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Why Flat Roof Extension Costs Vary

Thinking about adding a flat roof extension to your home in Ealing, Chiswick, Richmond, or West London?
One of the first questions that pops up is: “How much is this really going to cost me?”

At GLS Building Contractors Ltd — with over 20 years of experience — we’ve built and designed hundreds of home extensions. Flat roofs are popular for their modern look and space efficiency, but costs depend on many factors.

In this guide, we’ll break down real-world prices, what affects them, and how to plan your project smartly — so you can create more living space without overspending.

Typical Cost Ranges in London

Let’s talk numbers. Below are realistic average costs for flat roof extensions in West London areas like Ealing, Chiswick, Hammersmith, and Richmond.

Size / Description	Estimated Cost (£)	Notes
Small (15–20 m²)	£25,000 – £45,000	Ideal for a small kitchen or dining area
Medium (25–30 m²)	£40,000 – £65,000	Great for open-plan living
Large (35–40 m²+)	£55,000 – £85,000+	Includes premium finishes and glazing

Expect £1,200 – £2,500 + per m², depending on your finishes, materials, and access conditions.
Tip: Always budget 10–15% extra for contingencies — nearly every project needs it.

What You’re Really Paying For

Here’s what your quote actually covers:

• Foundations & Groundworks: digging, drainage, concrete slabs.
• Structural Work: walls, insulation, waterproofing, and steel support beams.
• Flat Roof Build: roof membrane, insulation, skylights, trims, and rooflights.
• Internal Finishes: plastering, flooring, painting, electrics, and heating.
• Professional Fees: architects, engineers, and building control.

So when comparing prices, remember — a low quote might mean corners are being cut.

How Location & Property Type Affect Cost

In areas like Ealing, Chiswick, and Westminster, prices are typically higher due to:

• Higher labour and material rates in London.
• Restricted access and parking, increasing logistics costs.
• Local planning permissions or conservation restrictions.

At GLS, we know the West London housing landscape — so we help clients navigate local planning smoothly while keeping costs predictable.

Choosing the Right Roof Type & Finish

Not all flat roofs are equal. You can choose between:

• EPDM Rubber Roofs – affordable, durable, great for extensions.
• GRP Fibreglass Roofs – sleek finish, long lifespan, minimal maintenance.
• Warm Roof Systems – energy-efficient insulation built into the roof.

Learn more about materials, finishes, and extension options on our
Roofing & Roof Extensions page.

Case Studies: Realistic Examples

Example	Size	Cost (£)	Best For
Kitchen Extension	20 m²	£30,000 – £45,000	Compact, bright kitchen-diner
Rear Extension	30 m²	£45,000 – £65,000	Open-plan family space
Wrap-Around	40 m²+	£60,000 – £85,000+	Large kitchen/living with bi-fold doors

Each home is unique — that’s why we offer personalised quotes based on your property, layout, and desired finish.

Hidden Costs You Might Not Expect

Here are some sneaky extras that can affect your budget:

• Party Wall Agreements (if adjoining neighbours)
• Planning permissions or building control fees
• Structural surprises in older properties
• Access issues that add labour and equipment costs

Want to budget like a pro? Check out this guide on
home renovation budgeting advice.

Budgeting & Smart Planning Tips

1. Always add 10–15% contingency for unknowns.
2. Get multiple detailed quotes from reputable builders.
3. Lock in your design and materials early to avoid mid-project costs.
4. Work with an experienced builder who handles design, planning, and construction.

We do exactly that at GLS — our in-house team manages every stage, keeping your project stress-free and transparent.

Is a Flat Roof Extension Worth It?

Absolutely. A well-built flat roof extension can increase your property value by 10–20% and provide the extra space your family needs — all without moving.

Homeowners in Ealing and West London love the flexibility: you can create a bright kitchen, lounge, or workspace — and with proper insulation, flat roofs are highly energy-efficient.

Learn more about your options on our
Roofing & Roof Extensions page.

Next Steps: Choosing the Right Builder

Picking the right builder is everything.
At GLS Building Contractors Ltd, we provide:

• Free on-site consultation & quote
• Detailed project planning
• Full management from concept to completion

Not sure what to look for? Read this guide on
how to choose a reliable builder in London.

FAQs

1. How much does a flat roof extension cost in London?
Usually between £25,000 – £85,000, depending on size, design, and finishes.

2. Do I need planning permission?
Some small flat roof extensions fall under permitted development — others need approval. GLS helps you handle it all.

3. How long does a flat roof extension take?
Anywhere between 8 to 16 weeks, depending on project scale and weather.

4. Is a flat roof cheaper than a pitched roof?
Yes, generally 15–25% cheaper, and often quicker to build.

5. Will it add value to my home?
Definitely — a modern, well-built extension can boost resale value and functionality.

6. What areas do you cover?
We work across Ealing, Chiswick, Richmond, Hammersmith, Kensington, and all of West London.

Ready to Transform Your Home in West London?

With 20+ years of experience, GLS Building Contractors Ltd is one of Ealing’s most trusted home renovation and building specialists.

Let’s bring your flat roof extension vision to life — on time, on budget, and built to last.

👉 Get Your Free Quote Today
👉 Explore Roofing & Roof Extensions