As the UK’s leading architectural design firm, we are often asked by our clients, “How much will my house extension cost?” In fact, it is the number one question asked by our clients and our answer is usually a very long and detailed explanation of costs and the wide range of factors that make up a home extension. Then we break out our construction cost calculator and add in these factors to create a total that is a good estimate of the final cost.

An average single-story extension about 4M X 5M would usually around £1,500 – £1,700 per m2 or more depending on location and other factors. A double story extension with the same footprint would cost, on average, around 50% more than the single-story extension.

So, what are these factors that could lead to a higher price per m2? Let’s take a look at some now:

Does It Include a Kitchen or Bathroom?

You would add around £5,000 for a bathroom and £10,000 for a kitchen depending on your needs and specifications.

Finishes and Fittings

Many homeowners choose premium finishes and fittings to personalise their home and this can lead to increased costs. These include tile finishes, glazing, premium carpeting and paint and sliding or folding door options.

Complex Site Constraints

Not every build site is ideal and complex site constraints can add to increased costs. Limited access, irregular soil type, drainage, pipework and meters as well as an existing structure can become expensive factors.


Where your home is located is a factor in the cost of your extension. If you live in London or the South East, costs would be considerably higher.

Materials and Construction Techniques

Finally, the type and quality of the construction materials used and the type of construction techniques employed by the builder could increase the cost of your extension by thousands.

How much will your home extension cost? For an accurate estimate, contact Lewis Visuals now and let one of our experts assist you today or click this link to use our construction cost calculator.