Drainage Requirements at Each Planning Stage

At outline, the authority is asking one question about drainage: can this site be drained at all? Not how, exactly — whether. The layout may not be fixed, and unit numbers may still move.

Typically needed:

• A drainage strategy in principle — the proposed method of surface water disposal and its justification against the drainage hierarchy
• A flood risk assessment, where the site is in Flood Zone 2 or 3, is one hectare or more, or has other flooding sources
• An indication of the likely discharge point and rate
• Enough storage estimation to show the required attenuation can physically fit on the site

Usually not needed yet: detailed pipe design, manhole schedules, or construction details. Percolation testing is sometimes worth doing early anyway — if infiltration turns out to be impossible, that changes how much land the drainage will occupy, and you would rather know before the layout is fixed.

With a full application the layout is fixed, so the drainage can be designed properly against it. Authorities vary in how much they want up front — some expect a complete design, others accept a strategy and impose a condition for the detail.

Typically needed:

• A surface water drainage strategy with supporting calculations for the required return periods and climate change allowance
• Percolation test results where infiltration is proposed
• A foul drainage strategy, and a foul drainage assessment (FDA1) where there is no public sewer connection
• A drainage layout drawing showing runs, storage, controls and outfall
• A flood risk assessment where the site triggers one
• Highway access and visibility details where a new or altered access is proposed

Submitting the strategy at application stage rather than accepting a condition is usually the cheaper route overall. A condition adds a separate application, a fee, and up to eight more weeks before you can start.

Where outline permission is already granted, reserved matters is where the detail lands. The principle of development is settled; what is being approved now is layout, scale, appearance, access and landscaping — and drainage has to work with all five.

Typically needed: a detailed drainage design consistent with whatever the outline strategy promised. This is where problems surface if the outline work was optimistic — if the outline said infiltration and testing later shows clay, the attenuation has to fit into a layout that was drawn without space for it.

A separate application to satisfy conditions already attached to a permission. Most drainage conditions are pre-commencement, meaning you cannot lawfully start until they are discharged.

Typically needed: exactly what the condition asks for and no more. The wording matters — a condition asking for a scheme “based on sustainable drainage principles” is asking for the hierarchy to be evidenced, not just a pipe layout. We work from the condition wording rather than a standard package.

Allow eight weeks for determination on top of the time to produce the information.

Related: example drainage conditions and what they require

Smaller works often need less, but not nothing. A single extension rarely needs a full drainage strategy, though it may still need a soakaway designed to BRE 365 if it adds impermeable area, and building regulations apply to the drainage regardless of whether planning does.

Where a site sits in a flood zone, permitted development rights can also be restricted, so it is worth checking before assuming no application is needed.