Building over or near buried infrastructure

Planning permission does not authorise building over a sewer, a tunnel or a railway. Those are separate consents, held by the asset owner rather than by the council, and they run on their own timescales with their own technical requirements. A scheme that treats them as a detail for the contractor discovers in month eight that the sewer under the car park needs diverting, that the diversion needs a year, and that the foundation layout depends on the answer.

The order of work is therefore: find the assets, establish the protection regime for each, then design the foundations. Not the other way round.

Finding out what runs under the site

Start with the statutory records and the searches that sit behind them.

• The sewerage undertaker's record of public sewers, obtained through a pre-development enquiry rather than from a conveyancing search, which usually shows only mapped public sewers and not private drainage. The practical steps are covered in ordering sewer records and pre-development enquiries.
• Utility searches from the electricity distribution network operator, the gas network, telecoms operators and any private network owner.
• The Coal Authority mining report where the site is in a coalfield referral area, and a British Geological Survey record for shafts and workings.
• Rail asset plans where the site adjoins or approaches a railway, and tunnel records from the relevant infrastructure owner in cities with deep tunnels.
• High pressure gas and fuel pipeline records, which bring consultation distances and land use planning advice from the Health and Safety Executive rather than a simple easement.
• Historic Ordnance Survey editions and any available ground investigation, which between them find culverted watercourses, filled basements and abandoned services that appear on no modern record.

The transfer of private sewers to the undertakers in 2011 caught many owners out, and it still does. A drain running through a back garden that serves more than one property is very often a public sewer now, whatever the deeds say.

Set the results out on a single constraints drawing with easements, protection zones and consultation distances shown as hatched areas rather than as lines. That drawing is what the Utilities Assessment and the layout design both work from.

Build over agreements with the sewerage undertaker

The Building Regulations deal with building over or near a public sewer, and the undertaker deals with whether it will agree. In broad terms, building within three metres of a public sewer needs the undertaker's agreement, and the closer the structure and the larger the sewer, the harder it gets.

Three outcomes are possible. A build over agreement, where the undertaker consents subject to conditions on foundation design, access, lintels over the pipe and a commitment not to obstruct future maintenance. A build near agreement, where the structure sits within the protection distance but not over the pipe. Or a diversion, which is the answer for larger sewers, deep sewers and anything the undertaker needs unobstructed access to.

Diversion is the expensive route. It is usually pursued under the statutory diversion powers in the Water Industry Act 1991, it needs design, approval, a legal agreement and an easement over the new line, and on a constrained site it can dictate the whole layout. The detail of what an agreement contains and what it commits you to is set out in building over or near a public sewer: the agreement.

One practical point. Undertakers assess the application against their own engineering standards, not against planning policy, and they are entitled to refuse. Do not design a scheme whose viability depends on an agreement you have not discussed with them.

Protection zones around tunnels

Where deep tunnels run below or near a site, the owner will operate a protection or safeguarding regime with a consultation zone measured from the tunnel, and the local planning authority will usually consult them on applications within it.

Inside that zone the owner's concern is not damage to your building. It is movement of, and load on, their asset: ground heave from excavation, additional surcharge from a new structure, vibration from piling, and any possibility of a drilled pile intersecting the tunnel itself. Deep foundations within a tunnel zone are frequently refused outright, and where they are accepted it is on the basis of a detailed assessment agreed in advance.

Safeguarding directions for major projects work differently again. Where a direction applies, the promoter is a consultee and the authority cannot grant permission contrary to the direction without following the process in it. The consequence is that the promoter's technical agreement becomes a de facto precondition of a workable consent, whatever the planning merits.

Asset protection agreements with rail operators

Development adjoining or near an operational railway brings the infrastructure owner into the process twice: as a planning consultee, and as the party whose agreement is needed for the works themselves.

The agreement, generally called an asset protection agreement or a basic asset protection agreement, covers the technical approvals, the owner's supervision costs and the conditions under which work proceeds. It typically bites on excavation near the boundary, piling, craneage that could oversail the railway, scaffolding, and anything that could cause trespass or debris on the line.

The costs are real and they are payable by the developer: the owner's engineering review, its supervision, possessions where work has to happen when trains are not running, and often a bond. Possessions are the item that reshapes programmes, because they are booked months ahead and are scarce.

Where the site is near a station, a level crossing or a structure carrying the railway, expect the review to extend to the permanent condition as well as to the construction phase.

Ground movement assessment and damage categories

For tunnels, rail structures and large sewers the technical conversation is about predicted ground movement and the damage it might cause, and it usually follows a staged approach.

1. A screening stage that asks whether the asset lies within the zone of influence of the proposed excavation or loading, using established volume loss and settlement contour methods.
2. A second stage that predicts settlement, horizontal strain and deflection ratio at the asset, and classifies the likely damage against the standard categories running from negligible through to very severe.
3. A detailed stage, usually numerical modelling, where the second stage indicates damage beyond the threshold the owner accepts.

Owners typically accept damage in the lowest categories and require mitigation above that. The mitigation options are staged excavation, stiffer retaining walls, propping sequences, or compensation grouting where the asset is sensitive and the movement cannot be designed out.

This work belongs with the geotechnical assessment and, on basement schemes, with the Basement Impact Assessment, and it needs the ground investigation to have been designed with the asset in mind. A borehole layout drawn for foundation design will not always give the parameters a tunnel owner asks for.

Foundation design responses

The engineering answers are limited in number and worth knowing before the layout is fixed.

Move the load. Shift the building footprint clear of the protection zone, which costs least and is the option most often dismissed too early.

Transfer the load. Piles outside the zone with a transfer structure, ground beams or a slab cantilevering over the asset, so nothing bears within the protected ground.

Spread the load. A raft or wide strip foundations designed so that the load spread from the foundation does not reach the asset, commonly assessed on a spread from the pipe invert or the tunnel crown.

Protect the asset. Lintels or a reinforced slab spanning over a shallow sewer, sleeves at crossings, or a concrete surround specified by the undertaker.

Or divert the asset, which converts a design problem into a programme and cost problem.

Timescales and what to start first

The sequencing point is the one that saves money. Asset consents are not on the planning critical path in theory, and they are in practice, because a planning permission whose foundations cannot be built is worth very little.

A workable order. Commission the asset searches at the option or feasibility stage, before the layout is drawn. Open a pre-application conversation with the undertaker or asset owner as soon as the constraint is known, in parallel with the planning pre-application. Design the ground investigation to answer the asset owner's questions as well as your own. Submit the build over or protection application once the layout is stable but before planning determination, so that any layout change comes before the consultation rather than after it.

And build the durations into the programme honestly. A build over agreement on a small sewer may take a few weeks. A diversion of a trunk sewer, or an asset protection agreement involving possessions, is measured in seasons.

This is general guidance, not legal advice. Asset protection requirements differ between owners and change, so confirm the position with the relevant undertaker or infrastructure owner and a qualified engineer before relying on it.

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