Underpinning a neighbouring property: the consents needed

Underpinning a neighbouring property is usually not development at all, which surprises owners who assume anything structural needs planning permission. The consents that do bite are the Party Wall etc. Act 1996 notices, building control approval, and whatever the planning conditions on the main scheme say about method statements and monitoring. Get the sequence right and it is a routine piece of temporary and permanent works. Get it wrong and you are in a dispute with a neighbour whose building has moved, with no award to fall back on.

The question that decides the paperwork is whose foundation is being strengthened: your own party structure, or the adjoining owner's building.

When foundation strengthening becomes necessary

Four situations recur on conversion and basement projects. Excavating below the level of an adjoining foundation, which removes the lateral support that foundation relies on. Forming a basement or lowered lower ground floor under or beside a shared wall. Loading a shallow Victorian footing more heavily than it was built for, typically by adding a storey. And remedying existing movement where the neighbour's building is already distressed and the works would make it worse.

The first is the common one and the least optional. A shallow strip footing next to a 3 metre excavation has nothing holding it, and the choice is between strengthening that footing, a piled retaining solution or a sequence of temporary support that keeps the ground in place.

Which of those is appropriate is an engineering judgement based on ground conditions, water, the neighbour's construction and the depth. It is not a judgement for a contractor on site, and it is the point at which a Geotechnical Assessment earns its fee rather than a point at which one is produced to satisfy a condition.

Whether underpinning is development

Section 55 of the Town and Country Planning Act 1990 defines development, and it excludes works of maintenance, improvement or alteration that affect only the interior of a building or do not materially affect its external appearance. Work of this kind normally falls within that exclusion, so no planning application is needed for the work itself.

Three exceptions matter. Where the building is listed, works affecting its character as a building of special interest need listed building consent, and this kind of work can engage that where it affects historic fabric or floor levels. Where the work forms part of a basement, the basement is the development and the permission covers both. And where a planning condition on an existing permission requires approval of a construction method statement, the method falls inside that condition even though it would not need permission on its own.

Building control is a separate matter and it does apply. Work of this kind is building work under the Building Regulations 2010, so it has to be notified and inspected, with structural calculations submitted.

Notices to the adjoining owner

Section 6(3) is the provision most often missed. It allows the building owner to underpin or otherwise strengthen the foundations of the adjoining building so far as may be necessary, and requires it where the adjoining owner asks for it. That is how work to a neighbour's foundations is authorised: it comes through the excavation notice and the award, not through a private agreement negotiated on the doorstep.

Serve the section 6 notice with the proposed method and drawings attached. A notice that describes an intention without a scheme gets dissented to and delays the award while the surveyors ask for what should have been sent first.

Temporary works design and the designer's duties

The permanent works and the temporary support that lets them be built are two designs, and both are designer outputs under the Construction (Design and Management) Regulations 2015. Mass concrete bays in short hit and miss sequences, pin piles, needle beams, or a contiguous piled wall built from your side: each carries a different sequence, a different risk of movement and a different set of hold points.

Three things belong in the design output. A sequence drawing with bay widths and the order of excavation. Stated limits on how long a bay may stand open. And a defined set of hold points where the engineer inspects before concrete is placed.

Where the scheme carries a neighbour's building, the engineer's professional indemnity cover is part of the commercial picture. Check the limit and whether it responds to third party property damage, because the sum insured on a small practice can be less than the cost of repairing a Victorian terrace.

Method statements the authority may condition

On basement schemes, and increasingly on larger conversions, authorities condition a construction method statement, a structural method statement or compliance with an approved basement impact assessment. In much of London a Basement Impact Assessment is a validation requirement in its own right, and it covers ground movement, hydrology and the effect on neighbouring structures.

Read the condition wording before you design. A condition requiring approval of a method statement before any works start will catch site set-up and demolition as well as the foundation works, and discharging it takes weeks that nobody allowed for. Where the condition requires the statement to be prepared by a chartered engineer, the authority will check the authorship.

Councils usually expect a suitably qualified author for this work, and professional review before submission is strongly advised. Where the basement creates habitable rooms, the policy questions about living space below ground are separate and are covered in our note on sleeping accommodation below ground.

Monitoring and insurance

Movement monitoring is the mechanism that converts an argument about damage into a record. Install targets on the neighbour's building and on your own, take a baseline set of readings before any excavation, and set trigger levels with defined actions at each level: continue, review, stop. Precise levelling plus crack monitoring is usually enough on a terraced house. Larger schemes add inclinometers and automated total stations.

The insurance picture has four parts. The contractor's public liability, which responds to negligence. Non-negligent liability cover, which responds to damage caused by the works where nobody was careless and which is the policy most often absent. The engineer's professional indemnity. And the neighbour's own buildings insurance, which the award will usually require you to notify.

Section 12 of the 1996 Act lets the adjoining owner request security for expenses before notifiable works start, and on works of this kind it is routinely requested. Treat it as working capital tied up for the duration rather than a fee.

Programme and access agreements

1. Survey the neighbour's construction and foundation depth, by trial pit where access allows, before fixing the design.
2. Produce the permanent and temporary works designs together, with the sequence drawing.
3. Serve the section 6 notice, and the section 3 notice where a party structure is being supported, with the method attached.
4. Allow the statutory period, the surveyor appointments and the award.
5. Carry out the schedule of condition and install the monitoring baseline.
6. Serve the section 8 notice for access, with the agreed working hours.
7. Build in the agreed sequence, with the hold points honoured and readings taken at the stated frequency.

Step one is the one owners try to skip because access is awkward. A design based on an assumed foundation depth is a design that changes on site, and a change on site after the award has been agreed means going back to the surveyors.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. To see which supporting documents a basement or conversion scheme is likely to need, enter your postcode on PlanScope.

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