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Extension Costs Outside of London

What contributes to the cost of an extension?

The main governing factor on the cost of an extension is square footage or square meterage – how big it is. So, the bigger it is, the more it’s going to cost. Typically if you worked out a square meter at £1500, that’s a good base price for basic quality; £1900 per square meter for good quality and £2200 per square meter for excellent quality.

Build Cost Guide

Single Storey Extension
For an average single storey extension of 20 square metres, the build cost would be from £30,000 for basic quality, from £38,000 for good quality and from £44,000 for excellent quality.

Two Storey Extension
Budget from £53,000

Loft Extension
Budget from £15,000 to £40,000 depending on conversion type

These costs per square meter are build costs only and so additional costs that you add on top of that would depend on questions such as;

Are you going to have a new kitchen?

Are you redoing the floor throughout the house and having underfloor heating?

What kind of floor finish do you want?

Are you putting in large bi-folds or sliding doors?

Are you putting in big roof lights?

Are you having new doors throughout? New radiators? New boilers?

All of these kinds of things contribute to the final cost of an extension.

Typically if you worked out a square meter at £1500, that's a good base price for the cost of an extension.

Interior Fit-Out Cost Guide

(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
  • Flooring
    Budget from £25 to £100 per square metre
  • Underfloor Heating
    Budget from £10,000 to £15,000
  • New Boiler
    Budget from £2,000 to £3,000 depending if the boiler is being fitted in an existing or new location
  • 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
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Inside this easy to read free guide, we will teach what you need to know as you consider extending your home

Additional Costs

Professional Fees
(Not all of these will be necessary depending on what you are planning)

  • Measured survey – £500 to £1,500
  • Planning and construction drawings – £2,700 to £4,000
  • Architect design fees – 3 to 7 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 builder/tradesmen) – 3 to 7 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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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
    Budget from £200 for 1-10 square metres
  • Party Wall Agreement
    (Covers shared walls/structures and protects both yourself and your neighbours whilst work is being carried out)
    Budget from £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


Total Cost

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 which have too many variables to be able to take into consideration at this stage.


For a two storey extension, if the second storey is the same size as the first then the generally-accepted rule is to add 50% extra to the cost of a single storey extension plus an additional 10% for professional fees and VAT. 

This gives you a minimum spend of about £87,000 including most fees, but excluding interior fit-out costs.


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