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How Much is a Single Storey Extension

A single story extension is a great way to transform your home to better suit your lifestyle and is typically the best way to add value to your property. They can also be a great opportunity to get more natural light into your home with the use of rooflights and bi-fold doors.

There are two types of single storey extensions:

Rear Extension

The rise of the kitchen at the heart of the house creates a very liveable home and a kitchen/dining/living space is easy to create with a rear extension. Up to 8 metres (for a detached house)/6 meters (for a semi-detached/terraced house) from the original rear of the property can be typically achieved via Permitted Development. However, larger single-storey rear extensions are subject to the Neighbour Consultation Scheme.

Side Extension

Side extensions are good for creating secondary type rooms – maybe a utility, a study or if you have the space, a bedroom. Side extensions can be built up to half the width of your original house.


How much does a single storey extension cost?

Build Costs

A good place to start is to work out how big the extension will be. This is the main governing factor of the cost of an extension – typically if you worked out a square meter at £1500, that’s a good base price for basic quality build costs.

For an average single storey extension of 20 square metres, the build cost would be from £30,000 for basic quality build costs

A single storey extension of 20 square metres of basic quality will will cost you a minimum of £44,000 including most fees, but excluding interior fit-out costs.

Interior Fit-Out Costs

(These costs are in addition to the build cost of the extension)

  • Kitchen
    Budget from £7,000 to £15,000 – the biggest cost factor is the appliances – the better the appliances, the higher the cost
  • Bathroom
    Budget around £5,000 assuming you can connect to existing pipework
  • Utility Room
    Budget from £1,500 to £11,000 depending on size and quality of appliances
  • New Boiler
    Budget from £2,000 to £3,000 depending if the boiler is being fitted in an existing or new location
  • Flooring
    Budget from £25 to £100 per square metre
  • Underfloor Heating
    Budget from £10,000 to £15,000
  • Wall and Ceiling Finishes
    Budget around £85 per square metre for plaster or dry-lining plus paint
  • Bi-fold or Sliding Doors
    Budget from £1,500 to £2,000 per linear metre

Professional Fees

  • Measured survey – £500 to £1,500
  • Planning and construction drawings – £2,700 to £4,000
  • Architect design fees – three to seven percent of the build cost (e.g. a fee of £900 to £2,100 for a £30,000 extension)
  • Structural engineer – £500 to £1,000
  • Contract management (employing someone to hire the builders/tradesmen) – three to seven percent of the build cost (e.g. a fee of £900 to £2,100 for a £30,000 extension)
  • Project management by a building contractor or architect will be 15 to 20 percent (up to £4,500) on top of the net cost of labour, materials and overheads
  • VAT at 20 percent of the labour, materials and services if applicable

An extension of 20 square metres at a build cost of £30,000 could potentially attract professional fees from £12,000.

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FREE: Home Extension Cost Guide

Inside this easy to ready free guide, you will learn how much extensions, loft conversions and other renovations to your home are typically priced.

Permission Fees

Additional reports that may be required

  • Tree report (requires an arboriculturist)from £300
  • Ecological ‘Phase 1’ report (required if your project might impact protected species)  – from £600
  • Flood risk/drainage/archaeological/environmental assessment – from £350
    (If problems are found, then this will add additional charges to rectify the issue)
  • Historic building report (if your home is a listed building) – £750 to £1,000 on average for a typical three-bedroom listed building


  • Building Regulations Approval
    You don’t need to get approval yourself if you use someone registered with a competent person scheme. If you are carrying out the building works yourself, or you’re working with someone who’s not registered with a competent person scheme, then budget from £400 – £750
  • Party Wall Agreement
    (Covers shared walls/structures and protects both yourself and your neighbours whilst work is being carried out) You can issue a Party Wall Notice yourself (for no charge) to each of your affected neighbours. If your neighbour agrees to the works in writing, you will not require a Party Wall Agreement.

    If the neighbour refuses to give consent or fails to respond within 14 days, then a surveyor will need to be appointed to arrange a Party Wall Award that will set out the details of the work which will carry a fee of £700 to £1,000 per neighbour
  • Build Over Agreement
    (If you’ve got a shared drain running across the back of your garden and you’re going to build over it or within three meters proximity)
    Budget around £335

All together, it’s worth setting aside at least £2,000 to cover any fees for permissions and sign off



If you are carrying out conversion/extension works you should arrange specialist insurance to cover the new works and the existing structure as most home insurers won’t cover works if contracts exceed £50,000. For expert advise on this and any other insurance exposures, please contact Gary Philip at GPS Insurance Brokers on 020 8207 7385. GPS Insurance

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Total Cost

Single Storey

For a single storey extension of 20 square metres of basic quality, this will give you a minimum spend of £44,000 including most fees, but excluding interior fit-out costs.


Getting An Accurate Figure

To calculate the cost of your project, Yoop Architects take a measured survey of your property, which captures all the information. Then we draw it all up in 3D and create sketches to show sometimes five or six options if it’s not entirely clear how you want to achieve your brief.

A lot of the times there are different ways of achieving your desired result and the more you work with the building, the cheaper, easier and faster your build will be. Once you’ve got a favorite scheme or a couple of favorite schemes, we have a phase called ‘Meet the Builders’. That’s when we can introduce you to a couple of builders, they visit the property and they give you a ballpark figure.


Thinking of your dream home improvement or how to start your project? 
Need your planning questions answered? Talk to an expert today.
Book a FREE 20 Minute Call with one of our home improvement Architects.

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Post Author: yoopadmin