Phasing drainage across a multi-phase site
An approved drainage scheme usually describes the finished site. On a phased development the interim conditions are where discharges and pollution incidents happen. What to draw, condition and document.
A phased drainage strategy has to work in every intermediate state, and the finished site is the easy part. The approved scheme usually describes the finished site: full attenuation volume, established swales, a constructed outfall and a landscaped surface. For two or three years before that exists, the site has bare earth, a larger effective runoff coefficient than the finished development, temporary haul roads and often no outfall at all. That interim condition is where discharges go wrong and where most pollution incidents on housing sites happen.
So the strategy needs two sets of drawings. The ultimate condition, which the calculations are based on, and the condition at the end of each phase, which is what the site actually builds.
Why phasing breaks a single site strategy
Four assumptions in a single-phase strategy fail as soon as the site is divided.
Storage is sized for the whole catchment, so a basin serving phases one to four may sit in phase four. The flow control is set for the ultimate discharge rate, which is wrong for a partially built catchment. Permeable surfaces and planted systems need an establishment period that construction traffic destroys. And the outfall, which is the one element nothing works without, is frequently in the last parcel to be developed because it is at the low point near the watercourse.
Add to that the ownership question. On a site assembled from several titles, or sold in parcels to different housebuilders, the drainage network crosses boundaries that will not all be in the same control at the same time. Easements and build obligations have to be in place before the first phase discharges anything.
Interim attenuation during construction
Construction stage water needs managing as a separate exercise with its own design. The usual package is a cut-off ditch or bund at the head of the works, temporary settlement lagoons, silt fencing on the downslope boundary, and a controlled discharge to the eventual outfall or to the existing drainage.
Four points decide whether it holds.
- Size the lagoon for residence time rather than for volume alone. Silt needs time to settle, and a lagoon that passes the design storm in twenty minutes passes the silt with it.
- Separate clean and dirty water. Roof water from completed plots and runoff from the untouched part of the site should be routed around the works rather than through the settlement system.
- Keep the discharge point consistent with what has been agreed. A temporary discharge to a different receptor usually needs its own permission.
- Plan the handover from temporary to permanent systems phase by phase, including who decommissions the lagoon and when.
Excavation dewatering is a separate consent question, not part of the surface water strategy, and we set it out in construction dewatering and the consents you actually need.
Outfall capacity available to each phase
Work backwards from the outfall. Three situations arise and each needs a different answer.
Where the outfall exists and has confirmed capacity, the phasing question is simply how much of the permitted discharge rate each phase may use. Set that out as a table of permitted rates per phase and keep the sum at or below the agreed total.
Where the outfall is to a public sewer, the undertaker's capacity assessment and the connection agreement normally relate to the whole development. Confirm whether a phased connection is acceptable and whether the agreed rate can be drawn down incrementally, because some undertakers will only permit the connection once.
Where the outfall does not yet exist, the first phase needs an interim route with its own consent, and the agreement to build the permanent outfall needs to bite before the phase that contains it starts. The design of the outfall itself, including bank stability and scour, is its own piece of work, covered in outfall design, bank stability and scour protection.
The arithmetic also needs doing at the interim condition. Phase one discharging at its share of the final rate, through a flow control sized for the final flow, will discharge faster than intended because the control is not operating at design head.
Conditions tying phases to drainage delivery
Expect the permission to secure delivery through a cluster of conditions, and expect to negotiate the triggers. The wording that works on a phased site has four features.
A phasing plan approved as a drawing, so the word "phase" has a defined meaning rather than a commercial one. Drainage details required phase by phase, with the submission for a phase allowed to come forward while an earlier phase is under construction. A trigger tied to a stage the site can evidence, such as development above slab level within the phase, rather than to a stage that is ambiguous. And a verification requirement per phase, with the final verification for the whole site at the end.
Two drafting traps recur. A condition requiring the whole site's drainage to be constructed before first occupation of any dwelling will stop the first phase, and it is sometimes imposed by accident on a scheme the officer has not read as phased. And a condition requiring the attenuation basin before any development above slab level is unworkable where the basin sits in a later parcel, so ask for interim provision to be an approved alternative.
Where a flow control may be partially obstructed in the interim condition, the modelling implications are the same as in the permanent case, and the blockage scenario should be run for the interim catchment as well as the final one.
Adoption boundaries between phases
Adoption is where phased sites accumulate liabilities nobody has priced.
Foul and surface water sewers offered for adoption under section 104 of the Water Industry Act 1991 are adopted on completion of the works to the undertaker's standard, which means a phase can sit with a privately maintained network for a long period. Surface water components that are not sewers, such as basins, swales and permeable pavements, usually fall to a management company or to the lead local flood authority where it adopts, and the arrangement for each phase should be identified on the phasing drawing.
Schedule 3 to the Flood and Water Management Act 2010 provides for a separate SuDS approval regime with automatic adoption of approved systems. Its provisions depend on regulations bringing them into force, and the position should be verified rather than assumed, because a change there would alter the adoption route mid-programme on a long site.
Set out, for each phase, who maintains each drainage element from practical completion until adoption, and who pays. A management company established at the end of phase one that inherits a basin built in phase four is a dispute in waiting.
| Element | Usual route | Phasing question |
|---|---|---|
| Foul and surface water sewers | Section 104 agreement with the undertaker | Whether the connection can be made phase by phase |
| Attenuation basins and swales | Management company, or local flood authority where it adopts | Which phase builds and which maintains |
| Permeable paving in private drives | Retained by plot owner | Whether the deed sets a maintenance duty |
| Highway drainage | Adopted with the road under a section 38 agreement | Timing of road adoption against phase completion |
Temporary works and silt management
Silt, not hydraulics, is the pollution risk on a part-built site. Suspended solids from exposed earthworks will discolour a watercourse at concentrations far below anything that causes a flooding problem, and that is what generates an enforcement visit.
Practical measures that work: stage the earthworks so exposed areas are limited at any one time, seed or sheet long-term stockpiles, keep a sweeper on site rather than on call, maintain the silt fence instead of installing it once, and hold stocks of straw bales and sediment bags for a wet week. Chemical flocculants can help and generally need prior agreement with the regulator, so do not plan on them without asking.
Rainfall assumptions for the temporary works should be stated and justified in the same way as for the permanent design, a subject we cover in which rainfall dataset to cite in a drainage calculation.
Documenting the phasing plan
The submission that gets approved first time contains a phasing drawing, a set of drainage layouts for each intermediate condition, a table of impermeable area and permitted discharge rate per phase, the interim arrangements with their consents identified, the handover and decommissioning sequence, and the maintenance responsibility matrix.
That is more drawing work than a single-phase scheme and it saves more time than it costs. A Drainage Strategy that shows the interim conditions explicitly rarely attracts a holding objection from the lead local flood authority, while one that shows only the finished site almost always does. And where the site sits in Flood Zone 2 or 3, the interim condition also needs addressing in the Flood Risk Assessment, because a part-built site with temporary levels can behave differently in a flood event from the one that was assessed.
Enter your postcode on PlanScope to see which flood and drainage reports your phased site is likely to need.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why does a phased site need interim drainage design?
Because storage, flow controls and planted systems are sized and specified for the finished development. A part-built site has bare earth, a higher effective runoff coefficient, haul roads and often no permanent outfall. The interim state can discharge faster and dirtier than the approved scheme ever would.
How should settlement lagoons be sized?
For residence time rather than volume alone. Silt needs time to settle, so a lagoon that passes the design storm within twenty minutes passes the suspended solids with it. Keep clean water separate by routing roof water from completed plots and runoff from undisturbed ground around the settlement system.
What if the outfall sits in the last phase?
The first phase then needs an interim discharge route with its own consent, and the obligation to construct the permanent outfall must bite before the phase containing it begins. Check also whether the sewer undertaker will permit a phased connection, because some will only allow the connection to be made once.
Which condition triggers work on a phased scheme?
Triggers tied to a stage the site can evidence within the phase, such as development above slab level, with drainage details required phase by phase and verification per phase. Avoid conditions requiring the whole site's drainage before first occupation of any dwelling, which stops phase one, and ask for interim provision to be an approved alternative.
Who maintains drainage between phases?
It varies by element. Sewers offered under section 104 of the Water Industry Act 1991 stay private until adopted, while basins, swales and permeable paving usually fall to a management company or to the lead local flood authority where it adopts. Set out the responsibility and the payer for each element on the phasing drawing.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.