House Extension Cost by Size: 3m x 3m to 6m x 4m Price Guide (Essex 2026)

If you’re trying to work out your house extension cost by size, the first thing I need to know is how big the extension is. After that, I’ll ask a few more questions, such as whether you’re having a new kitchen, knocking through to create an open plan space, whether you need steels, underfloor heating or a bathroom. Once I know that, I can give you a realistic cost.

The size of the extension is what affects the build cost the most. We price extensions by the square metre because that’s the fairest and most accurate way to do it. The other way is to use a quantity surveyor, but for most domestic extensions, pricing by square metre works well.

The figures below are typical build costs for Essex in 2026.

House Extension Cost by Size Comparison

Extension Size	Floor Area	Estimated Build Cost
3m x 3m	9m²	£27,000 + VAT
3m x 4m	12m²	£33,545 + VAT
4m x 3m	12m²	£33,545 + VAT
3m x 5m	15m²	£38,864 + VAT
5m x 3m	15m²	£38,864 + VAT
4m x 4m	16m²	£40,364 + VAT
4m x 5m	20m²	£45,000 + VAT
5m x 5m	25m²	£56,250 + VAT
6m x 3m	18m²	£42,955 + VAT
6m x 4m	24m²	£54,000 + VAT

*These prices include the shell build, bifold doors, roof windows, plastering, plumbing and electrics. VAT is additional.

3 metre extension cost

A 3 metre extension is often used to make the kitchen bigger, extend the living room or create a games room or cinema room. The exact cost depends on the width of the extension and what’s included inside it.

3m x 3m extension cost

A 3m x 3m extension gives you 9m² of additional space. Because it’s a smaller extension, the cost per square metre is higher than a larger build. The build cost is typically £27,000 + VAT.

3m x 4m extension cost

A 3m x 4m extension provides 12m² of space and has a typical build cost of £33,545 + VAT. You’ll also see people searching for the average cost of a 4m x 3m extension.

4m x 3m extension cost

A 4m x 3m extension is exactly the same size as a 3m x 4m extension. They’re both 12 square metres, so they generally cost the same.

Average cost of a 4m x 4m extension

A 4m x 4m extension gives you 16m² of additional living space. The average build cost is around

£40,364 + VAT.

6m x 3m extension cost

A 6m x 3m extension gives you 18m² of space and has an estimated build cost of £42,955 + VAT.

6m x 4m extension cost

A 6m x 4m extension is 24m². The estimated build cost is £54,000 + VAT.

Frequently Asked Questions

Why isn't size the only thing that affects the price?

The build cost is based on the size, but the final price depends on what you’re putting inside the extension. Kitchens, bathrooms, underfloor heating, boilers, upgraded sliding doors, electric opening roof windows, flooring and decorating all increase the cost.

Why do larger extensions cost more?

A bigger extension needs more materials and more labour, so it costs more to build.

What do homeowners forget to budget for?

People often forget about flooring, decorating, landscaping, reconfiguring spotlights and other miscellaneous items that come up during the project.

How can you keep extension costs down?

Budget properly from the beginning and get a detailed quotation that includes everything. An extension costs what it costs. The important thing is knowing exactly what’s included before you start.

Are extensions in Essex more expensive?

Generally, yes. Material prices are fairly consistent across the UK. The difference usually comes down to labour costs.

How much will my extension cost?

The first question I’ll ask is what size extension you’re planning. Then I’ll ask whether you’re having a new kitchen, knocking through, whether you need steels, underfloor heating or a bathroom. Once I know those details, I can give you a realistic price.

Ready to Plan Your House Extension?

Whether you’re considering a compact 3m x 3m extension or a spacious 6m x 4m family room, our experienced Essex team can provide honest advice, realistic pricing, and a free, no-obligation quotation tailored to your home.

Jamie Lindsay is the founder of Kirkwood Build and has over 15 years of experience in residential construction, architectural design, and home extensions across Essex. He specialises in helping homeowners navigate the design, planning, and building process, sharing expert advice on extensions, renovations, and property improvements through the Kirkwood Build blog.