Underpinning, party walls and a basement next door

Basement underpinning and the party wall process are separate from your planning permission, and the order in which people discover that is usually the wrong way round. Planning permission says the council accepts a basement here in principle. It gives you no right to touch your neighbour's wall or to excavate beside their foundations. That right comes from the Party Wall etc. Act 1996, and the neighbour who objected to your planning application has far more say under that Act than they ever had at the council.

For a basement under a terraced house you will very likely be serving two notices under two different sections.

What planning controls and what it does not

The planning system deals with land use and amenity: the principle of the basement, the lightwell in the front garden, the rooflight in the rear garden, construction traffic, noise and hours.

Where a council requires a basement impact assessment, it is assessing ground conditions, groundwater, stability and predicted ground movement as planning matters, and it may impose conditions on monitoring and on the construction method. Many London boroughs have adopted policies that do exactly this.

What the council cannot do is grant you rights over your neighbour's property. It also cannot refuse permission because your neighbour has not consented under the Party Wall Act, since private rights are not a material planning consideration. Those two facts together mean a permission can be granted for a scheme you have no legal ability to build until the party wall process concludes.

The two notices a basement triggers

Section 2 of the Act gives rights over party structures. Section 2(2)(a) covers works that support a party structure from below, thicken it or raise it, which is exactly what a basement does to the wall between two terraced houses, and the notice required is a party structure notice.

Section 6 deals with adjacent excavation. It bites in two situations:

• Excavation within 3 m of a neighbouring building or structure, to a depth lower than the bottom of its foundations. This is the common case for a basement.
• Excavation within 6 m of a neighbouring building, where any part of the excavation would cut a line drawn downwards at 45 degrees from the bottom of that building's foundations. This catches deeper basements further away, including those on the other side of a garden wall.

A basement under a mid terrace house usually engages both sections, for the party walls on each side and for the dig itself. Each has its own notice and its own period.

Section 6 notices have a specific content requirement. They must be accompanied by plans and sections showing the site and depth of the proposed excavation, and they must state whether you propose to strengthen the foundations of the neighbouring building. A notice without those is invalid, and an invalid notice wastes the whole period.

Notice periods, dissent and the surveyors

Silence is not consent. If the adjoining owner does not reply within 14 days, the Act treats a dispute as having arisen, which means surveyors must be appointed and an award produced before work starts. In practice most basement projects end up in this route whatever the neighbour's attitude, because agreeing to a basement without an award leaves the neighbour with less protection and most are advised against it.

The appointment is either one agreed surveyor acting for both owners, or two surveyors, one appointed by each owner, who then select a third surveyor to resolve any disagreement between them. Identify all the adjoining owners before serving, since leaseholders with unexpired terms are owners for the purposes of the Act and a flat above a neighbouring shop may mean three notices on one property.

The award, and what it can decide

The award is the document that gives you the right to proceed, and it is wider in scope than people expect.

It determines the right to carry out the work, the time and manner of doing it, and any other matter arising out of or incidental to the dispute, including the costs of making the award. On a basement that normally means it will specify the sequence of the works below the wall, the maximum length of each pin and the order in which pins may be excavated, the propping, the monitoring regime with trigger values, the working hours, access arrangements and the schedule of condition.

Either owner may appeal to the county court within 14 days of the award being served. That is a short window and it is the only route to challenge it.

The award does not replace Building Regulations approval, and it does not replace planning permission. All three are needed, and none of them substitutes for the others.

Costs, security and who pays for what

The default position under the Act is that the building owner, meaning you, pays. That includes the fees of the adjoining owner's surveyor, which you do not choose and cannot control beyond reasonableness.

Two provisions worth knowing before you budget.

The Act allows the adjoining owner to request security for expenses before the work begins. On a basement that request is reasonable and increasingly common, and it can mean placing a sum in escrow or providing a bond. Programme it in.

The Act also obliges the building owner not to cause unnecessary inconvenience and to compensate the adjoining owner for any loss or damage resulting from the works. Damage caused by basement excavation is the single largest source of claims, which is why insurance specific to the works, naming the adjoining owners, is standard practice.

Schedules of condition and movement monitoring

A schedule of condition is not required by the Act and no sensible project goes ahead without one.

It records the state of the neighbouring property before work starts: every crack, every settled door frame, every stained ceiling, photographed and described room by room with the date. Six months later, when a hairline crack appears above a door, that document decides whether it is yours. Have it prepared by the party wall surveyors and have both owners sign it.

Monitoring then runs alongside. The usual arrangement is precise levelling or total station readings on targets fixed to the neighbouring property and to the party walls, read at a stated frequency, with amber and red trigger values and a defined action at each. Amber typically means increased reading frequency and a review by the engineer. Red means work stops.

Where the planning permission also imposes a monitoring condition, align the two regimes rather than running them separately. The piece on settlement monitoring conditions sets out what councils tend to ask for, and a structural survey of both properties is the starting point for either.

Sequencing the two regimes

The order that works:

1. Obtain planning permission, so the design is fixed in principle.
2. Complete the ground investigation and the detailed structural and temporary works design, including the pin sequence.
3. Identify every adjoining owner, including leaseholders, from the title registers.
4. Serve the notices, with the section 6 plans and sections, allowing two months for party structure works and one month for excavation.
5. Appoint surveyors on deemed dissent and agree the schedule of condition.
6. Obtain the award, then allow the 14 day appeal period to pass.
7. Discharge the planning conditions that must be satisfied before work begins, and obtain Building Regulations approval.
8. Start on site.

Serving notices before the structural design is settled is the common mistake, because the notice has to describe the actual works. A revised sequence means a fresh notice and a fresh period. The practical sequencing of the excavation itself, once all of this is in place, is covered in the piece on sequencing a basement excavation in a terraced street.

Build four to six months of consents and notices into the programme before anyone digs. Projects that treat the party wall process as a formality to be dealt with during mobilisation lose more time than that.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional 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.

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