Extensions are a good alternative to moving house – just think of the savings you'll make in fees alone. 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.
But what does a single storey extension cost? And how should you go about planning it? Follow our expert, step-by-step guide to getting the planning and budgeting right first time.
How much does a single storey extension cost?
There are different ways to break down the cost of a single storey extension.
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 10-15% 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.
Get a comprehensive extension cost calculation using our free extension cost calculator
What does it cost to do an interior fit on an 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.
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.
How much will it cost to add heating to an 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.
How much are professional project fees for extensions?
Budget for 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.
What does planning permission cost for an 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 £28 per request. Bear in mind that other reports may be needed:
- Tree report, £800 upwards
- Flood risk assessment within flood zones, £800 upwards
- Ecology report that may be required by your local authority, from £800, 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
Will you need planning permission?
When planning a more ambitious single storey extension, you will need planning permission if:
- Your extension covers more than half the area of land surrounding your home
- You are extending towards a road
- You are increasing the overall height of the building
- You are extending more than six metres from the rear of an attached house
- You are extending more than eight metres from the rear of a detached house
- Your single-storey extension is taller than four metres
- Your single-storey extension is more than half the width of your house
- You are using materials that differ from the original style of the house
- You plan on building a balcony or raised veranda
The cost of a planning application in England and Wales is £206, an expenditure that you don’t want to have to make several times. 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.
You may be able to make use of permitted development (PD) rights for your single storey extension, which allow you to build without having to make a planning application.
However, under PD, you cannot use contrasting materials for an extension — they have to be similar to that of the existing house. If you want something different, permission will be needed. The footprint and height of the extension will also have to meet PD criteria.
If building under PD, you may want to apply for a certificate of lawful development from your local authority. You can build without one, but it will give you paperwork to prove that your scheme met requirements and did not need planning permission.
The general PD criteria is detailed on the official government website, but check with your local authority as well, because some areas have more restricted rights, such as in Conservation Areas. Local authorities can also use Article 4 Directions to remove rights.
What do building regulations for an extension cost?
Building regulations approval is essential. ‘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 £250 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?
‘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 Michael.
How to find an architect or designer for your extension
‘As well as finding a designer you can talk to easily, make sure that they will carry out small-scale work,’ says Alan Cronshaw of Acronym Architecture & Design.
‘Choose someone who can demonstrate completion of similar schemes that impress you. You may want to choose a firm that is located fairly near to your house. Some designers charge for an initial visit; others do this for free. This may inform how many you decide to get into an initial dialogue with.’
How to design a single storey extension
‘It is good to have a brief for your designer, but this can be loose,’ says Alan. ‘If you want an open-plan layout, a better relationship between house and garden or simply more space, your designer will advise you on how to achieve this, perhaps with options. If you want expensive items, such as a certain brand of kitchen, or bi-fold or large sliding doors, let the designer know so the added cost can be factored into your budget.’
Think about the future, too. ‘Single-storey extensions usually need less substantial and, therefore, cheaper foundations and steelwork/lintels over openings than a two-storey design. However, if you think you may build another storey above your extension one day, it may be worth planning for this expansion from the outset,’ says Alan. ‘If you are thinking of doing this, make it clear to the professionals you hire.’
Finding a builder for your single storey extension
Recommendation, as well as trade bodies such as the Federation of Master Builders, can help you to find reliable contractors for building your single storey extension.
‘Decide if you want to use a larger, faster but more expensive firm that may be more flexible, or a smaller firm that will be less expensive,’ says Marta de Sousa of property developers Lux Reality.
‘Make sure that any contractors you use have warranties in place, such as Masterbond. Draw up a watertight contract using a template, such as a JCT homeowner contract, and make sure to state in it that payments be made following the completion of specific parts of work and not at different stages, as it is usually very hard to define when you are halfway or a quarter of the way through an extension project.’
Creating an indoor-outdoor link
Why merge indoor and outdoor spaces? Creating a visual link between your new extension and the garden will make both spaces feel bigger and cohesive. Here's how:
Ensure good sightlines
Opt for the widest panel possible when you’re choosing bi-fold doors for uninterrupted outlooks to your garden space. ‘The maximum panel width for this door style is 1.2 metres, providing a large expanse of glass and maximising the view outside,’ explains Neil Ginger, chief executive officer at Origin.
Opt for bi-fold doors
‘There are a range of different options available for bi-fold doors in a single storey conversion, from two-door models through to large eight-door configurations, set-ups for bays and 90-degree corner sections,’ explains Neil Ginger, CEO at Origin. ‘The price of doors varies but, as a guide, a bespoke, aluminium design would cost from £1,500 per door leaf.
‘The space allowance for the doors to open outside depends entirely on their width, which can be as narrow as 40cm, protruding less than half a metre outwards. You will need to allow just over a metre of space for doors with a width of 1.2 metres.
Choose continuous flooring
Having a stone floor that looks as good in the extension as on the patio can create an expansive feel. ‘Make sure the stone you choose will work outside and in, and make sure both are fitted to a solid substrate,’ says Jo O’Grady, marketing director at Stone Age.
‘Often the exterior stone will be thicker than the internal and it may require a slightly coarser finish. Depending on the climate and surroundings of the area you live in, bright and dazzling limestones work well, as they help to open up the space and illuminate when the sun hits it — try creamy Brabazon or Piedra Plana Extra.’
Make materials blend
Try to think of your furniture choices for the interior and exterior together. ‘Choosing matching finishes, colours and textures for outdoor and indoor furniture is a great way to keep a coherent look that flows from within the home and out into the garden or patio,’ says John Sims Hilditch, co-founder of Neptune.