Building a home extension in London can be one of the most rewarding investments you make — increasing your living space, boosting property value, and improving your home’s functionality. However, costs can vary significantly depending on a range of factors.
Whether you’re planning a kitchen extension, a loft conversion, or a multi-storey addition, understanding the key cost drivers will help you plan effectively and avoid unexpected surprises.
What Affects the Cost of an Extension?
Here’s a breakdown of the main factors that influence the overall cost of building an extension in London:
Size and Complexity
Naturally, a larger or more complex extension will cost more. A basic single-storey extension is more affordable than a double-storey or wraparound extension. Intricate designs or bespoke elements can also raise costs.
Location Within London
Construction costs in London are generally higher than elsewhere in the UK, and prices can vary between boroughs. Central or high-demand areas often come with increased labour and material costs.
Materials and Finishes
Premium materials, glazing, or bespoke finishes will elevate the cost. Choosing quality, durable materials is wise — but it’s important to strike a balance with your budget.
Architectural Design & Planning
Custom architectural designs and the need for planning permission will increase project costs. Planning applications come with fees and may involve professional consultancy costs.
Site Conditions
Your property’s existing structure and ground conditions can affect foundations and structural design. Poor soil or drainage, for example, may require more extensive groundwork.
Labour and Regulations
London’s skilled labour market is competitive, and tradespeople charge accordingly. Your project must also meet UK Building Regulations, which may affect design and material requirements.
Utilities and Internal Works
Connecting plumbing, electrics, and heating into the new space adds further cost — especially if systems need upgrading or rerouting.
Doors, Windows, and Insulation
Energy-efficient installations such as large bi-fold doors or triple glazing offer long-term savings but cost more upfront.
Interior Finishes
Flooring, lighting, kitchen fittings, and décor vary greatly in price, depending on personal taste and project standards.
Contingency
It’s wise to budget an additional 10–15% of your total project cost to cover unexpected issues or design changes during construction.
Typical Costs per Square Metre
To provide a general estimate:
Basic single-storey extension: £1,500–£2,500 per m²
Mid-range spec: £2,500–£3,500 per m²
High-end finishes or complex design: £3,500–£4,500+ per m²
Double-storey extensions generally cost 50–75% more than single-storey, depending on layout and structural demands.
These figures are rough guidelines — always obtain detailed, written quotes from professionals based on your specific project.
Choosing the Right Builder for Your Extension
Once you understand the cost, selecting the right builder is vital. A good builder brings not only craftsmanship, but also reliability and transparency to your project. Here are tips for choosing wisely:
Do Your Research
Look for builders with a strong local reputation. Read reviews, check their website, and ask friends or neighbours for recommendations.
Check Credentials
Ensure your builder is licensed, insured, and compliant with UK standards. Ask for proof and confirm that they work with trusted suppliers and subcontractors.
Review Their Experience
Look for builders with proven experience in the type of extension you’re planning. Ask to see a portfolio of completed projects.
Ask for References
Request references and speak to previous clients about their experience — not just the results, but also communication, timeliness, and professionalism.
Demand Transparency
A reputable builder should provide a clear, written quote including labour, materials, timelines, and payment terms.
Get a Contract
Always sign a written agreement that outlines the full scope of work, deadlines, payment schedule, and warranties.
Avoid Red Flags
Be cautious of unusually low quotes or large upfront payments. These can be signs of financial instability or corner-cutting.
Don’t Overlook Your Structural Engineer
Before building begins, a structural engineer is essential to ensure your extension is safe, legal, and structurally sound. They assess your architect’s plans and calculate the loads, materials, and supports required — particularly for tasks like removing load-bearing walls or creating large open-plan areas.
At Neasden Structural Engineers, we provide expert structural calculations and drawings for home extensions of all types — from modest kitchen builds to complex double-storey additions.
Contact Neasden Structural Engineers to ensure your house extension is going to stand the test of time. A structural engineer will look at your architectural drawings and work out the building calculations for your new construction. We can help homeowners in Camden, Chiswick, Ealing, East Finchley, Edgware, Hampstead, Harrow, Kilburn, Neasden, Shepherds Bush, Wembley, Willesden.
Call us on 020 8088 3834 for a consultation or to request a quote for your next project.
