Basement Extensions and the Party Wall Act: Understanding Section 6 Requirements in London

Published January 2026 | Reading time: 18 minutes

Basement extensions are among London's most valuable home improvements, adding substantial living space and significantly increasing property value. However, they're also the most complex party wall matters you'll encounter. Party Wall Agreement London provides this comprehensive guide to Section 6 of the Party Wall etc. Act 1996, explaining excavation rules, underpinning requirements, costs, and how to ensure your basement conversion proceeds smoothly.

Why Are Basement Extensions Party Wall Matters?

Almost every basement extension in London triggers party wall requirements for two main reasons:

1. Section 6: Excavation Near Adjoining Buildings

Section 6 of the Party Wall Act applies when you excavate close to your neighbour's property. This is virtually unavoidable in London's densely built terraced and semi-detached areas. The excavation rules are:

• 3-Metre Rule: If you excavate within 3 metres of your neighbour's building and go deeper than the bottom of their foundations, Section 6 applies
• 6-Metre Rule: If you excavate within 6 metres and a 45-degree line drawn downwards from the bottom of their foundations would pass through your excavation, Section 6 applies

Most Victorian and Edwardian properties in London have foundations only 600mm-1000mm deep (2-3 feet). When you excavate for a basement, you're typically going 2-3 metres deep, which means you'll almost certainly trigger Section 6 if working within 3 metres of your boundary.

2. Section 2: Work to Party Structures

Basement extensions also involve Section 2 work, including:

• Underpinning the party wall to support it at the new, lower level
• Exposing the party wall foundations during excavation
• Installing tanking (waterproofing) that affects the party wall
• Cutting into party walls for new basement doorways or openings
• Supporting party fence walls during excavation

The combination of Section 2 and Section 6 makes basement extensions the most complex party wall projects, requiring experienced surveyors and careful planning.

Understanding Section 6: The Excavation Rules Explained

The 3-Metre Rule in Detail

If any part of your excavation is:

1. Within 3 metres horizontally of your neighbour's building
2. AND extends below the bottom of their foundations

Then Section 6 applies and you must serve notice.

Example: Your neighbour's Victorian terrace has foundations 800mm deep. You're excavating for a basement 2.5 metres deep, starting 1 metre from the boundary wall. Because you're within 3 metres horizontally and going 1.7 metres deeper than their foundations, Section 6 applies.

The 6-Metre Rule in Detail

This rule uses geometry to determine if your deeper excavation could affect your neighbour's foundations. If:

1. You're excavating within 6 metres horizontally of their building
2. AND a 45-degree line from the bottom of their foundations would pass through your excavation

Then Section 6 applies.

Example: Your neighbour's foundations are 1 metre deep. You're excavating 4 metres from the boundary to a depth of 2.5 metres. Drawing a 45-degree line from their foundation bottom: the line would reach 5 metres horizontally at 2.5 metres depth. Since you're only 4 metres away, the line passes through your excavation, so Section 6 applies.

The Basement Extension Party Wall Process

Timeline: Plan for 14-16 Weeks

Basement extensions require longer party wall procedures than other projects:

1. Weeks 1-3: Complete structural engineer's design, including excavation and underpinning details
2. Week 4: Serve party wall notices (Sections 2 and 6) to all adjoining owners
3. Week 6: Neighbours respond (14-day deadline)
4. Weeks 7-10: Extensive schedule of condition prepared (often takes 3-4 weeks for basements)
5. Weeks 11-13: Party wall award drafted, reviewed, revised
6. Week 14: Award served on both parties
7. Week 16+: 2-month notice period ends, work can commence

Step 1: Get Professional Structural Design

Before serving party wall notices, commission a structural engineer who specializes in London basement extensions. They'll design:

• Excavation sequence: In what order excavation will proceed
• Temporary works: Shoring, props, and support systems
• Underpinning method: How party walls will be supported at the new level
• Permanent structure: Retaining walls, slabs, and waterproofing
• Foundation impact: Calculations showing neighbouring properties won't be affected

Your party wall surveyor needs these detailed drawings to properly assess the proposed work and prepare appropriate protective conditions in the party wall award.

Step 2: Serve Combined Notices

For basement extensions, you'll serve combined notices covering:

• Section 6 Notice: For excavation work within 3m/6m
• Section 2 Notice: For underpinning and other party wall works
• Section 1 Notice: If building new walls at boundaries (e.g., light wells)

Notice must be served at least 2 months before work starts (Section 2 governs the timing). Include:

• Detailed excavation plans showing depths and distances from boundaries
• Cross-sections showing relationship to neighbours' foundations
• Underpinning drawings and methodology
• Structural engineer's calculations
• Building regulations application reference

Step 3: Appoint Surveyors

For basement extensions, Party Wall Agreement London strongly recommends using two surveyors (one for each party) rather than an agreed surveyor. Basements are too complex and high-risk for a single surveyor to manage without potential conflicts.

Building Owner's Surveyor (your surveyor):

• Reviews structural design for party wall compliance
• Ensures your excavation can proceed legally
• Negotiates reasonable protective conditions
• Monitors work to confirm compliance with award
• Defends against unreasonable adjoining owner demands

Adjoining Owner's Surveyor (your neighbour's surveyor):

• Protects neighbour's property from damage
• Ensures adequate safeguards in place
• Prepares comprehensive schedule of condition
• May require structural monitoring during works
• Represents neighbour's interests throughout

Third Surveyor:

• Selected by the two surveyors at the outset
• Only involved if the two surveyors can't agree on something
• Acts as arbitrator on disputed issues
• Fees only incurred if actually called upon (rare)

Step 4: Comprehensive Schedule of Condition

Basement schedules of condition are extensive and take several weeks to complete. The adjoining owner's surveyor will document:

External Elements:

• Entire external walls (not just party walls)
• Foundations where visible
• Drainage systems and manholes
• Paving and external ground levels
• Garden walls, outbuildings, and structures
• Trees within the zone of influence

Internal Elements:

• All rooms, particularly those adjacent to party walls
• Basement or ground floor (most at risk from settlement)
• Ceilings and cornicing (vibration damage indicators)
• Existing cracks with measurements and crack monitors
• Window and door frames (distortion indicates settlement)
• Staircases (vulnerable to structural movement)

The surveyor may also install crack monitoring stations – small markers that allow precise measurement of any crack widening during your excavation work.

Step 5: The Party Wall Award

Basement extension party wall awards are detailed documents, often 20-30 pages plus drawings. They specify:

Excavation Sequencing:

• Maximum depth per excavation lift (often 500mm-1000mm per stage)
• Order in which excavation proceeds
• Required temporary supports before each lift
• Approval points where surveyor inspects before work continues

Underpinning Requirements:

• Underpinning method (mass concrete, mini-piles, etc.)
• Length of underpinning sections (typical 1-1.5m "legs")
• Concrete specifications and curing times
• Shoring and temporary support requirements
• Testing and inspection provisions

Monitoring Provisions:

• Regular site inspections by surveyors
• Crack monitoring and reporting procedures
• Vibration monitoring if required
• Settlement monitoring using levels
• Immediate notification of any issues

Protective Measures:

• Party wall protection (boarding, waterproofing)
• Existing foundation exposure and temporary support
• Water management and drainage during excavation
• Dust and noise control measures
• Vibration limits for machinery

Access Rights:

• When and how builders can access adjoining property
• Notice periods for access requests
• Limitations on access times and duration
• Liability and insurance requirements

Step 6: The Excavation Phase

During excavation, your contractor must:

• Notify surveyors: Before starting each excavation stage
• Follow sequencing: Excavate in the order specified in the award
• Install supports: Temporary shoring as excavation proceeds
• Stop for inspections: At specified depths or approval points
• Monitor neighbours: Check for any signs of movement or damage
• Report issues: Immediately notify surveyors of any concerns
• Maintain access: Allow surveyors to inspect at any reasonable time

Party Wall Agreement London recommends weekly surveyor visits during active excavation and underpinning phases. This frequency allows early detection of any issues before they become serious.

Step 7: Underpinning the Party Wall

Underpinning is the process of extending party wall foundations to the new, lower basement level. Common methods include:

Traditional Mass Concrete Underpinning:

• Excavate small sections (legs) under existing foundations
• Pour concrete in alternating legs (skip one, pour one)
• Allow each leg to cure before excavating adjacent sections
• Slower but reliable method
• Typical leg length: 1-1.5 metres

Mini-Pile Underpinning:

• Drill piles down to stable ground
• Create pile caps and beams under existing foundations
• Faster but requires specialized equipment
• Better for poor ground conditions
• Higher cost but less disruptive

Your party wall award will specify the underpinning method based on your structural engineer's design and the adjoining owner's surveyor's requirements for protecting their property.

Step 8: Final Inspections

After basement works complete, the adjoining owner's surveyor conducts final inspections comparing current condition against the original schedule. For basements, this involves:

• Re-measuring all documented cracks
• Checking crack monitoring stations
• Surveying for new cracks or damage
• Testing drainage systems
• Checking door and window operation
• Reviewing structural monitoring data

If damage has occurred, the party wall award provides the mechanism for resolving it – either your contractor makes repairs or you compensate your neighbour to arrange their own repairs.

How Much Do Basement Extension Party Wall Procedures Cost?

Basement extensions have the highest party wall costs due to complexity and risk:

Surveyor Fees

• Simple single-storey basement: £2,500-£4,000 total (both surveyors)
• Double basement or large area: £4,000-£7,000 total
• Complex multi-property basement: £7,000-£12,000+ total

These fees cover:

• Initial design review and advice
• Notice preparation and service
• Extensive schedules of condition (both sides)
• Party wall award preparation
• Weekly site inspections during excavation
• Underpinning inspection and approval
• Crack monitoring and reporting
• Final inspections and reports

Additional Costs

• Structural monitoring: £2,000-£5,000 if required (precise leveling and movement tracking)
• Crack monitoring stations: £500-£1,500 installation and reading
• Third surveyor fees: £1,000-£3,000 if called upon (rare)
• Specialist reports: £2,000-£5,000 if disputes require expert assessment

As the building owner, you pay for:

1. Your own surveyor's fees
2. All adjoining owners' surveyor fees
3. Third surveyor fees if needed
4. All monitoring and testing costs

For a typical London terraced house with basement on both sides of you, budget £4,000-£8,000 total for party wall costs. This represents approximately 2-4% of a £200,000 basement extension budget.

Common Basement Extension Party Wall Issues

Ground Conditions Surprises

London's varied geology can reveal surprises during excavation:

• London Clay: Shrinks when dried, causing settlement
• Made ground: Unstable fill materials requiring different underpinning
• High water table: Increased hydrostatic pressure and waterproofing challenges
• Old wells or cellars: Unexpected voids requiring filling
• Tree roots: Large roots affecting excavation sequence

A good party wall award includes provision for variations if ground conditions differ from the structural engineer's assumptions. This prevents work stoppages while surveyors negotiate changes.

Multi-Property Complications

In terraced streets, your basement affects multiple properties:

• Both immediate neighbours (left and right)
• Potentially neighbours behind if extending under garden
• Properties above if you're in a converted house with flats

Each adjoining owner can appoint their own surveyor, multiplying the complexity and costs. Some owners consent, others appoint surveyors, and some don't respond (requiring you to appoint on their behalf).

Existing Basements Next Door

If your neighbours already have basements, party wall matters become more complex:

• Their basement walls are now party structures
• Your excavation might go deeper than theirs
• Waterproofing systems must be compatible
• You may need to underpin their basement retaining walls
• Access for work could require going through their basement

The party wall award must carefully address these interactions to protect both basement structures.

Rear Garden Basement Extensions

Extending basements under rear gardens raises additional issues:

• Light wells and ventilation affecting neighbouring gardens
• Excavation deeper than neighbours' garden walls
• Drainage and water runoff changes
• Tree protection zones
• Access for construction through gardens

Your surveyor must serve notice on all affected neighbours, which can include properties behind yours if excavation extends close to their boundaries.

Best Practices for Basement Extension Party Wall Success

1. Start Early

Begin party wall procedures 4-5 months before your planned excavation start date. This allows:

• Time for complex schedules of condition
• Negotiation of award conditions
• Resolution of any disputes
• The full 2-month notice period
• Buffer for unexpected delays

2. Choose Experienced Basement Surveyors

Not all party wall surveyors have basement experience. Look for surveyors who:

• Have completed 20+ basement projects
• Understand geotechnical engineering
• Know London ground conditions
• Can read structural calculations
• Have relationships with structural engineers
• Provide references from recent basement projects

3. Use a Specialist Basement Contractor

Your contractor must have:

• Extensive experience with London basements
• Knowledge of party wall award requirements
• Track record of working with party wall surveyors
• Proper insurance (£5-10 million minimum for basements)
• Competent site managers who understand party wall procedures

4. Budget Adequately

Include in your budget:

• Party wall surveyor fees: 2-4% of construction costs
• Contingency for ground conditions: 10-15% of construction costs
• Potential repairs to neighbours' properties: 2-5% contingency
• Delay costs if issues arise: adequate cash reserves

5. Communicate Openly

Keep neighbours informed:

• Discuss plans informally before serving formal notice
• Explain the process and their rights
• Provide construction timeline and key milestones
• Give advance notice of noisy or disruptive phases
• Respond promptly to concerns
• Consider goodwill gestures (arranging alternative parking, noise-canceling provisions)

6. Monitor Proactively

Don't wait for problems:

• Check neighbours' properties regularly for new cracks
• Photograph your work daily to document proper procedure
• Maintain detailed site diaries
• Address minor issues immediately before they escalate
• Keep surveyors informed of progress and any variations

What Happens If Damage Occurs?

Despite careful procedures, some damage can occur during basement excavation. The party wall award provides the resolution mechanism:

1. Documentation: Your neighbour or their surveyor reports the damage
2. Assessment: Surveyors visit to inspect and photograph
3. Comparison: Current condition compared to schedule of condition
4. Causation: Surveyors determine if damage resulted from your works
5. Remedy: Two options offered to your neighbour:
   • Your contractor makes repairs (most common)
   • Cash payment allowing neighbour to arrange own repairs

If there's disagreement about damage or repair costs, the surveyors resolve it. If they can't agree, the third surveyor makes the final decision.

Can I Do My Basement Extension Without Party Wall Procedures?

Absolutely not. Basement extensions without party wall procedures are:

• Unlawful: You're breaking the law
• Dangerous: Risk undermining neighbours' foundations
• Expensive: Injunctions and legal costs far exceed party wall fees
• Uninsurable: Your insurance won't cover damage from unauthorized works
• Unmortgageable: Future buyers can't get mortgages if party wall procedures weren't followed

Some builders suggest "your neighbour will never notice" or "we'll be careful" – ignore this dangerous advice. Excavation work ALWAYS affects adjoining properties to some degree, and without proper procedures, you're liable for all consequences.

Conclusion: Basement Extensions and Party Walls

Basement extensions are complex, high-value projects that require expert party wall handling. Key takeaways:

• Section 6 applies to nearly all London basement extensions (3m/6m rules)
• Budget £4,000-£8,000 for party wall procedures (2-4% of construction costs)
• Start party wall process 4-5 months before planned excavation
• Use two experienced surveyors (not an agreed surveyor)
• Ensure comprehensive schedules of condition on all adjoining properties
• Choose contractors with proven basement and party wall experience
• Budget contingencies for ground conditions and potential damage
• Monitor proactively throughout excavation and underpinning
• Maintain good neighbour communication throughout
• Never attempt basement work without proper party wall procedures

Party Wall Agreement London specializes in basement extension party wall matters throughout London. Our surveyors have extensive experience with Section 6 excavation rules, underpinning procedures, and the unique challenges of London's Victorian and Edwardian housing stock. We work efficiently to prevent project delays while ensuring your neighbours' properties are fully protected.

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