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House Extension Cost

How much does a house extension cost in London and the south east?

My analysis of actual costs for competitively tendered projects by reputable local building contractors in the south east within commuting distance of London suggests a current ‘ball park’ figure of £3,000/m2 for a single storey extension and a further £2,000/m2 where there is a second storey. These figures are based on a full fit-out to a reasonable standard of specification including bathrooms. They will go up if the project is located closer to central London and down if the project is further out. Tender prices have been rising steadily over the last few years. The tender price inflation forecast in this month’s Building Magazine is for a 5% increase over the coming year however builders that I have spoken to are suggesting that it will be more than this. House extensions attract VAT at 20% and a client contingency of say 10% is advisable.

Single storey extension at 'Charwin' under construction

Single storey extension at ‘Charwin’ under construction

Whilst m2 area costs are a good guide, design complexity and the level of specification can make a big difference.  There are also project specific costs such as the cost of demolitions where required and the cost of other work within the existing house. The cost of the foundations can increase beyond the cost of the standard depth of one metre if the ground conditions are poor. External drainage costs can increase if extensive reconfiguration is required. If the design requires a structural steel frame rather than individual steel members this would be an extra as would the cost of a ground source heat pump installation with its associated ground loops and / or bore holes. The biggest single cost that is not included above is the cost of a new kitchen which can vary enormously. Several years ago I had two concurrent projects where the cost of the similarly sized kitchens differed by a factor of ten.

Two storey extension at 'Fir Tree Cottage' under construction

Two storey extension at ‘Fir Tree Cottage’ under construction

Architects provide ‘ball park’ estimates based on m2 rates but if cost ‘is of the essence’ a quantity surveyor or construction cost consultant / estimator can produce a more detailed estimate on an ‘elemental’ basis where every element of the project is costed. Typically for a project of this size clients opt to rely upon the architect’s estimate until the tenders are returned. At the end of the day it is the tender price at which a building contractor is willing to carry out the work that matters.

I am usually the starting point for a house extension project. The full architectural service is typically in three roughly equal parts which are concept design and developed design to Planning Application / technical design up to Building Regulations and tender / contract administration and construction to handover and close out. Partial services are available (eg just the first part or just the first two parts). I normally provide the ‘principal designer’ role as part of the architectural service. During the technical design stage the core team typically comprises a structural engineer to provide calculations for Building Regulations purposes, an underground drainage engineer to carry out the design for the underground drainage and an energy assessor to demonstrate energy efficiency in compliance with the Building Regulations. Consultant fees for this core team will typically be around 13% of the building cost. There will also be local authority fees for Planning (where required) and Building Regulations.

A house extension can be a viable alternative to moving house as it avoids the disruption and the cost of moving, stamp duty, legal and estate agents fees etc. On top of the desire for extra living space and bedrooms, information technology is driving a move towards living and working from home with the demand for additional space for a home office. As well as this the process, if handled properly, can be very rewarding.

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Doer Upper Do’s and Don’ts

‘Demand for the countryside soars as Londoners seek to flee the capital’ says the headline. I can believe it more on account of the positive impact of information technology than the negative impact of Covid-19.

For those looking for not just a purchase but a ‘project’ here are some tips:

Before purchasing anything be sure the town planning position is checked out with the council to make sure that what is proposed is possible in policy terms. Is there scope to construct a new house or replace the existing house with something larger or to extend it? Has the existing property already been extended up to the permissible limits?

New build projects. Finding a decent virgin site is easier said than done and many of those that do emerge are blighted by being either undersized pieces of existing gardens or fragments of land never previously considered suitable for development. IMO it is better to upcycle a decent sized, well located site that has an existing undersized house such as a bungalow on it. Whilst it might cost more initially it secures a mature location and avoids the considerable cost to both the public utility companies and the contractor in making network connections and bringing the likes of electricity, gas, water and telephone / broadband onto the site and taking sewerage out of the site. Another considerable cost payable to both the highways authority and the contractor is the creation of a new access to a public highway.

Fir Tree Cottage - Before

Fir Tree Cottage – Before

Fir Tree Cottage - During

Fir Tree Cottage – During

Fir Tree Cottage - After

Fir Tree Cottage – After

Refurbish and / or extend projects. Whether to demolish what is there or not can be driven by the VAT position. Whilst it is not very sustainable, as it encourages demolition rather than re-use, VAT on new build houses is zero rated whereas VAT at 20% is payable on other building work. It is permissible to incorporate existing foundations and anything else below ground level into a new build house and still avoid the VAT which can save a considerable amount if the new design can be configured accordingly. Where refurbishing or extending a property don’t ‘pay’ twice for things that are going to be removed as part of the work – be happy so long as their poor state is reflected in the price since they are coming out anyway. Externally this could be the roof tiles and external windows and internally the kitchen and bathrooms. In the new design retain the kitchen and bathroom locations (but not the fittings etc) as moving the intakes and drainage is comparatively expensive whereas the creation of new bedrooms is comparatively cheap.

Lastly, employ an architect. The few % spent on an architect at the outset is probably the best investment you’ll make over the course of the ‘project’.