Dig your basement with your neighbours’ buildings protected

Blueprint cutaway of two terraced houses with a new basement dug under one, the party wall taken down in teal and a crack gauge

For homeowners and developers in England and Wales digging or deepening a basement next to neighbours. Compare quotes from party wall surveyors who work alongside your structural engineer.

Digging a basement takes the ground out from beside a neighbour’s foundations, so it is one of the most closely watched kinds of party wall work. In England and Wales the Party Wall etc. Act 1996 covers it through a notice of adjacent excavation (for digging near and below a neighbour’s foundations) and, where a shared wall is underpinned, a party structure notice. The excavation must be designed by a qualified structural engineer before any digging begins, and the neighbour’s surveyor will want to see that design, including how the ground and the neighbouring building are supported. The award then sets the method, monitoring and protection. A party wall surveyor handles the notices, records and award, and works with your engineer.

How a professional can help

Notices for digging and underpinning: Working out whether you need a notice of adjacent excavation, a party structure notice for underpinning shared walls, or both, and serving them with drawings the neighbour’s surveyor can follow.
Explaining the engineer’s design: Putting the engineer’s sequence of excavation, underpinning and temporary support into the notice and award, so the neighbour’s surveyor can see how their building is supported at each stage.
A thorough schedule of condition: A detailed record of the neighbour’s walls, floors, cellar, garden structures and any existing cracks, with photographs, made before the digging starts.
Monitoring movement: Agreeing movement monitoring on the neighbour’s building, such as crack gauges or survey targets, with trigger levels at which work pauses and the engineers review the readings.
Security for expenses: In some cases the neighbour can ask for security, a sum held in reserve, to cover making good if something goes wrong. The owners or their surveyors agree the amount and how it is held.
Joining up with the wider design team: Making sure the party wall terms match the structural design, the ground investigation, the waterproofing designer and any basement impact assessment that your council asks for.

Is this right for you?

You plan to dig a new basement or deepen an existing cellar under or beside a house that shares a wall with a neighbour.
Your structural engineer’s design includes underpinning of a shared wall.
Your council has asked for a basement impact assessment, and you need the party wall side to match.
Your neighbour has served notice for a basement and you want your property protected.

How the project usually runs

Excavation designed first: A qualified structural engineer designs the excavation, underpinning sequence and temporary support before any digging starts, informed by ground investigation. The notices can then describe it accurately.
Excavation and structure notices: A notice of adjacent excavation is served at least one month before digging, and a party structure notice at least two months before work on a shared wall, with the drawings.
Surveyors and records: If the neighbour does not consent within 14 days, surveyors are appointed. They inspect the neighbour’s property and prepare a detailed schedule of condition.
Award with monitoring: The award sets the method, working hours, protection and monitoring, including points where work must stop and be reviewed. Security for expenses may be agreed too.
Dig, monitor, re-inspect: While work is on, monitoring readings are shared with the neighbour’s surveyor. Afterwards, the properties are re-inspected and any damage is dealt with under the award.

What affects the cost

The depth, size and shape of the basement, and how close it is to neighbouring buildings.
How many neighbours are affected, since each needs notices and may appoint a surveyor.
Whether movement monitoring is needed and how long it must run.
The amount of recording needed in neighbouring buildings, including cellars and gardens.
Whether security for expenses is requested, and the time needed to agree the design with the neighbour’s surveyor.

Planning, basements and the Act

The Party Wall etc. Act 1996 applies to basement digs in England and Wales. Scotland and Northern Ireland have different rules, though party wall surveyors work across the UK.
Basements often need planning permission, depending on the council and the property, and some councils ask for a basement impact assessment. Party wall rights are separate and replace neither.
Building control normally needs the engineer’s calculations and drawings, along with a plan for temporary works and waterproofing.
Basement rules differ across England, Scotland, Wales and Northern Ireland, and between councils, so ask yours early what it expects alongside the party wall papers.

Questions people ask

Do I need party wall notices to dig a basement?

Very often, if the house shares a wall or the dig is close to a neighbour’s building and deeper than its foundations. In England and Wales, expect a notice of adjacent excavation, and a party structure notice if shared walls are underpinned. A surveyor can confirm which.

Will the neighbour’s building be monitored for movement?

Often, yes. The award can require monitoring of the neighbour’s building during excavation, with agreed trigger levels at which work stops while engineers review. What is required depends on the design, the ground and how close the digging is.

What is security for expenses?

It is a sum held in reserve to pay for making good if work is left unfinished or damages the neighbour’s property. A neighbour can ask for it in some cases, and the owners or their surveyors agree the amount and how it works.

Is a cellar conversion different from a new basement?

Yes. Converting an existing cellar without deepening it may involve little or no excavation, so fewer notices. Lowering the floor or enlarging it means digging beside foundations and is treated much more seriously. Your engineer and surveyor can tell which you have.

How long does it take to agree an award for a basement?

It can take longer than for simpler work, because the neighbour’s surveyor will want to understand the engineer’s design and monitoring. Serving notices as soon as the design is firm enough to describe the work leaves more time before digging is due to start.

Available across the UK

Party wall surveyors cover the whole of the UK. The Party Wall etc. Act 1996 applies in England and Wales; Scotland and Northern Ireland have different rules, and a professional can tell you what applies to your property.