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Drainage for small developments

Small developments

Small developments (for example individual or small groups of houses or extensions to larger buildings) can present many problems as far as drainage is concerned. There follows a very brief overview of the design of the drainage for smaller developments in urban areas in England and Wales, see also Drainage for new developments in rural areas. The information below should not be regarded as a definitive guide and accurate under all circumstances.

Firstly it is suggested that drainage matters should be considered at an early stage in the overall design process. These can take a significant time to resolve not least due to the need to consult the county or unitary council and possibly the Environment Agency (EA) or Natural Resources Wales (NRW) together with the local water company. Delaying gaining the relevant drainage approvals can result in an almost complete new house being unable to be occupied for months because its drainage arrangements have not been finalised.

Background − flooding

The need for a Flood Risk Assessment (for use in England) or a Flood Consequence Assessment (for use in Wales) should be considered based on the size and location of the site. Areas prone to flooding are shown on the EA or NRW flood maps.

Background − sewer adoption

Very briefly, sewer adoption provides a way for a developer to transfer the ongoing liability for the maintenance of sewers to the relevant water company. The water company, under section 104 of the Water Industry Act 1991 (WIA, 1991), agrees to accept this responsibility if the sewers are designed and constructed to a suitable standard. This standard is set out in 'sewers for Adoption'. Section 102 of the same act provides a way for existing sewers to be adopted.

The mandatory adoption of some new drains and most new sewers is in operation in Wales. This is a relatively costly and lengthy process in relation to a small development. In England mandatory adoption of some new drains and most new sewers does not apply at present but may be implemented at some time in the future. Please see the following section for more details.

Based on the requirements of Dwr Cymru Welsh Water in Wales, mandatory adoption requires the developer to pay a number of additional costs including the legal costs associated with drawing up an agreement under section 104 of the WIA, 1991. The developer is also required to provide a non-performance bond which the water company can use to remedy matters if the work is not completed satisfactorily. The developer also has to pay for the water company to carry out a technical assessment and to supervise the work (2.5% of the cost of the works, minimum ₤500). At least two CCTV sewer surveys of the completed sewer or lateral drain are also required.

It should be noted that a 'Small Developments' version of 'Sewers for Adoption' was published by WRc in September 2013 and this is available to download free of charge from their web site.

Background − mandatory sewer adoption

Following on from the previous section, in Wales mandatory adoption applies to both foul and surface water sewers but does not apply to SUDS systems which would require approval under Schedule 3 of the Flood and Water Management Act 2010 if and when that schedule is implemented. The overall situation is as set out below.

If the drain proposed to serve the development were wholly within a single property it would be designed in accordance with the Building Regulations.

In Wales, if the proposed drain passed under the boundary of the property, then it would be subject to mandatory adoption downstream of the point where it passed into the third party's land and the Building Regulations upstream. A sewer serving two or more properties would also be the subject of mandatory adoption. At present in England, in similar circumstances and where the developer chose not to have the drainage adopted (assuming that this was not a condition of planning approval) then it would mainly be subject to the Building Regulations. If mandatory adoption where implemented in England, the situation would then be similar to that at present in Wales.

Background − sewer connections

Permission needs to be obtained to connect directly or indirectly to a public sewer (section 106 of the WIA, 1991). Indirectly means when a connection is made to an existing private drain or sewer which itself already connects to a public sewer. Permission will also be required from the owner of the private drain or sewer.

The water company will probably insist on the developer employing an approved contractor to make the physical connection to the public sewer but on occasion they can insist on making the connection themselves. In the case of an indirect connection to a public sewer, the water company will probably not make the developer use an approved contractor but they may insist on inspecting the connection once made.

The water company will charge for all these approvals and inspections and once connected to the public sewer the development will attract an infrastructure charge and annual sewerage rates.

Background − crossing third party land

A proposed development may be close to a watercourse or sewer but its use might require pipework to cross another party's land which could result in a problem with a 'ransom strip'. This might be circumvented by requestioning a lateral drain or sewer from the water company under section 98 of the WIA, 1991 but this is likely to be slow and relatively costly. The water company will also require that the developer gains the consent of the riparian owner of any watercourse that the development is intended to directly discharge into.

Work in the highway will require the approval of the highway authority.

Surface water − SUDS

The government encourages the use of Sustainable Drainage Systems (SUDS) for the disposal of surface water from new developments. SUDS measures include a variety of options, including soakaways and attenuation ponds, which reduce the peak run-off flow from a development.

Surface water − Building Regulations

According to the Building Regulations, ('Approved Document H'), the disposal of surface water from the roofs of new buildings, in order of preference should be,

•  to soakaways

•  to a watercourse

•  to a sewer

It will probably be necessary to carry out a soakaway test at the site either to determine the appropriate parameters to use in sizing the soakaway or to demonstrate to the building inspector that soakways are not feasible.

Surface water − other possibilities

It is possible to discharge surface water to a highway drain if there is one close by. A highway drain is a pipe which receives run-off from a highway maintainable at public expense and which 'belongs' to the highway and so is the responsibility of the highway authority. Permission to connect would be required from the highway authority but they are in no way obliged to agree to the proposal and would probably require payment.

Foul drainage

The design and construction standards for the foul drainage would be determined by whether it would be subject to the Building Regulations (and so remain privately owned) or be adopted by the water company. The relevant design and construction standards are, in England for the Building Regulations 'Approved Document H' and for adoptable sewers 'Sewers for Adoption'. In Wales the former document is still applicable but the 'Welsh Ministers' Standards for gravity foul sewers and lateral drains' replaces the relevant parts of 'Sewers for Adoption'. It should be noted that the latter two documents are very closely related. See also Design of sewers for section 104 agreement page.


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