How Much Does a House Extension Cost?

Can You Price an Extension Per Square Metre?

You will often see extension costs quoted as a price per square metre. While this can be useful as a very rough guide, it doesn’t reflect how projects are actually costed in practice. Two extensions of the same size can vary significantly in cost depending on what’s involved behind the scenes.

For example:

• The amount of structural work required
• Whether walls are being removed or altered internally
• The type and size of windows and doors
• The style of extension (simple vs more architectural designs)
• The level of internal finishes and specification

Because of this, relying solely on a cost per square metre can be misleading. A more accurate understanding comes from looking at the specific details of your project, rather than just the size of the space.

Do You Need Planning Permission for an Extension?

One of the most common questions homeowners have is whether planning permission is required. In many cases for UK homeowners, single-storey rear extensions can be built under Permitted Development rights, meaning formal planning permission may not be needed. As a general guide:

• Detached homes can often extend up to 4 metres from the original rear wall
• Semi-detached and terraced homes are typically limited to 3 metres

However, these allowances come with conditions. Factors such as the height of the extension, proximity to boundaries, materials used and whether the property has already been extended can all affect what is permitted.

It’s also important to note that even if planning permission isn’t required, the project will still need to comply with Building Regulations. Because of this, it’s always worth speaking with an experienced design team early on. A professional assessment will confirm what is possible for your specific property and help avoid delays or complications later in the process.

How Do You Get an Accurate Quote for an Extension?

One of the most common frustrations homeowners experience is receiving quotes that are unclear, incomplete or difficult to compare.

The most accurate way to understand the cost of an extension is to begin with a properly considered design and detailed plans. At Adamsons, we start with a home design visit, where we take the time to understand your ideas, assess your space and take accurate measurements. From there, our in-house architectural designers develop plans that reflect both your vision and the practical requirements of the project.

This allows us to provide a clear, detailed quotation based on a defined scope of work rather than estimates or assumptions. Once you decide to move forward, we provide a fully detailed fixed price, so you know exactly what your project includes before work begins.

How to Find a Reputable Company for Your Extension

Choosing the right company is just as important as the design itself. A well-managed project can feel smooth and predictable, while a poorly managed one can quickly become stressful.

When researching companies, it’s worth taking the time to:

• Read recent, verified customer reviews
• Ask to see examples of completed projects
• Request references from previous clients
• Look for consistency in the quality of their work
• Ensure they offer a structured, clearly managed process

If you’re unsure what to look out for, we’ve also written a guide on how to spot the warning signs of unreliable or “cowboy” builders, which you may find helpful when starting your search.

Planning Your Extension with Confidence

Every extension project is different, which is why clear planning and detailed pricing are so important from the outset. With the right design, the right team and a clear understanding of costs, the process becomes far more straightforward — and far less stressful.

If you’re considering an extension and would like to explore your ideas further, we’d be happy to help you understand what may be possible for your home.