How to check a new estate's drainage

Updated 20 October 2026

A new estate's drainage is set by its planning permission: the conditions on the decision notice, and the drainage design the council approves before building starts. All of it is public, on the council's planning website, and anyone can read it.

What a drainage condition is

Planning permission comes with conditions. A drainage condition usually says that building may not start until the council has approved a detailed scheme for surface water, the rain that falls on roofs and roads, and often that no home may be lived in until the drainage serving it has been built. The developer applies to "discharge" the condition by sending in drawings and calculations. The council asks the lead local flood authority, which in West Sussex and Hampshire is the county council, whether the design is good enough, and then approves or refuses it.

Where to find the documents

• The main application. Search the council's planning website for the site's address or reference. The decision notice lists the conditions. The flood risk assessment and drainage strategy explain the design.
• Discharge of condition applications. Each condition is usually signed off in its own application (Chichester gives these a "DOC" reference). They hold the final drawings, the calculations, the flood authority's comments and the council's decision.
• Committee papers. If councillors decided the application, the officer's report and the minutes are on the council's committee website.
• An appeal. If the permission was granted on appeal, the inspector's decision and conditions are on the Planning Inspectorate's site.

How to read the numbers

• Discharge rate. The litres a second that may leave the site, usually through a flow control valve in a manhole (a Hydro-Brake is a common make). It is compared with "greenfield runoff": what the field gave off before it was built on, worked out per hectare.
• The area it is worked out on. Check whether the per-hectare rate is applied to the whole site or only to its roofs and roads. The answer changes the total a lot.
• The design storm. The usual test is a storm with a 1 in 100 chance in any year, made 40% or 45% heavier to allow for climate change. The calculations show how high the water rises in each basin and pipe in that storm.
• Basin sizes. Drawings give each basin's area at the bottom, its area at the top and its depth. Its volume is roughly the depth times the average of the two areas. A "detention" basin is dry most of the time and fills only in heavy rain; a "retention" pond holds water all year.
• Groundwater. If the ground is wet in winter, the design should say how high the water table rises, from monitoring over winters, and how basins and pipes are protected from it, for example by lining them.
• Who maintains it. The maintenance plan says who looks after the basins, pipes and ditches, and the estate's management company may charge for it. Sewers and some drains are adopted by the water company; the rest stay private.

What a verification report is

Some councils ask the developer, once the drainage is built, to send a verification report: surveys and photographs showing the basins, pipes and flow controls were built as approved. Where a condition asks for one, it is filed on the planning website like any other document. Where a condition does not, its own wording is the test, such as "no building shall be occupied until" the drainage serving it is in place.

Worked example: Highgrove, Bosham

Highgrove is 300 homes on land north of Highgrove Farm, Main Road, Bosham. Permission 21/00571/FUL was granted on appeal on 9 November 2023, and the homes are being built under 24/01012/FUL, granted on 15 August 2024, which carried the drainage conditions across.

• The condition. Condition 4 of 24/01012/FUL says: "No development shall commence until details of the proposed overall site-wide surface water drainage scheme has been submitted to and approved in writing by the local planning authority", and "No building shall be occupied until the complete surface water drainage system serving that property has been implemented in accordance with the approved surface water drainage scheme." It does not ask for a separate verification report.
• Sign-off. The council discharged condition 4 on 6 May 2025 (application 25/00205/DOC), after consulting West Sussex County Council as the lead local flood authority. The county had not supported earlier versions of the design.
• The basins. The approved scheme has four lined, dry detention basins along the A259 frontage, about 1 metre deep (the first is 1.2 metres). The largest measures 2,980 square metres at the bottom and 3,808 at the top. Underground storage crates and permeable paving with a sealed base hold more water. The design storm is 1 in 100 years plus 45% for climate change.
• The outflow. All surface water leaves through one outfall into the ditch on the western boundary. A Hydro-Brake limits it to 10.3 litres a second. That is the field's average-year greenfield rate in the flood risk assessment, 1.94 litres a second per hectare, applied to the 5.32 hectares of roofs and roads, not to the whole 14.6-hectare site.
• Groundwater. Winter monitoring from 2016 to 2024 recorded groundwater as close as 0.16 metres below the ground, in 2023, on the western boundary. The basins are lined with a clay liner, and the crates and paving are sealed; the committee report says the basins "will necessarily need to be lined to prevent groundwater ingress".
• Maintenance. The approved maintenance plan gives the four basins to the estate's management company.

If something looks wrong

• Ask the council's planning officer named on the application, or the county's flood risk team for surface water.
• Anyone can report a suspected breach of a planning condition to the council's planning enforcement team; Chichester publishes an enforcement register.
• Report sewer problems to the water company, which for this coast is Southern Water.
• If you are buying, ask your conveyancer to raise questions with the developer about drainage, adoption and the management charge before you exchange.

Sources

• Chichester District Council planning website: 21/00571/FUL, 24/01012/FUL (decision notice, condition 4) and 25/00205/DOC (decision of 6 May 2025, approved drawings and calculations, winter groundwater monitoring, maintenance plan, flood authority responses). Basin sizes are from the drainage construction plan 6813.583 on 26/00323/FUL.
• Appeal decision APP/L3815/W/23/3322020, Planning Inspectorate, 9 November 2023.
• Planning Committee, 9 November 2022: officer's report, paragraph 8.33.
• Chichester District Council planning enforcement register.
• Documents were read on 20 October 2026. The council's direct document links expire, so each link above opens the application's documents list.