A Homeowner’s Checklist on How to Choose the Right Rear Extension Builder in London

Rear Conversion by RS D&M - Exterior Angle

Building a rear extension in London is one of the most rewarding upgrades you can make to your home. It increases living space, improves functionality, and can add substantial value to your property. But the success of your project relies heavily on choosing the right builder - especially in London, where older housing stock, tight access and complex planning rules require a highly skilled team.

Whether you live in a Victorian terrace, Edwardian semi or modern home, here is a step-by-step checklist designed to help London homeowners select a trustworthy, high-quality rear extension builder who can deliver a seamless, stress-free project.

1. Choose a Builder Who Specialises in Rear Extensions (Not Just “General Building”)

Rear extensions are complex - especially in London.

You need structural steel installation, excavation, foundation work, drainage upgrades, glazing systems, and integration with the existing property. A “general builder” often lacks the experience required for:

• Structural engineering coordination

• Steel framework installation

• Managing London’s tight access sites

• Working within conservation or restricted zones

• Integrating period buildings with modern extensions

Look for a builder who regularly completes rear extensions, ideally with examples they can show you.

2. Check Their Recent London Projects

A legitimate, skilled builder will proudly show:

• Before & after photos

• Drawings and structural plans

• Videos or walkthroughs

• A portfolio of completed extensions in your area

  
  

Even better, ask for addresses you can drive past. Most London builders will have done work in:

• Clapham

• Balham

• Wandsworth

• Richmond

• Wimbledon

• Fulham

• East/West London terraces

  
  

If a builder avoids showing past work - that’s a red flag.

3. Only Work With a Builder Who Provides a Detailed, Transparent Quote

A good quote should include:

✔ Clear scope of works

Demolition, foundations, steelwork, insulation, electrics, plumbing, finishes, glazing -all listed clearly.

✔ Itemised pricing

Not just “£xx for extension”.

✔ Separate allowances for finishes

Such as:

• Kitchen budget

• Flooring budget

• Lighting allowance

• Tiles & bathroom fixtures

  
  

✔ Project timeline

A week-by-week breakdown is ideal.

If you receive a vague quote with no detail, expect hidden costs later.

4. Ask Whether They Handle Planning Permission or Permitted Development

Rear extensions in London often require:

• Planning permission

• Lawful Development Certificates

• Party Wall Agreements

• Structural engineering drawings

  
  

Some builders only build, meaning you must organise the architectural and planning work yourself, adding stress, cost and risk.

A better option is a design-and-build company that manages:

• Architectural design

• Structural calculations

• Planning permission

• Building Control

• Party Wall

• Construction

• Final sign-off

• 
  

This single-team approach prevents miscommunication and delays.

5. Look for Builders Experienced With Structural Steel Installations (RSJs)

Rear extensions almost always need steel beams to open up the rear wall and support the upper floors.

Your builder must be capable of:

• Coordinating steel calculations

• Ordering steel to specification

• Installing RSJs safely

• Working with Building Control inspectors

• Managing temporary supports

This is a non-negotiable skill.

A builder who doesn’t frequently handle steelwork can cause major structural issues.

6. Check If They Have Experience With Modern Glazing Systems

Internal and External Glazing by RS D&M

London homeowners love big, open spaces with natural light, meaning your rear extension may include:

• Sliding doors

• Slimline aluminium frames

• Rooflights

• Lanterns

• Corner glazing

• Fixed picture windows

  
  

Not all builders know how to integrate these correctly with:

• Steel supports

• Thermal breaks

• Flat roof details

• Drainage

• Insulation

  
  

Ask to see past glazing installations. Poor glazing = leaks, condensation and heat loss.

7. Ask for a Project Manager - Not Just a “Builder in Charge”

A dedicated project manager (PM) ensures:

• Daily communication

• Timelines are kept

• Site coordination

• Problem solving

• Minimising disruption

• Quality checks

  
  

Without a PM, you’ll end up managing subcontractors, schedules and decisions yourself.

High-end rear extension builders always include a PM, it’s standard practice.

8. Confirm They Have Insurance, Warranty & Certifications

Your builder should have:

✔ £5m Public Liability Insurance

✔ Employers’ Liability Insurance

✔ Structural Warranty (10 years)

✔ Gas Safe certification (plumbing/heating)

✔ NICEIC certification (electrics)

✔ Waste carrier licence

Never accept excuses. These documents protect you, your home and your investment.

9. Check How They Handle Unexpected Issues (Because London Houses Hide Surprises)

In older properties, you may encounter:

• Poor rear drainage

• Weak foundations

• Damaged brickwork

• Hidden structural issues

• Asbestos

• Damp or rot

  
  

Ask your builder:

“How do you deal with unexpected work? What does your process look like?”

Top builders will show you transparent pricing. Cheap builders will simply say: “Don’t worry -we’ll sort it.”…which usually means financial surprises later.

10. Read Genuine Reviews and Testimonials

Along with a company’s testimonials remember to look at things like:

• Google reviews

• Trustpilot

• Houzz

• Checkatrade (with caution -many fake profiles exist)

  
  

Look for keywords like:

✔ Extensions

✔ Project management

✔ Communication

✔ Transparency

✔ Build quality

✔ On budget / on time

Avoid builders with only generic comments like “nice guy” or “good job”.

11. Ask for a Clear Timeline and Compare It Across Builders

A realistic timeline for a high-quality London rear extension is:

• Design + Planning: 8-14 weeks

• Construction: 10-18 weeks

  (depending on size, glazing, complexity, and access)

  
  

If someone claims they can complete everything in 6–8 weeks -it’s a lie.

12. Choose a Builder Who Understands Your Property Type

London has:

• Victorian terraces

• Edwardian semis

• 1930s homes

• Townhouses

• Flats with freeholders

• Modern infill properties

  
  

Each requires different structural and architectural approaches. Choose a builder familiar with your home’s area and layout.

Final Advice: Don’t Choose the Cheapest Builder -Choose the Right One

Rear extensions involve structural changes, roofing, electrics, plumbing, glazing and finishing. If built poorly, the repair costs can exceed the original budget.

Choose a builder who is:

• Experienced

• Transparent

• Properly insured

• Well-reviewed

• London-focused

• Design-and-build capable

• Proven with past rear extensions

  
  

This guarantees your project runs smoothly, stays on budget, and delivers an exceptional finish.

Need a Reliable Rear Extension Builder in London?

Our team at RS D&M Limited specialises in:

• Rear extensions

• Side returns

• Wraparounds

• Loft conversions

• Full refurbishments

• Planning-to-completion design + build

We manage everything for you; design, planning, structural work, and construction.  Have any more questions visit our FAQ page or give contact us today to book your free no obligation survey.

RS D&M Limited delivers prime, high-quality design, build and construction services with a focus on quality, innovation, and client relationships in Barnes, Battersea, Belgravia, Clapham, Chessington, Claygate, Chelsea, Chiswick, Esher, East Sheen, Earls Court, Epsom, Ewell, Fulham, Hampton, Hammersmith, Holland Park, Isleworth, Kensington, Kingston, Kew, Knightsbridge, Leatherhead, Mayfair, Merton, Mitcham, Morden, New Malden, Notting Hill, Paddington, Parsons Green, Pimlico, Putney, Raynes Park, Shepherds Bush, South Kensington,