Costing your home extension

Home extension costshome extension cost

It is impossible to give a simple guide to the cost of a house extension because there are so many factors that affect it. For a very basic round figure of building costs £1000/m² of floor area is sometimes quoted but this can vary significantly particularly upwards depending upon the exact nature of the scheme. We have listed some of the main factors that can affect home extension costs but obviously there may be others in some circumstances.

 Where you live

There are some regional differences in home extension costs, in particularly areas such as London where overheads and labour rates will tend to be higher it will be more costly. There could however be individual circumstances, such as being particularly inaccessible that might make some homes more costly to extend than others in the general locality.

Size and Shape

In general terms very small house extensions will tend to be proportionately more expensive than larger ones, however beyond certain sizes (depending upon the construction) it may be necessary to have additional beams or columns and thicker walls etc. which would tend to reduce some of the savings due to its size. In general a simple floor plan of square or rectangular shape will be more economical to build than some highly complex scheme of interlocking octagons with an equally complicated roof!

The number of storeys of the home extension

A building of several storeys will often be cheaper to build than an equivalent area of single storey; this is in part due to there often being little difference in the foundations and roof by the addition of another storey. It should be borne in mind though that the Building Regulations have additional requirements particularly in relation to fire regulations, the higher the extension goes. At second floor level it will normally require a protected stairway (fire doors and walls around the stair area leading to a final exit door) and above this a separate means of escape. This could affect the existing building as well as the house extension. Where building over someone else’s property, which may occur with some extensions to flats, it should be borne in mind the floor will need to be built to a high sound proofing standard with implications in terms of cost.

Work to the existing house

Few home extensions involve just the building of the new part, it will often require some interconnection with the original house but there may be additional works as well, perhaps rearranging the layout in the existing part such as creating corridors, en-suites etc. It may be that with the additional accommodation the heating, plumbing and electrical systems will be stretched or in need of renewal anyway. Where there is a significant alteration to the roof it may be time to consider whether the existing tiles still have a reasonable lifespan, as such these additional expenses need to be added to the home extension costs.

Quality of materials and finish in the home extension

There can be considerable differences between the cost of materials between for example a very basic brick and tile and high quality hand made bricks and tiles. These differences can sometimes be even more marked with the internal finishes. A home extension containing a kitchen or bathroom will inevitably be more expensive than a fairly clear space such as a living room, but in addition to which there can be a vast difference between budget lines from DIY retailers and high end luxury products.

Extensions partly within the existing structure

On a price per square metre loft conversion costs and garage conversion costs will tend to be less than for new buildings provided the work required to upgrade them is not too extensive.

With basement costs, converting an essentially useable existing basement may be fairly economical whilst creating a totally new basement will tend to more expensive than other types of extension.

 Extent of the work done by yourself

From an extreme case of a full DIY project, some people chose to do a part of the work. In most cases this will be just the final decoration but where one has particular skills such as plumbing, electrics, plastering then the builders costs will be less if these are not included in his quote. Be realistic though of your abilities and make sure you do not hold up the work the builder is contracted to do.

Some people manage the project using individual tradesmen and buying the materials themselves, this obviously saves the overheads and profit of a main contractor but can involve considerable time in sourcing labour and materials if you are not used to it and any mistakes maybe down to you.

Don’t forget the VAT!

Most home extensions will be subject to VAT at the normal rate, new houses are zero rated whilst some types of conversions to residential use from other uses and houses that have been empty for two years or more are charged at a lower rate. These are complex areas that need to be researched for the individual circumstances. See HMRC website. Occasionally builders will not be registered for VAT, this will not be very often as it would not take many simple extensions in the course of a year to reach the threshold. Where it occurs the saving will not generally be the full amount of VAT as they will probably have had to pay VAT on the materials and probably to some sub-contractors.

Others costs

In addition to the actual home extension costs there are a number of other charges etc. that will form part of most home extension projects. We have listed many of the typical ones although the list is not necessarily exhaustive, on the other hand for many fairly straightforward schemes a lot of the potential costs listed will not apply.

  • Design Costs: Architectural and sometimes structural, interior, heating etc
  • Council fees including planning and building regulations,
  • Party wall surveyor and associated costs,
  • Covenant approvals etc.
  • Costs associated with specific aspects of a scheme such as sewer build over agreement, arboricultural reports etc.
  • Supervision or project management cost
  • Quantity surveyor.
  • Other costs you may incur as a result of the work taking place, perhaps renting another property, renewal of carpets, curtains etc.


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