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Building Over Sewers


Public sewers (the sewers which are the responsibility of the local water and sewerage company) are often located under private land and in these circumstances there can be a conflict between interests of the landowner and the company. The landowner wants to be able to maximise the use of his or her property, possibly by constructing buildings, while the company wants to protect the sewer from possible damage and to maintain access to allow maintenance or repairs to be carried out if needed. Under certain circumstances a 'building over agreement' is a way of resolving this conflict to the satisfaction of both parties.

The government transferred the responsibility for private foul and combined sewers and some private drains to the water companies in October 2011 and this means that the majority of existing private sewers became public sewers. Since the former private sewers and drains tend to be located close to the properties they serve, there is likely to be a substantial increase in the number of householders wanting to build over public sewers.

This document examines the problem of building over public sewers from the point of view of the householder who might be considering a home extension and with particular reference to the transfer of the responsibility for private sewers. A section is also included concerning the diversion of public sewers, since that is often the only alternative to building over a sewer.

Building Regulations

Section H4 of 'Building Regulations, Approved Document H', 2002 (incorporating 2010 amendments) provides guidance on when it is permissible to build over, or within 3m of sewers and on the protective measures to be employed if this is done. It should be noted that H4 is applicable to drains, sewers and disposal mains shown on the public sewer records even if the pipe is not a public sewer. The guidance includes such matters as special measures where there is an undue risk to the sewer (for instance in the case of certain soils and a high water table), the maintenance of access, the protection of the sewer during construction and the protection of the sewer from settlement. Sewers will only be allowed to be built over if there is a satisfactory route for subsequent diversion of the sewer around the building. This is because it will be extremely difficult and costly to repair the sewer once it is under a building.

The building control section of the local council and other bodies carrying out building control functions can be expected to check that proposed buildings will not be within 3m of a sewer or drain shown on the sewer records. If the building inspector does find that there is a sewer close to the proposed building they will inform the applicant. This may then mean having to amend the plans for the proposed building so it is advisable to check the sewer records before designs are produced. It is worth noting that if a public sewer is located within 3m of the proposed building then a Building Notice will not be allowable and a full Building Regulations application will be required.

Paragraphs 1.5 and 1.6 of section H4 of the Approved Document says that the length of the sewer or drain built over should not be greater than 6m without the permission of the drain or sewer's owner. In addition sewers and drains deeper than 3m and larger than 225mm diameter should not be built over, again, without the approval of the drain or sewer's owner. These parameters tend to be reflected as the upper bounds of acceptability with water companies for building over agreements.

Section H4 also says that manholes should not be built over although it adds that a manhole may sometimes be removed provided that satisfactory alternative access is then provided. This sounds simple but will require the demolition and removal of the existing manhole and its replacement with a length of suitable pipework and then the construction of the replacement manhole at the new location. This work will need to be carried out to a standard required by the water company. Manholes located at the junction of three sewers are likely to be problematical and will involve the diversion of at least one of the sewers, see the later section.

Sewer Records

Earlier it was advised that the householder should consult the relevant sewer records when considering a house extension and at first sight this seems simple, however, this is often not the case.

Sewer records are notoriously variable in quality, in some areas they are quite good but in others they are a work of fiction, but it is not generally known which area is which. Current digitised sewer records tend to be based on the information from old paper based sewer records. The information may or may not have been updated but the digitisation process will probably have resulted in a significant decrease in the overall accuracy of the data.

Under section 199 of the Water Industry Act, 1991 water companies are generally obliged to keep records of the position of their sewerage equipment. Sewers laid before 1989, however, are not required to be recorded if the company does not know of their existence or it is not “reasonably practicable” to discover the course of the sewer and it has not done so. This then includes 'former section 24 sewers' (of the Public Health Act, 1936) which were private sewers (ie serving more than one property) laid before 1 October 1937 and which were made into 'full' public sewers in 1989. These were probably not shown on the old paper based records and have not been systematically mapped since 1989 due to the cost of such an undertaking. Another category unlikely to be shown on the sewer records is that containing the former private sewers which were made into public sewers on 1 October 2011. It is understood that these in turn will not be systematically mapped, again for cost reasons. The water companies will be expected to update their records as the sewers are discovered during the course of other operations. Finally the author knows of one water company which had its agents systematically add likely/guessed routes of 'former section 24 sewers' to its sewer records. Unfortunately there was no record made of which were likely/guessed and which were actually surveyed.

It is in the interest of the householder to confirm the position of any public sewer during the design process for a new building. If for example a house extension is constructed over a public sewer without the agreement of the water company, the company ultimately has the power to have the extension removed since the Building Regulations have not been complied with. Copies of the sewer records are available for inspection at local authorities while it is possible to buy an extract of the sewer records from the company for a fee.

Building Over Agreements

A building over agreement is a legal agreement between the water company and the landowner. Under the agreement, the water company allows a building of approved size and position to be constructed over an existing public sewer if the other party agrees to meet certain conditions. It should be noted that these conditions will probably largely follow the requirements of the Building Regulations, see above, although interpretation of the regulations varies.

Many water companies appear at first sight to discourage the use of building over agreements although there appears to be a dual approach by the parties involved. For example the building control section of one local authority in the Severn-Trent Water area says that there is a presumption that the water company will agree to building over a sewer in some limited circumstances. Severn-Trent Water itself says in bold print in its 'Policy Guidance Notes and Requirements' “Wherever possible we prefer to see a solution that involves repositioning of the proposed development or diverting the sewer” but then goes on to say in normal print “However in some cases where risk to public sewer is small, Severn-Trent Water is normally willing to permit building over or close to sewers where diversion is not practical and the extension cannot be redesigned.” Defining the meaning of “not practical” is likely to be a problem here.

If there is an existing easement or covenant covering the sewer (in other words an existing legal agreement that the owner of the land will not build over the sewer) then the water company may not agree to a building over agreement. An alternative to a building over agreement might be to divert the sewer prior to the building work and this is covered in a later section.

To enter into a building over agreement with a water company the householder needs to fulfil various requirements. For example in the case of Severn-Trent Water, these include paying an application fee based on the sewer diameter (150mm diameter; ₤169.50, 225-300mm diameter; ₤194.70) and carrying out both pre and post-construction CCTV sewer surveys to the requisite standards. If the sewer is found to be in poor condition, the householder will also be required to carry out repairs to the sewer at his own expense to bring it up to an acceptable standard before constructing the extension. Costs for building over agreements vary considerably between companies, for example, the application fee for Southern Water is ₤867.60 which is considerably larger than the figures given above for Severn-Trent.

To summarise, it is likely that the majority of former private sewers will fall into the category where the extension will not pose a high risk to the sewer. Householders can probably build over the sewer in this case but if the sewer is in poor condition this will involve significant additional costs.

Further Guidance

The Department for Communities and Local Government (DCLG) acts as a 'referee' in matters where there is disagreement regarding building control matters in England. A similar role is carried out by the Welsh Government in Wales. The DCLG web site has two examples of the application of building over agreements but unfortunately the cases do not give locations or the water companies involved so the cases will be referred to by the dates of the judgements. The first was determined by the present Department's predecessor, the Office of the Deputy Prime Minister (ODPM), in September 2004 while the second dates from January 2011.

The 2004 case concerned a proposal to construct a two storey house extension on the footprint of an existing detached single storey garage. The garage was constructed over a 225mm diameter public foul sewer 1.96m deep which served approximately 100 properties. There was also understood to be a drain from the adjacent property connecting to the public sewer under the garage. The length of sewer affected was 8m and there was no adequate route to divert the sewer if required later. The ODPM determined that the building over proposal was not acceptable and it was rejected. It was held that if the sewer subsequently collapsed under the garage, it '...would be a general nuisance and potentially be a hazard to public health.' On the other hand, the demolition of the garage while the sewer was repaired would '...not cause extreme inconvenience to the occupier' but that would not be the case with a two storey extension.

The 2011 case involved an existing single story garage plus a toilet but this time located close to, but not above two sewers, a 375mm diameter foul sewer, 3.5m deep, and a 900mm diameter surface water sewer, 3.1m deep. Again there was no diversionary route available. The distance between the existing garage and the wall of the adjacent house was approximately 3.2m and this area contained a footpath. This suggests that the distance between the walls of the two houses (excluding the garage) was approximately 5.6m. It was proposed to replace the existing garage and toilet with a two storey extension of slightly larger plan area and utilising piled foundations, so taking the building loading below the level of the sewers. The centreline of the piles was to be set back 600mm from the line of the boundary. The length of the existing constriction of the access to the sewer due to the presence of the garage was 4.5m and this was to be increased to 7m with the proposed extension. The water company had already accepted the proposed extension on the footprint of the garage. The Department determined that the proposed house extension was acceptable since the single storey garage had shallow foundations and so the proposal was an improvement over the existing situation.

There appears to the writer to be a significant difference in the attitude of the Department to the two cases. In the 2004 case the ODPM took into account the number of residents who would suffer considerable disturbance due to the delay in repair of the sewer if the proposal were accepted and the sewer subsequently collapsed. They also took into account the possibility of demolishing the garage to allow the repair of the sewer. In the 2011 case the number of residents effected was likely to be considerably larger judging by the size of the pipes involved. The writer wonders if there was a lack of understanding in the second case of the practicalities of dealing with the repair of such large pipes and dealing with the significantly larger flows in the meantime.

Sewer Diversions

The diversion of a public sewer is carried out under section 185 of the Water Industry Act, 1991. A person with interest in a plot of land or adjoining land can require the water company to divert a public sewer passing under the land. There is a proviso that the request is reasonable and reasons for the request to be unreasonable include that it is not feasible from an engineering point of view and that the diverted sewer would stop other parties from developing their land. A sewer which is wholly located in the highway cannot be diverted under section 185.

The water company will charge all reasonable costs to the person requiring the diversion work. It will also probably require that a sum of money is deposited with the company to cover abortive costs incurred in the event that the person eventually decides not to carry out the diversion. All work is likely to have to be carried out to an appropriate standard, usually, 'Sewers for Adoption'. The water company is likely to carry out design and construction work in high risk situations but may allow approved designers and contractors to carry out work in straightforward cases where the work is wholly within the land owned by the developer. In any event the cost is born by the person requesting the sewer diversion. The need to divert the sewer can represent a significant additional cost and this is particularly the case for anyone constructing a small extension such as a single storey single room extension.

Attitude of different water companies

A comparison of the attitude of the different water companies to building over agreements shows a significant difference as to what are regarded as an acceptable diameter and depth of sewer to be built over. For some companies there appears to be a hard and fast cut-off point, but for others it is a bit woolly. This latter point combined with the discrepancies between the two cases given in the Further Guidance section means that there may often be some degree of latitude which the house holder may be able to exploit in particular cases.


This report has briefly examined various aspects of building over agreements for public sewers. The report has also shown that they can be a convenient way of resolving the possible conflict of interests between the householder wanting to construct an extension and the requirements of the water company. The report has also shown, however, that there may still be many associated problems to overcome.

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Copyright, 2012, Morton-Roberts Consulting Engineers Limited.
Last Modified 2 July 2012.