Semi-Detached Properties and Party Walls: What Owners Must Know

Semi-detached properties have unique party wall considerations that differ from terraced houses or detached homes. With just one shared wall and one neighbour to deal with, the party wall process is often simpler – but understanding your specific situation is essential. This comprehensive guide covers everything semi-detached homeowners in London need to know about party walls.

Understanding Your Semi-Detached Party Wall

In a semi-detached property, the party wall is the shared structural wall that separates your home from your neighbour's. This wall typically:

• Runs the full depth of both properties (front to back)
• Supports both roofs and upper floors
• Is load-bearing and structural
• Shared ownership – both properties own half
• May include chimney stacks built into the wall

Common Features of Semi-Detached Party Walls:

• Usually 9-inch (225mm) brick thickness (older properties)
• Modern properties: cavity walls with separate leaves
• Often contains flues for chimneys or boilers
• May have different finishes on each side
• Loft space above wall may be party structure

Common Building Works in Semi-Detached Properties

1. Single-Storey Rear Extensions

Party Wall Implications:

• Section 2 Notice Required: If cutting into party wall for steel beam support
• Section 1 Notice Required: If building new walls up to the boundary
• Timeline: 2 months' notice for Section 2 works
• Typical Cost: £800-£1,500 party wall surveyor fees (one neighbour)

2. Two-Storey Side Extensions

Many semi-detached owners extend to the side (non-party wall side) to create a wider, more detached-style home.

Party Wall Considerations:

• Usually NO party wall implications (building on your own land away from shared wall)
• Exception: If excavating foundations within 3m or 6m of neighbour's property (Section 6)
• Check boundary carefully – some side extensions ARE near boundaries

3. Loft Conversions

Very Common in Semi-Detached Properties:

• Section 2 Notice: Required if raising party wall height or cutting into wall for beams
• Roof Structure: If roof structure spans across party wall, work to it may require notice
• Chimney Removal: If removing shared chimney breast, Section 2 notice essential
• Typical Scenarios: Dormer windows, roof lights, raising ridge height

4. Basement Conversions

High-Risk Works:

• Section 6 Notice: Excavating below neighbour's foundation level
• Section 2 Notice: May need to underpin party wall foundations
• Complex Process: Structural engineers and experienced surveyors essential
• Schedule of Condition: Absolutely critical for basement works

Step-by-Step: Party Wall Process for Semi-Detached Homes

Step 1: Initial Planning (3-6 Months Before Start)

• Finalize architectural plans
• Identify which party wall notices you'll need
• Talk informally with your neighbour about plans
• Appoint a party wall surveyor

Step 2: Serve Notices (2-3 Months Before Start)

• Section 2 Notice: 2 months before work starts (structural works to party wall)
• Section 1 Notice: 1 month before work starts (new boundary walls)
• Section 6 Notice: 1 month before excavation starts
• Keep proof of service (recorded delivery recommended)

Step 3: Neighbour Response (14 Days)

Your neighbour has three options:

• Consent: Work can proceed after notice period expires
• Dissent: Party wall award process begins (this is normal and not confrontational)
• No Response: Deemed dissent after 14 days

Step 4: Award Process (4-8 Weeks if Dissent)

• Neighbour appoints their own surveyor OR agrees to an Agreed Surveyor
• Schedule of condition prepared for neighbour's property
• Surveyors prepare party wall award detailing:
  • Permitted works and methods
  • Working hours
  • Access arrangements
  • Protective measures
  • Damage remediation procedures
• Award served on both parties (14-day appeal period)

Step 5: Works Commence

• Only start after notice periods expire AND award is served (if required)
• Follow award conditions strictly
• Maintain good communication with neighbour
• Surveyor may inspect at critical stages

Step 6: Completion and Sign-Off

• Notify neighbour and surveyor when works complete
• Post-completion inspection against schedule of condition
• Any damage identified must be repaired
• Final sign-off from surveyor

Specific Issues for Semi-Detached Properties

Chimney Breasts and Flues

Many semi-detached properties built before 1970 have shared chimney stacks within the party wall.

Removing Your Chimney Breast:

• Section 2 Notice Required: You're altering the party structure
• Structural Implications: May need to support your neighbour's remaining breast
• Professional Advice Essential: Structural engineer + party wall surveyor
• Cost Responsibility: You pay for supporting works and party wall fees

If Neighbour Has Already Removed Theirs:

• They may not have followed correct procedures (common in older works)
• Doesn't exempt you from serving notice
• Check if proper support was provided for your breast

Loft Party Walls

The party wall continues through the loft space in most semi-detached properties.

Loft Conversion Considerations:

• Wall may need raising to new roof height
• Party wall timber frame may need fire-stopping upgrades
• New steel beams often bear onto party wall
• Roof structure may cross the party wall line

Different Property Styles and Ages

Sometimes semi-detached neighbours are different:

• Your side: 2-storey / Neighbour: Bungalow → Raising your party wall requires Section 2 notice
• Your side: Original / Neighbour: Already extended → Check existing works didn't compromise your wall
• Different ages: Victorian paired with modern → Wall may have different construction each side

Cost Breakdown for Semi-Detached Party Wall Works

Conclusion: Simpler But Still Essential

Semi-detached properties often have simpler party wall requirements than terraced homes, but compliance remains essential. With one shared wall and one neighbour, the process is typically faster and less expensive – but skipping proper procedures can still lead to serious consequences.

Key takeaways for semi-detached homeowners:

• One shared party wall means one neighbour to deal with
• Costs typically 50% of mid-terrace equivalents
• Side extensions usually don't require party wall notices (unless excavating near boundary)
• Loft conversions almost always require Section 2 notices
• Chimney breast removal always requires Section 2 notice
• Follow the same legal process – no shortcuts