How much does a house extension cost?

If you are thinking about extending your home, it is vital to work out the cost of your home extension before you start the work. Therefore this article will discuss benefits of building your home extension and then provide an indicative cost levels for works in South East of England in 2011.

In the current UK economic climate more people are opting to build an extension rather than move, as it can be a cost effective way of adding value to your property. Depending on the size of the extension you are planning to build, it can add around 25% to the value of the property and basement or loft conversion will typically boost the property value by 10% to 15%.

Estimated cost of a house extension

Providing cost estimation for an ‘average’ home extension can be tricky – since there really is no average! Issues such as ground conditions, access to the site, location and proximity of services are a huge factor in the build budget. Our indicative costs are provided assuming that all of these issues are reasonable to deal with. The cost of building a home extension greatly depends on its size and style. As a rough guide, a single storey (20 metre sq) extension costs 22,340GBP or 1,117GBP per metre sq; two storey (40 metre sq) extension costs 46,080GBP or 1,152GBP per metre sq; a (32 metre sq) loft conversion costs 25,536GBP or 798GBP per sq metre; and a (32 metre sq) basement conversion costs 27,168GBP or 849GBP per sq metre.

These figures will be more variable and depend on the preferences of the homeowner. For example, stone flooring will have a greater cost implication than cheaper timber alternatives. A more detailed estimation for your extension may be obtained at the planning stage of your project. A builder will then be able to give an indicative estimate for the work.

Contingency fun

Of course, you can’t foresee unexpected problems but you can prepare for them. You should always set aside a contingency fund to deal with added costs that can, and often do, arise.


Extending your home is a good cost effective way to increase the value of your home and with the right design it will be a source of joy and satisfaction for years to come.

Aleksandra Bryla is RIBA Part I Architect
and founder of Young Architecture Architects in Kent

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Aleksandra is a RIBA Part 1 Architecural Assistant

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