A house extension is one of the biggest home-related decisions you’ll make, especially since it costs a lot of time, money, and effort.
According to statistics, the global remodelling market was around $3.5 trillion in 2024 and is expected to reach $5.2 trillion by 2034.
However, before you start planning for any renovations, it’s important to know how much a typical house extension costs.
Depending on where you live and what sort of extensions you want, the typical cost can range anywhere between £2500 and £4000 per square metre.
If you want interesting tips and tricks on home extensions and want to have the question “How much does a house extension cost?” answered, then let’s keep reading!
1. Build Cost of a Single-Storey Extension
Approximately, a single-storey house extension costs around £1200 to £1500 for every square metre. For example, if you build a home extension of 5×5 metres, you’ll have to spend about £30,000.
This, of course, is the standard pricing for areas in the United Kingdom outside London.
In London, the cost will be considerably more, somewhere around £50,000 or even more.
Along with this, you also need to consider additional costs that come with house extensions, such as architect fees, fees for planning and building permissions, and the cost of hiring a structural engineer.
On average, this will be around 10-15% of the overall build cost of the extension.
2. Build Cost of a Two-Storey Extension
For a two-storey extension, the general rule of thumb is to add around 50% on top of the build cost and another 10% for the fees you’ll have to pay to professionals.
For example, if the average cost to extend a house is around £39,000, then adding another 50% to it will total £58,500. Now, if you include the costs of professionals and fees, then another 10% will bring it to £76,050.
3. Build Cost of a Basement
If you have an existing basement with a good ceiling height, you can think of extending it.
In the United Kingdom, the average cost of extending a house’s basement starts from £975 per square metre for a completely waterproof space that also has a floor screed laid out.
However, to lower the floor or make any other major changes before the extension project begins, you might have to spend a significantly greater amount.
For example, underpinning the existing foundations or renovating the existing house can quickly become one of the most expensive forms of house extensions.
Typically, a full basement rework project will start from £2875 per square metre for a waterproofed basement with floor screed.
4. Build Cost of a Loft
When you’re planning a loft conversion, the most important factor is the available head height.
A habitable loft space and an internal floor area of at least 40 square metres to provide enough support for an extension are a must.
The basic costs will assume that the roof tiles are in good condition and don’t require replacing.
Generally, the price of a fully furnished roof light loft conversion is £1800 per square metre. If you have big windows inserted, then the cost will rise to an additional £1200 per square metre.
For a dormer loft conversion, the cost can be slightly higher in the United Kingdom, somewhere around £2000 per square metre.
A hip-to-gable or Mansard loft conversion will cost you even more, starting from £2550 per square metre.
5. House Extension Stages
The Royal Institute of British Architects published a document called Plan of Work, which divides the entire house extension process into different parts, starting from calling up the architect to conducting a building inspection. Overall, there are six main stages that you need to be aware of:
Feasibility Study
This is where a builder or contractor surveys the existing structure of the house and draws up an estimate for the project, such as telling you how much a house extension costs.
Planning Application
Here, the builder makes the planning application and deals with the negotiations department regarding the extension price per square metre.
Building Warrant
Here again, the building or contracting company draws up a plan that adheres to the local and national regulations. The design is also coordinated with structural engineers and an energy consultant.
Pre-Construction
In the pre-construction stage, negotiations are held with contractors and other people involved in the extension.
Construction
As the name suggests, the agency draws up a contract between the builder and the client. This contract involves all the major clauses of the extension, such as what materials will be used, how much to extend the house, and any other important pointers.
The agency might also inspect the site once per week and deal with any delays that might occur with time and payment.
Post-Construction
The architect will conduct a final inspection of the site after construction. If the inspecting officer has any problems with the extension portion, the architect will also deal with them.
6. Prices for Each Stage of Extension
Most architects in the United Kingdom will charge fixed prices for all stages of the house extension except for the first one.
For the feasibility study stage, architects typically charge by the hour since clients might ask them to repeatedly draw up inspection plans later on.
On average, these are the prices for different stages of the building extension process that an architect might ask:
| Feasibility study | Planning application | Building warrant | Preconstruction stage | Construction | Post-construction stage |
| £90 per hour for 18-25 hours | £2500 | £4500 | £2500 | £350 per week for 16-18 weeks | £900 |
| £90 per hour for 20-35 hours | £3250 | £5000 | £3000 | £350 per week for 18-22 weeks | £1000 |
| £90 per hour for 25-40 hours | £4000 | £6000 | £3500 | £350 per week for 20-26 weeks | £1100 |
The first row of quotes is for a house extension budget of £150,000 to £200,000, while the second row is for a budget of £200,000 to £300,000, and the third row is for a budget of £300,000 to £400,000.
Quite naturally, the bigger and more complex the extension project, such as a fancy single-storey side extension, the more costly it’ll be.
7. Whom to Contact
When you’re planning house extensions, and you want to know how much extensions cost, there are four main options to choose from:
Main Contractor
A reliable main contractor will give you the pricing estimates and their building firm includes a team of architectural designers, structural engineers, and much more. They will manage the entire extension process.
Using a standard contract procedure, you’ll get a quote, although you can bring down costs by doing some of the extension work yourself.
Builders and Subcontractors
You can reduce the overall costs by around 10% through this route. After the main contractor finishes the major portion of the work, such as completing the structure or the watertight stage, you can take over from there by letting subcontractors handle the rest.
Self-Management
If you plan on this route, you’ll save nearly 20% of your building costs. However, you’ll have to manage the entire project scheme yourself.
You won’t do the construction on your own, but rather, you’ll manage the subcontractors directly. You may supply the machinery, tools, and other equipment required while also managing deadlines.
DIY Extension
This route gives you maximum savings of up to 25% of the total costs. Undertaking a large part of the extension building can help you greatly reduce unnecessary costs, but you’ll also carry out many of the second-fix tasks like landscaping, general labouring, and decorating.
Conclusion
House extension is undoubtedly a time and money-consuming affair, so it’s always best to talk to a professional architect or builder agency before starting the project yourself.
Wondering where to get started? Contact us at DOMENECH DESIGN LTD to consult renowned experts and get your free quotation now!
