangements, and receiving watercourses or sewers. Soil permeability data from infiltration testing (BRE Digest 365 soakaway tests or falling head permeameter tests) is essential for determining whether infiltration-based SuDS are viable. The site description must also identify any constraints such as contaminated land, high groundwater, or proximity to sensitive receptors.
Drainage hierarchy	The drainage strategy must follow the drainage hierarchy set out in the NPPF and Building Regulations Approved Document H. Surface water should be discharged in the following order of priority: (1) infiltration to the ground (soakaways, permeable paving, infiltration basins), (2) discharge to a watercourse, (3) discharge to a surface water sewer, (4) discharge to a combined sewer. Each option higher in the hierarchy must be demonstrated to be unfeasible before the next option is considered. Evidence of infiltration testing is required to rule out infiltration.
Design storms	The drainage system must be designed to accommodate rainfall events up to and including the 1 in 100-year storm event plus a climate change allowance. The climate change allowance for peak rainfall intensity is currently 45% for the upper end estimate (used for sensitivity testing) and 40% for the central estimate in most of England. The design must demonstrate that no flooding occurs on the site for events up to the 1 in 30-year storm and that flooding in events up to the 1 in 100-year plus climate change storm is safely managed on site without affecting third-party land.
Pre-development greenfield run-off rate	The pre-development greenfield run-off rate must be calculated using a recognised methodology (typically the IH124 method or the FEH statistical method for the QBAR greenfield rate). Post-development discharge rates must be limited to the pre-development greenfield rate for all events up to and including the 1 in 100-year plus climate change storm. Where the pre-development site is brownfield, the LLFA may still require greenfield run-off rates to be achieved, or may accept a betterment over the existing brownfield rate.
SuDS features proposed	A description of the SuDS features proposed, including permeable paving, rain gardens, swales, detention basins, attenuation ponds, green roofs, rainwater harvesting, filter strips, and bioretention systems. Under the 2025 National SuDS Standards, the SuDS design must deliver water quality treatment (using the Simple Index Approach or equivalent), biodiversity benefits (habitat creation and enhancement), and amenity value (public open space, visual amenity, recreation). Underground attenuation tanks alone will not satisfy the new standards.
Hydraulic model	A hydraulic model of the proposed drainage system, typically prepared using industry-standard software such as MicroDrainage or InfoDrainage. The model must demonstrate that the system has adequate capacity for the design storms, that discharge rates are limited to the agreed rate, and that any on-site flooding during extreme events is safely contained and does not affect buildings or third-party land. The model outputs (network diagrams, long sections, flood volume summaries) must be included in the drainage submission.
Exceedance route	An assessment of exceedance flow routes for rainfall events that exceed the design capacity of the drainage system. The exceedance route analysis must demonstrate that surface water flows safely away from buildings, does not pond in areas where it could cause damage, and does not flow onto neighbouring land or the public highway. Exceedance routes must be shown on a site plan and must be designed to convey flows safely to a point of discharge.
Maintenance	A maintenance plan setting out the maintenance activities required for each SuDS feature, the frequency of maintenance, and the responsible party. The maintenance plan must cover the lifetime of the development and must be secured through a planning condition, section 106 agreement, or adoption by the local authority or water company. The LLFA will not accept a drainage strategy without a credible maintenance plan, because SuDS features that are not maintained will fail and cause flooding.
LLFA submission	The complete drainage strategy submission to the Lead Local Flood Authority, including all calculations, hydraulic model outputs, SuDS design drawings, infiltration test results, maintenance plan, and a covering report that demonstrates compliance with the NPPF, local drainage policy, and the National SuDS Standards. The LLFA will review the submission and provide comments to the local planning authority. Pre-application engagement with the LLFA is strongly recommended to identify any site-specific requirements before the formal submission.

3. Common Mistakes

Proposing sewer connection without demonstrating infiltration is infeasible

The drainage hierarchy requires infiltration to be the first option considered for surface water disposal. A drainage strategy that proposes connection to a surface water sewer or combined sewer without first demonstrating that infiltration is not feasible will be rejected by the LLFA. Infiltration feasibility must be demonstrated through site-specific infiltration testing (BRE Digest 365 soakaway tests at the proposed SuDS locations and depths). Simply stating that the ground is clay or that infiltration is not feasible without providing test data is not acceptable. The LLFA expects to see infiltration test results showing measured infiltration rates and a reasoned assessment of whether infiltration-based SuDS can be used on the site.

Not including climate change in the design storm

All drainage calculations must include a climate change allowance applied to rainfall intensity. The current central estimate climate change allowance for peak rainfall intensity is 40% in most areas of England. Drainage strategies that design the system for the 1 in 100-year storm without adding the climate change allowance are significantly under-designing the system. The LLFA will reject any drainage submission that does not include climate change allowances. The allowances must be applied to both the attenuation storage calculation and the discharge rate assessment, and the drainage system must be shown to function safely for the 1 in 100-year plus climate change event.

No maintenance plan or unclear maintenance responsibilities

A drainage strategy without a credible, funded maintenance plan is incomplete and will be rejected by the LLFA. SuDS features require regular maintenance to function correctly — permeable paving needs vacuuming and jet washing, swales need mowing and desilting, detention basins need vegetation management and inlet/outlet clearance, and all SuDS features need inspection after major storm events. The maintenance plan must specify the maintenance activities for each SuDS feature, the frequency of each activity, and the party responsible for carrying out the maintenance. The LLFA will require evidence that the maintenance arrangements are legally secured and funded for the lifetime of the development, whether through a management company, adoption agreement, commuted sum, or service charge.

4. Frequently Asked Questions

When did SuDS become required for all developments?▾

The revised National Planning Policy Framework published in December 2024 extended the requirement for sustainable drainage systems to all new developments in England, not just major developments. Prior to this revision, the NPPF required SuDS for major developments (10 or more dwellings, or sites of 1 hectare or more), and the decision on whether to require SuDS for minor developments was left to local planning authority discretion. The December 2024 NPPF now states that all developments should incorporate SuDS unless there is clear evidence that this would be inappropriate. The National SuDS Standards, which come into force in June 2025, will set mandatory minimum requirements for the design and performance of SuDS on all developments, including requirements for water quality treatment, biodiversity enhancement, and amenity provision that go beyond the existing hydraulic performance requirements.

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