Extension Costs | House Extensions | Kitchen Extensions |

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.

Heating

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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