How much does it cost to build and extension?

The cost of a single storey extension will vary depending on where you are in the UK, the complexity of the build and the standard of build quality you are aiming for.

Build costs for a single storey extension can be broken down as follows:

If you’re looking to build an extension on a budget, you can achieve a single storey extension for around £1,500 to £1,900/m2 for basic quality.

For good quality, expect to pay between £1,900 to £2,200/m2.

For a high spec extension, you’ll pay between £2,200 to £2,400/m2.

Before you begin planning your single storey extension’s design, you need to set your budget and ensure you can afford what you’re planning. On top of the build cost, you’ll also need to factor in the following:

Architects’ fees: These work out at around 3-7% of the construction cost, with planning drawings around £2,700, and construction drawings at a similar rate

Structural engineer: If roof joists and foundations need to be specified you’ll need a structural engineer. This would cost in the region of £500 to £1,000

Survey: Between £500 and £1,500 if a survey of the existing house is required

Project manager: Factor in a fee of 3-7% of the build cost for project management (you can also agree a daily or hourly rate). If you are looking to keep costs down, you could always manage your own project

VAT: This is at a rate of 20% of the labour, materials and services

Planning fees: For a residential single storey extension in England, the cost of an application is £206. If you need a certificate of lawful development, you’ll pay £103; and it costs £34 per request for discharging planning conditions

Building control charges: These will vary according to your extension’s size; plan for between £200 (for an extension of 10m2) to £900 (for 80 to 100m2)

A party wall agreement: This typically costs from £700 to £1,000 per neighbour.

Additional fees: These can include a tree report (£250 upwards); a food risk assessment within food zones (£250 upwards); an ecology report, possibly required by your local authority (from £400); an archaeological report if your home is in an area of archaeological interest (possibly several thousand pounds); a historic building report, likely if your home is listed

The costs involved in fitting out a single storey extension very much depends on the room type you’re adding, a kitchen will be more expensive to equip than a home office , for example, but expect to pay:

Between £5,000 and £30,000 on your kitchen design (higher spec kitchens can cost considerably more)

From around £4,500 to £11,000 for your bathroom design, depending the quality of fittings; a shower room will cost a similar amount

Factor in between £25 to £100 per square metre for your chosen type of flooring

Add around £85 per square metre for plaster or dry-lining, plus paint if these finishes are not included in the build quote

Expect to spend between £1,500 to £2,000 per linear metre for sliding or bifold doors

Don’t forget to include the cost of adding heating to your new room. Extending an existing central heating system may only need two days’ work by a plumber, at around £150 per day (excluding materials).

Underfoor heating will be more expensive.

Electric under floor heating is a cheaper installation choice. However, water-fed underSoor heating, although more expensive to install, and possibly requiring the addition of a new boiler to cope with the demand, is cheaper to run in the long term. Expect to pay around £2,500 for a new boiler

A single storey extension is cheaper to build than a two storey extension because it needs less substantial foundations and steelworks, but building a double-storey extension is the most budget effective way to gain the most space in one project.

If now is not the time, but you might want to add a second storey in the future, now’s the time to up the spec of the extension to allow for this in future.

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