For any London homeowner considering a home extension, one question precedes all others: „How much is this going to cost?” It’s the most critical factor in any decision-making process, yet it’s often the hardest to get a straight answer for. The internet is filled with vague estimates, hidden costs are a common fear, and the prospect of a spiralling budget can be enough to stop a dream project in its tracks.
The truth is that while a simple per-square-metre rate can provide a starting point, the real cost of a high-quality London home extension is far more nuanced. It’s influenced by everything from your local soil conditions to your choice of kitchen taps.
This guide is designed to demystify the process. We will transparently break down the key factors that truly determine the final cost of your project. Our goal is to empower you with the knowledge to budget effectively and to understand how a modern, professional construction process can provide the one thing every client deserves: financial certainty.
The "Per Square Metre (£/m²)" Rate: A Starting Point, Not a Final Answer
You will often see builders and articles quote a per-square-metre rate for extensions. As of 2025, a realistic range for a standard home extension in London is typically £2,500 to £4,500 + VAT per square metre.
It is crucial to understand why this range is so broad and what it usually covers. This „basic” rate generally accounts for the construction of the structural shell and making it watertight. This includes:
Foundations and groundworks
Structural walls and steelwork
The roof structure and covering
Basic plumbing and electrical first-fixes
Standard windows and external doors
Plasterboarded internal walls
However, this rate almost always excludes the biggest variables that define the quality and cost of your finished space. These exclusions are often where budgets become unpredictable in a traditional build. They include the kitchen, bathrooms, flooring, tiling, decoration, professional fees, and any high-end features like bespoke joinery or large glazed doors. A £/m² rate is a useful initial gauge, but to get a real cost, you must look deeper at the key drivers.
THE COST DRIVERS
The 5 Key Factors That Determine Your Final Project Cost
The final price of your property extension is a combination of five key areas. Understanding these will give you a much clearer picture of your potential investment.
1. Size and Type of Extension
This is the most obvious factor. A larger extension will naturally cost more. However, the type of extension is just as important. A simple, single-storey rear extension on flat ground is the most cost-effective. A two-storey extension is more complex, requiring deeper foundations and more extensive structural work. A wraparound extension (combining a side and rear extension) is the most complex of all, involving multiple structural junctions and extensive reconfiguration of the existing house, which is reflected in the cost.
2. Structural Complexity
No two sites are the same, and the structural work required is a major cost driver. Key factors include:
Ground Conditions: London’s clay soil can require deeper or more engineered foundations, adding to the cost.
Steelwork: The amount and complexity of steel beams needed to support the house above an open-plan space is a significant cost. Large „box-frame” designs are more expensive than simple beam-and-post constructions.
Glazing: While flooding your home with light is a key goal, large-span bi-fold or sliding doors and structural glass skylights are high-value items that have a significant impact on the budget compared to standard windows.
3. The Specification Level (The Finishes)
This is where you, the client, have the most control and where budgets can vary the most dramatically. The quality of the finishes you choose defines the character and final cost of your home renovation. Key areas include:
The Kitchen: This is often the single biggest-ticket item. A budget kitchen from a high-street supplier might cost £15,000, while a bespoke, handmade kitchen with high-end appliances can easily exceed £100,000.
Bathrooms: The cost is determined by your choice of tiles, sanitaryware, brassware, and any custom joinery.
Flooring, Tiling, and Decoration: The choice between engineered wood, luxury vinyl, or large-format porcelain tiles will significantly affect the cost per square metre.
Bespoke Joinery: Custom-made wardrobes, media units, and storage solutions add a huge amount of value and quality but are a considerable investment.
4. The "Unseen" Essentials
Homeowners often forget to budget for the necessary upgrades to the home’s infrastructure that an extension triggers. These can include:
A new, more powerful boiler and/or an unvented hot water cylinder to service the new space and extra bathrooms.
Upgrading your home’s main consumer unit (fuse box) to handle the additional electrical load.
Significant drainage alterations or new connections to the main sewer.
External works such as landscaping, new patios, and garden walls to integrate the extension with your outdoor space.
5. Professional Fees
A high-quality project requires a team of experts. These fees are a crucial part of the overall investment. They include costs for architectural design, BIM modelling, structural engineering calculations, planning application fees to the council, Building Control inspection fees, and potentially Party Wall surveyors. A key benefit of a turnkey service like ours is that these professional services are managed and integrated into a single, cohesive project plan.
THE SOLUTION
The Real-World Benefits of a Design & Build 2.0 Home Extension
Understanding these variables can feel overwhelming. It highlights how, in a traditional process, costs can easily spiral as unforeseen issues arise. This is precisely the problem our Design & Build 2.0 model is designed to solve.
Our process delivers financial certainty through technology and accountability. By creating a hyper-detailed BIM „digital twin” of your project, we can plan and quantify everything in advance. Our 3D model contains every material, every steel beam, and every light fitting. This data-driven approach allows us to:
Prevent Errors: We solve structural and service clashes digitally, so they never become expensive problems on-site.
Provide Accurate Quantities: We can calculate exactly how much material is needed, reducing waste and allowing for precise costings.
Offer a Fixed-Price Contract: The culmination of this detailed digital planning is our ability to offer you a guaranteed fixed-price contract for the construction work. Once the design and specification are finalised and agreed upon, the price is locked in.
This is our commitment to absolute transparency. It removes the single biggest source of stress from any home extension project and gives you the confidence to invest in your home without the fear of the unknown.
(Ready to learn more? Explore our specialist Home Extension services to see how we deliver.)
Conclusion: From Variable Costs to Guaranteed Value
While the cost of a London home extension has many variables, the process of budgeting for it should not be a mystery. By understanding the key drivers – from the structural form to the final finishes – and by partnering with a firm that uses modern technology to deliver a fixed-price guarantee, you can move forward with confidence. A well-planned, professionally managed extension is not just a cost; it is one of the most reliable and rewarding investments you can make in your London home.
Ready to Understand the Cost of Your Vision?
The first step to a realistic budget is a professional consultation. Contact our team today for a free, no-obligation discussion about your project. We can provide an initial estimate based on your goals and show you how our process delivers the financial certainty you need.
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Ready to start your project with complete clarity and confidence? Get in touch with the Reltic BIM team today to see how our process delivers better results with a guaranteed fixed price.
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