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How Much Does a House Extension Cost?

It’s not an easy question to answer and obviously depends on a number of factors, here are some general guideline, a starting point for you to use that will allow you to do some basic estimations for your project.


  • Standard quality £1,250 to £1,550

  • Good quality £1,550 to £1,750

  • Excellent quality £1,750 to £2,050                                                                                                                                     

Let’s start with a standard single storey extension

for example 3m x 5m

A realistic figure for a single storey would be around £1,400 - £2,050 per m2 

So, for your 3 x 5m single extension, the build cost at the lowest end of these estimates would be roughly £21,000

  • Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer)

  • VAT 20%

Single storey extension = £28,000

Small extensions, under 15–20 square meters, have higher costs per square meter


How about a double storey extension 3m x 7m

The cost of a double storey extension would be around £1,250 - £2,050 per m2

So, for your 3 x 7m double extension, the build cost at the lower end of these estimates would be roughly £52,500

  • Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer)

  • VAT 20%

Double Storey Extension = £69,000


The above figures would apply to a typical, uncomplicated extension in most parts of the UK. However, you should consider that these figures would creep up if any of the following apply:


Does it include a kitchen or bathroom?

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

  • For a kitchen, budget from around £4,500 to £16,500, depending on the specification; if you go high end, costs can increase significantly on this.

  • For a bathroom, factor in from around £4,000 to £10,000, 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 £24 to £96 per square meter.

  • For wall and ceiling finishes not included in the build costs, set aside from around £77 per square metre for plaster or dry-lining plus paint.


Finishes and fittings

Above costs assume modest finishes, fittings, electrics and lighting. White wash painted walls, carpet floors or possibly engineered oak.

  • If you’re wanting higher-end finishes e.g. tile finishes, fitted joinery etc. the costs will increase.

  • If you’re including bi-fold or sliding doors in your single storey extension, factor in £1,400 to £1,800 per linear metre.                                                                                                                                                              


It may be possible to extend your existing central heating system, Or you may opt for underfloor heating. If you need a new boiler to meet the demand, the cost of a gas boiler replacement at around £2,400.  


Professional project fees

Budget for design fees from three to seven per cent of the construction cost, with a minimum of around £2,400 to £3,600 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.

Are there any complex site constraints?

Your builder will need to include additional costs in their quotation if any of the following apply:

  • Your soil type and ground conditions may present more costly construction methods / techniques.

  • Limited access to the site. Will it be easy for the builder to bring in tools and machinery easily?

  • Existing structure. Are you removing any structural walls? If so there will likely be additional costs for steelwork.

  • Are you moving any pipework, drainage, gas meters?

Preliminaries 10%

Foundations 15%

Structure 30%

External Envelope 20%

Mechanical & Electrical 15%

Finishes 10%




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