Single storey extension: how to plan and cost your build in 2018

Improve the functionality of your home with this expert advice on adding a single storey extension to your home

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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.


Related articles: The ultimate guide to extending your home | 10 ways to cut the cost of your extension | How to build a two storey extension | How to build a glass extension

How much will your single storey extension cost?

Building work

The costs of the single storey extension depends on the result you’re after. Use these figures, based on a finished cost per square metre (£/m²), from experienced renovator Michael Holmes:

  • Basic quality £1,500 to £1,900
  • Good quality £1,900 to £2,200
  • Excellent quality £2,200 to £2,400

Get a comprehensive extension cost calculation using our free extension cost calculator

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)

The cost of fitting out your single storey extension will depend on the rooms within it.

  • 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

Bi-fold or sliding doors

If you’re including bi-fold or sliding doors in your single storey extension, factor in £1,500 to £2,000 per linear metre.


‘It may be possible to extend your existing central heating system,’ says Alan. ‘Or you may opt for underfloor heating.’ If 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.

kitchen extension with glazing from Origin

An extension to this West Midlands home allowed the kitchen and living room to be incorporated into a more family-friendly, contemporary space. Triple-glazed bi-fold doors from Origin were chosen to connect the new room to the garden. Windows and doors cost £13,000 in 2013

(Image: © Origin)

Single storey extension: Professional project fees

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.

Planning permission 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 £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

glazed single storey extension

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,’ says Alan. ‘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.

single storey extension completed under permitted development by Architect Your Home

This Middlesex house lacked a dining space, so the owners asked Architect Your Home to create an area that opened up to the garden. Designed to fall within permitted development rights, a reverse mono-pitch roof finished in semi-weathered zinc was combined with sliding glazed doors and flush glazed rooflights. It cost £110,000 including construction, fixtures and fittings

(Image: © Architect Your Home)

Building regulations for a single storey extension:

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.

Party walls

‘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.’

You can find architectural technologists on and architects via or RIBA.

side return single storey extension

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 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.’

single storey traditional kitchen extension

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.

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.

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.’

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.