Last Updated on July 8, 2021

Glass Box Extension Cost – UK Prices for 2021

Zac Houghton

Zac Houghton

Construction expert and founder of Loftera.

Did you know that installing a glass box extension in your property can increase the value of your home by up to 7%?

Let’s do some quick maths to figure out exactly how much that 7% increase could work out at for different house values.

Initial House Value

7% of Initial Value

House Value After Extension



















If you already know where your property value falls on the above table, you should now have a clearer understanding of how much your value could increase by after the work to install a glass box extension has been carried out.

Comparing quotes could save you
glass box extension costs

Table of Contents

How Much Does a Glass Extension Cost?

Glass box extensions are something of a modern-day phenomenon, and their increase in popularity isn’t difficult to comprehend. When built correctly, they look absolutely stunning, and provide something entirely different to any other type of extension out there.

The name gives it away, but a glass box extension involves an awful lot of windows, meaning that natural sunlight is plentiful with such a design. No matter the weather, you truly are a part of the outside world.

All the above is great, but you’re here to have your question answered – exactly how much does a glass extension cost?

Depending on the size of the extension being built, you can expect to pay between £14,000 and £70,000. We know, that’s a huge leap, but such a difference in cost comes down to one main factor – size. We’ll get into the difference in price between a large and small glass box extension shortly, but before then, let’s discuss your average glass box extension price.

Materials, design, size and location will have a say on your final price, but the average outlay for a medium sized glass box extension is between £25,000 and £35,000.

If you go into the project with a clear idea of what you would like your glass box extension to look like, you’ll be able to hold open discussions with your builders and maybe even agree a price before work begins. This way, there won’t be any unwanted surprises once the work has finished.

Comparing quotes could save you

Large Glass Box Extension Price

If you’re committing to a larger scale glass box extension, it goes without saying that you’ll have to pay more. The job will require more materials and the increased length of time will add to the overall labour costs. Generally, you can expect to pay between £35,000 and £70,000 for a large glass box extension. The larger the area covered, the higher on that scale you can expect to be.

Of course, you have to work to your budget, but building a large glass box extension can really change your home life for the better.

The bigger the area to work with, the more creative freedom you have to make the extension look exactly how you like. You know what the experts say – invest as much as you can into your home.

Small Glass Box Extension Cost

Working to a stricter budget? Or just prefer the look and feel to a smaller extension? You can have a small glass box extension built for between £14,000 and £27,000. Think back to our chart with the 7% value increase added on, and this quickly becomes a solid investment.

A major selling point of a smaller glass box extension is that it is unlikely to require planning permission, meaning that you can get to work without the hassle of intervention from the authorities.

If you aren’t looking to completely renovate the way your house looks, a small glass box extension is the ideal way to do just that.

Factors Impacting Your Glass Box Extension Price

We have touched on this, but there are a number of factors that can impact your final glass box extension price. Carry on reading below, and you’ll have a better idea of where you can make those all-important savings along the way.

Size and Layout

When it comes to extensions, size matters. How large you want the building to be comes down to personal preference, but the bigger the extension, the more materials required, and hours of labour taken, meaning that your final cost will increase.


If you live in London, expect to pay a premium compared to those outside of the capital. Don’t feel bad, this is the case with most aspects of like in the UK.


Whether you opt for double or triple glazed windows will be a deciding factor on your costs. Having said that, this isn’t an area where you should compromise on quality.


Living in the UK, we aren’t blessed with year-round sunshine. With this in mind, you’ll want to install adequate heating in order to get the best out of your extension for 12 months of the year.


Carpeting, laminate or tiled – the choice is yours. Each come with their own unique selling points, and differ in price as a result. Plan ahead, and budget for your preference.

Existing Condition

If the area that you are hoping to build on is in good condition already, then you shouldn’t have to worry too much. However, if it is in poor condition, there is every chance that you’ll have to pay extra to keep your extension secure.


Once the building work is complete, it’s time to decorate the inside of your glass box extension to your taste. You can hire a professional decorator, but where is the fun in that?

Comparing quotes could save you

Benefits of Building a Glass Box Extension

If you are already debating building a glass box extension, then you are probably well aware of the benefits of doing so. Be that as it may, there is no harm in us discussing them further.

We have already been over the increase in property value as well as an improved home life with natural light entering your home effortlessly. There are also the basics like enjoying the outside views of your garden and immersing yourself in nature. In addition, the warmer months will see an increase in ventilation within your home.


To summarise, glass box extensions look absolutely stunning. They can vary in price massively, so have a long think about the size and layout you are after, and don’t be afraid to shop around to compare prices.

If you wish to know more on how to finance an extension of this size, take a read of our recent blog.

Turn your dream extension into reality. Save up to 40% on your quote: