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.
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 - £3,000 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
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Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer)
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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 - £3,000 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
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Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer)
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VAT 20%
Double Storey Extension = £69,000
Does it include a kitchen or bathroom?
The cost of fitting out your single storey extension will depend on the rooms within it.
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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.
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For a bathroom, factor in from around £4,000 to £10,000, according to the level of fittings.
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For a shower room, plan for between £4,500 and £11,000, again depending on your level of fittings.
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For flooring, budget in the region of £24 to £96 per square meter.
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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.
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If you’re wanting higher-end finishes e.g. tile finishes, fitted joinery etc. the costs will increase.
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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.
Preliminaries 10%
Foundations 15%
Structure 30%
External Envelope 20%
Mechanical & Electrical 15%
Finishes 10%