Single storey extensions: costing, planning and designing your single storey extension

Improve the layout and how you live in your home with our expert advice on costing, planning and designing a single storey extension

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Building a single storey extension is the perfect way to create extra living space in your home, while transforming your space to better suit your lifestyle. It is also the perfect opportunity to get more natural light into interiors with the addition of rooflights and much sought after bi-fold doors. Single storey extensions are a good alternative to moving house, too – just think of the savings you'll make in fees like moving van hire and stamp duty alone. 

But how to plan and design a single storey extension? And what does a single storey extension cost? Follow our expert, step-by-step guide to getting the planning, design and budgeting of your single storey extension right first time. 

Find much more advice on extending a house in our ultimate guide.

How much does a single storey extension cost?

There are different ways to break down the cost of a single storey extension, but here are some rough costs. 

Building work costs depend on the results you're after, but reckon to pay around £1,500 to £1,900 per square metre for basic quality; £1,900 to £2,200 per square metre for good quality; and  £2,200 to £2,400 per square metre for excellent quality.

For an average extension of 4m x 5m, the build cost would therefore be £30,000 upwards for basic quality; from £38,000 for good quality; and from £44,000 for excellent quality. 

Remember, this is without professional fees (see more on this below), which come in at around 10 to 15% and VAT. With these added, the lowest cost for a 4m x 5m single storey extension would be just shy of £40,000, with the upper end nearer £58,000.

Bear in mind too that a single storey extension is cheaper to build than a two storey extension because it needs less substantial foundations and steelworks. However, two storey extensions are more cost-effective per square metre, so consider your options carefully. And, if you think you may want to add a second storey in the future, now’s the time to up the spec of the extension basics (such as the foundations) to allow for this with minimal extra expense.

For an accurate estimate of how much your extension will cost, use our extension cost calculator.

What does it cost to do an interior fit-out on a single storey extension?

The cost of fitting out your single storey extension will depend on the rooms within it, but these interior fitting costs for extensions will give you a good start:

  • For a kitchen, budget from around £5,000 to £20,000, depending on the specification; if you go high end, costs can increase significantly on this.
  • For a bathroom, factor in from around £4,500 to £11,000, according to the level of fittings.
  • For a shower room, plan for between £4,500 and £11,000, again depending on your level of fittings.
  • For flooring, budget in the region of £25 to £100 per square metre.
  • For wall and ceiling finishes not included in the build costs, set aside from around £85 per square metre for plaster or dry-lining plus paint.

light filled single storey kitchen extension

A glazed roof brings light into the heart of this single storey kitchen extension

How much do bi-fold or sliding doors cost?

If you’re including bi-fold or sliding doors in your single storey extension, factor in £1,500 to £2,000 per linear metre. They are a great way to bring in light and connect your home to the garden – this also creates the feeling of even more space.

How much will it cost to add heating to a single storey extension?

It may be possible to extend your existing central heating system, which could mean a couple of day's work by a plumber, at around £150 per day excluding materials. 

Or you may opt for underfloor heating. Electric underfloor heating might be a simpler, less expensive option when it comes to fitting (it can be fixed in place by a DIYer but needs to be connected to the mains by a qualified electrician; day rates are around £150 per day), but water-fed underfloor heating will be less expensive to run over time. 

However, this may mean you need a new boiler to meet the demand; the Energy Saving Trust estimates the cost of a gas boiler replacement at around £2,300.

Side return extension exterior

(Image: © Tim Mitchell)

How much are professional project fees for single storey extensions?

Budget for architect design fees from three to seven per cent of the construction cost, with a minimum of around £2,700 to £4,000 for the planning drawings. Construction drawings will typically cost the same and you might need a structural engineer to size roof joists and foundations. Factor in £500 to £1,000.

A measured survey of the existing house will cost from £500 to £1,500 depending on the size of the property.

If you are employing your designer to put the contract out to tender, appoint the builder and administer the contract, budget for a further three to seven per cent of the build cost. Alternatively, agree a day or hourly rate for ad hoc site attendance.

modern single storey extension to a north London terraced home

This contemporary extension to a north London home by Lipton Plant Architects would cost in the region of £3,000 per square metre

(Image: © Lipton Plant Architects)

What does planning permission cost for a single storey extension?

The planning fee for a residential single storey extension in England is £206. A certificate of lawful development will cost you £103 and the fee for discharging planning conditions is £34 per request. 

Make sure you have gone through your plans thoroughly with an architect or builder who is familiar with the local planning authority and their preferences. 

Find out how to make a planning application with our easy guide. 

Bear in mind that other reports may be needed:

  • Tree report, £250 upwards;
  • Flood risk assessment within flood zones, £250 upwards;
  • Ecology report that may be required by your local authority, from £400, but can be much more;
  • Archaeological report if in an area of archaeological interest, possibly several thousand pounds;
  • Historic building report, likely if your home is listed.

glazed single storey extension

This glass roofed, single storey extension makes for a wonderful kitchen diner

What do building regulations for a single storey extension cost?

Building regulations approval is essential for a single storey extension. ‘Any extension will have to demonstrate compliance and this usually comes in the form of making a Building Control application to the local authority,’ explains Alan. ‘Each authority has its own table of charges or you can use a private certified Building Control firm if you prefer.’

The size of your extension will affect the fees for building regulations approval. Plan for from £200 for one to 10 square metres, and £900 for 80 to 100 square metres, whether you go for a full plans application with everything approved before you start, or a building notice application, where you need only give 48 hours’ notice of commencement.

What do party wall agreements cost?

Click to go to extension planning

‘If your neighbours consent formally to your single storey extension project, you won’t need a party wall settlement and can save money here. If not, having a surveyor arrange party wall agreements for you typically costs from £700 to £1,000 per neighbour,’ says renovations expert Michael Holmes.

Kitchen by Cave Interiors

(Image: © Cave Interiors)

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