The average cost of building an extension in the UK is £27,000 for a single-storey rear extension measuring 20 square metres in ground floor area. A 20 square metre two-storey rear extension would usually cost in the region of £43,000.
How much does an extension cost?
Although the above is an average extension cost for the UK, extensions do vary significantly in their dimensions and specification and there are a couple of variables which will determine your total house extension cost.
Size of area
Generally, the larger your extension, the less it will cost per square metre due to economies of scale coming into force on greater building projects. On average, a small single-storey rear extension measuring 12 square metres in ground floor area can cost as much as £1,700 per square metre. Whereas a large 60 square metre single-storey rear extension can cost in the region of £1,200 per square metre.
Number of storeys
In the same manner as economies of scale come into effect on extensions with a larger floor area, two-storey extensions (or more) do not cost double of equivalent single-storey extensions. As a general rule, a two-storey extension will cost 60% more than an equivalent single-storey extension. For example, where a single-storey rear extension measuring 25 square metres would cost in the region of £33,000, a two-storey rear extension with an equivalent ground floor area would cost in the region of £53,000.
It is increasingly popular to install 3 metre (10 foot) and over sliding-folding doors within a wall of an extension as part of the design to bring the outside, inside. As beautiful as these doors are, their installation costs do start from around £4,000 so are not recommended if you are on a very tight budget.
Function and fixtures
Your requirements for how you will use your new extension will also play a role in your total house extension cost. For example, a kitchen installation will require a greater element of plumbing, waste and electrical services to be extended from your existing home than what would be required if your new extension was to function as a dining room. In this scenario, you would also need to account for the cost of a kitchen installation.
The cost of all your furniture and fixtures is entirely ruled by your budget. You can easily spend £70,000 on new designer furniture and fixtures, especially if you are wanting a bespoke kitchen, or you can spend £2,000 for cheaper high-street equivalents. Hence, it is hard to provide an average cost for these elements of an extension.