Section 104 sewer adoption and the standards behind it

A section 104 adoption agreement is the contract under which a sewerage undertaker agrees to take over the foul and surface water sewers you are about to build, so that on a date after completion they stop being your liability and become the water company's. It is made under section 104 of the Water Industry Act 1991, it is negotiated before construction starts, and it is backed by a bond. Get it wrong and you hand a management company a drainage network it cannot afford, or you hold a maintenance liability on a site you sold years ago.

What the agreement actually covers

The agreement covers gravity sewers, rising mains, pumping stations and, in some circumstances, lateral drains that serve more than one property. It does not cover private drains inside a plot boundary, highway drainage adopted by the highway authority, or watercourses.

Three parties matter. The undertaker signs the agreement and inspects the works. The developer builds to the approved design and pays the fees. The surety, usually a bank or a bond provider, stands behind the bond so that if the developer fails the undertaker can complete the works.

A well-run agreement has a technical approval stage, a construction stage with inspections at defined hold points, a vesting date when the sewers transfer, and a maintenance period before the bond is released. Programme it backwards from the first plot sale, because a plot that discharges to an unadopted sewer with no agreement in place is difficult to sell and harder to mortgage.

The design and construction standards applied

Since 1 April 2020 sewer adoption in England has run on a single national code of practice rather than the patchwork of company standards that preceded it. The design and construction guidance sets out pipe sizes, gradients, cover depths, manhole spacing, materials, testing regimes and the minimum information an application must contain.

Expect the technical reviewer to check the same short list every time:

1. Pipe gradients against the self-cleansing velocity for the design flow, with any shallow runs justified.
2. Manhole positions at every change of direction, gradient or diameter, and at defined maximum spacings.
3. Cover levels and depth to invert, including where a sewer runs outside adoptable highway and needs an easement.
4. Hydraulic design for the design storm, with no flooding of the network at the return period the code requires.
5. Structural design of the pipe under the loading it will actually see, including construction traffic.
6. Pumping station design, including storage for pump failure and the access arrangements for a tanker.

The separate question of how surface water leaves the site, and at what rate, belongs in the Drainage Strategy submitted with the planning application. A drainage strategy that assumed an attenuation rate the undertaker will not accept is the single most expensive thing to discover at technical approval stage.

Where SuDS components are and are not adoptable

This is where developers lose most time. A sewerage undertaker will adopt some sustainable drainage components and refuse others, and the split is not intuitive.

Broadly, piped systems, below-ground attenuation tanks, flow control devices and some permeable paving serving more than one property fall within what an undertaker will consider. Open features such as swales, basins, ponds and rain gardens are usually adopted by someone else, or by nobody, because they read as landscape rather than sewer. That leaves a maintenance gap that has to be closed by a management company, a highway authority agreement or a planning obligation.

Decide who holds each component before the layout is fixed. Our note on SuDS adoption and maintenance sets out the routes in more detail, and the position on the statutory approving body in England is still unsettled, which is covered in the piece on Schedule 3 SuDS approval.

One practical test: if a component needs a mower, a person with a rake, or a planting replacement budget, assume it is not going into the section 104 agreement.

Bonds, inspection fees and the timeline

The undertaker charges an administration and technical approval fee, an inspection fee usually calculated from the length and complexity of the network, and requires a bond set as a percentage of the estimated construction cost. Figures vary between companies and are revised annually, so take them from the current published developer services charges for your undertaker rather than from a previous scheme.

The bond is released in stages. A substantial part comes back at the vesting date, with a residue held through the maintenance period against defects found on the post-construction survey. That survey is normally a CCTV inspection of the whole network, and it is the moment a contractor's sloppy jointing becomes the developer's problem.

Timeline, realistically: technical approval takes weeks rather than days, the legal agreement runs in parallel, and neither should be started after groundworks. Build a sewer before the agreement is signed and you have built an asset the undertaker is entitled to refuse.

Interaction with the build programme

Planning conditions, the section 104 process and the housebuilding programme pull against each other. A condition requiring drainage details to be approved before work starts cannot be discharged until the design is settled, and the design cannot be settled until the undertaker has commented. Meanwhile the site needs surface water routed away from the works from week one, which is a temporary arrangement, not the adopted network.

Sequence it like this. Fix the outfall and the discharge rate during the planning application, using the Flood Risk Assessment and drainage strategy. Submit for technical approval as soon as the layout is frozen. Sign the agreement before the first pipe goes in. Keep as-built records from day one, because the undertaker will ask for them.

Common reasons a technical submission is rejected

Rejections cluster around a handful of avoidable faults. Levels that do not tie in with the highway design. Sewers routed through private gardens with no easement shown. Attenuation sized on a different storm from the one the planning drainage strategy used. Pumping stations with no adoptable access for a tanker, or sited so close to a plot that the odour and noise standoff cannot be met. Missing structural calculations for deep runs. And, frequently, a submission that simply contradicts the approved planning drawings.

The fix is boring and effective: one drainage model, one set of levels, one layout, shared between the planning consultant, the highway engineer and the drainage designer.

What happens if adoption is never completed

Sewers built without an agreement do not vanish. They may be adopted later through a different route, they may be taken on by a private management company, or they may sit with the freeholder. Each option has a cost and each affects the residents who bought the houses.

A site left in that position tends to surface years later, when a manhole collapses and nobody can establish who owns it. That conversation is much easier if the as-built drawings, the CCTV survey and the correspondence with the undertaker were kept. This is general guidance, not legal advice; check the position with your sewerage undertaker and your local planning authority 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. Enter your postcode on PlanScope to see which drainage and flood reports your site is likely to need.