home office

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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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Your Home Office

Although it came into being in the 1990’s it has taken the current Covid-19 crisis to push the ‘you don’t have to be there to be there’ spirit of video conferencing into the business mainstream through the technology of Zoom / Microsoft Teams / Google Hangouts and so on and with it the idea of the home office. Working from home some, or even all, of the time has previously been a bit of a novelty but now it is firmly established and I doubt we’ll go back to where we were just a few months ago.

Home Office 1 photo Herman Miller

Home Office 1 photo Herman Miller

This change will be bound to have consequences for the design of our homes in terms of the ensuing ‘live / work’ arrangements. As it is less than ideal to get the laptop out to work on the sofa, the kitchen table or the spare bedroom there is a clear need for a new building type which combines home and family with office and work. Just as we should have a good mattress because we spend so much of our lives on it we should have a good home office space.

Home Office 2 photo Neville Johnson

Home Office 2 photo Neville Johnson

My approach to residential design is that it is better to have a few spectacular set-piece spaces (living room, garden room, kitchen / dining room etc) rather than lots of nondescript cellular spaces (unused bedrooms etc). The home office should be another such space. IMO it should be attached to, but separate from, the house with a separating door and a separate wc and a tea / coffee point. It should be architecturally distinctive and spacious with sufficient room for best quality, ergonomically designed furniture including a comfortable office chair, large desk and a meeting / layout table and chairs. It should have hardwearing surfaces but at the same time good acoustics. Network cabling for fast and smooth internet access and data retrieval and space for storage, printing, filing and a hard copy library all need to be integrated into the design to ensure that it is clutter free. Quality architectural and task lighting are needed as is good natural lighting, with adjustable blinds, arriving from more than one direction so that the light quality changes as the day progresses. Oh – and ideally a view.

Sounds almost too good to be true and we can but hope that in due course another of the gifts of modern technology will be a reduced four day week!